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Message-Id: <1471878705-3963-2-git-send-email-sumit.semwal@linaro.org>
Date: Mon, 22 Aug 2016 20:41:44 +0530
From: Sumit Semwal <sumit.semwal@...aro.org>
To: linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
linaro-mm-sig@...ts.linaro.org, linux-doc@...r.kernel.org
Cc: corbet@....net, linux-kernel@...r.kernel.org,
markus.heiser@...marit.de, jani.nikula@...ux.intel.com,
Sumit Semwal <sumit.semwal@...aro.org>
Subject: [PATCH v2 1/2] Documentation: move dma-buf documentation to rst
Branch out dma-buf related documentation into its own rst file to allow
adding it to the sphinx documentation generated.
While at it, move dma-buf-sharing.txt into rst as the dma-buf guide too;
adjust MAINTAINERS accordingly.
v2:
- Removed authorship as suggested by Jani,
- Address review comments from Jonathan Corbet and Markus Heiser.
Signed-off-by: Sumit Semwal <sumit.semwal@...aro.org>
---
Documentation/DocBook/device-drivers.tmpl | 41 ---
Documentation/dma-buf-sharing.txt | 482 ----------------------------
Documentation/dma-buf/guide.rst | 507 ++++++++++++++++++++++++++++++
Documentation/dma-buf/intro.rst | 82 +++++
MAINTAINERS | 2 +-
5 files changed, 590 insertions(+), 524 deletions(-)
delete mode 100644 Documentation/dma-buf-sharing.txt
create mode 100644 Documentation/dma-buf/guide.rst
create mode 100644 Documentation/dma-buf/intro.rst
diff --git a/Documentation/DocBook/device-drivers.tmpl b/Documentation/DocBook/device-drivers.tmpl
index 9c10030eb2be..a93fbffa9742 100644
--- a/Documentation/DocBook/device-drivers.tmpl
+++ b/Documentation/DocBook/device-drivers.tmpl
@@ -128,47 +128,6 @@ X!Edrivers/base/interface.c
!Edrivers/base/platform.c
!Edrivers/base/bus.c
</sect1>
- <sect1>
- <title>Buffer Sharing and Synchronization</title>
- <para>
- The dma-buf subsystem provides the framework for sharing buffers
- for hardware (DMA) access across multiple device drivers and
- subsystems, and for synchronizing asynchronous hardware access.
- </para>
- <para>
- This is used, for example, by drm "prime" multi-GPU support, but
- is of course not limited to GPU use cases.
- </para>
- <para>
- The three main components of this are: (1) dma-buf, representing
- a sg_table and exposed to userspace as a file descriptor to allow
- passing between devices, (2) fence, which provides a mechanism
- to signal when one device as finished access, and (3) reservation,
- which manages the shared or exclusive fence(s) associated with
- the buffer.
- </para>
- <sect2><title>dma-buf</title>
-!Edrivers/dma-buf/dma-buf.c
-!Iinclude/linux/dma-buf.h
- </sect2>
- <sect2><title>reservation</title>
-!Pdrivers/dma-buf/reservation.c Reservation Object Overview
-!Edrivers/dma-buf/reservation.c
-!Iinclude/linux/reservation.h
- </sect2>
- <sect2><title>fence</title>
-!Edrivers/dma-buf/fence.c
-!Iinclude/linux/fence.h
-!Edrivers/dma-buf/seqno-fence.c
-!Iinclude/linux/seqno-fence.h
-!Edrivers/dma-buf/fence-array.c
-!Iinclude/linux/fence-array.h
-!Edrivers/dma-buf/reservation.c
-!Iinclude/linux/reservation.h
-!Edrivers/dma-buf/sync_file.c
-!Iinclude/linux/sync_file.h
- </sect2>
- </sect1>
<sect1><title>Device Drivers DMA Management</title>
!Edrivers/base/dma-coherent.c
!Edrivers/base/dma-mapping.c
diff --git a/Documentation/dma-buf-sharing.txt b/Documentation/dma-buf-sharing.txt
deleted file mode 100644
index ca44c5820585..000000000000
--- a/Documentation/dma-buf-sharing.txt
+++ /dev/null
@@ -1,482 +0,0 @@
- DMA Buffer Sharing API Guide
- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-
- Sumit Semwal
- <sumit dot semwal at linaro dot org>
- <sumit dot semwal at ti dot com>
-
-This document serves as a guide to device-driver writers on what is the dma-buf
-buffer sharing API, how to use it for exporting and using shared buffers.
-
-Any device driver which wishes to be a part of DMA buffer sharing, can do so as
-either the 'exporter' of buffers, or the 'user' of buffers.
-
-Say a driver A wants to use buffers created by driver B, then we call B as the
-exporter, and A as buffer-user.
-
-The exporter
-- implements and manages operations[1] for the buffer
-- allows other users to share the buffer by using dma_buf sharing APIs,
-- manages the details of buffer allocation,
-- decides about the actual backing storage where this allocation happens,
-- takes care of any migration of scatterlist - for all (shared) users of this
- buffer,
-
-The buffer-user
-- is one of (many) sharing users of the buffer.
-- doesn't need to worry about how the buffer is allocated, or where.
-- needs a mechanism to get access to the scatterlist that makes up this buffer
- in memory, mapped into its own address space, so it can access the same area
- of memory.
-
-dma-buf operations for device dma only
---------------------------------------
-
-The dma_buf buffer sharing API usage contains the following steps:
-
-1. Exporter announces that it wishes to export a buffer
-2. Userspace gets the file descriptor associated with the exported buffer, and
- passes it around to potential buffer-users based on use case
-3. Each buffer-user 'connects' itself to the buffer
-4. When needed, buffer-user requests access to the buffer from exporter
-5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
-6. when buffer-user is done using this buffer completely, it 'disconnects'
- itself from the buffer.
-
-
-1. Exporter's announcement of buffer export
-
- The buffer exporter announces its wish to export a buffer. In this, it
- connects its own private buffer data, provides implementation for operations
- that can be performed on the exported dma_buf, and flags for the file
- associated with this buffer. All these fields are filled in struct
- dma_buf_export_info, defined via the DEFINE_DMA_BUF_EXPORT_INFO macro.
-
- Interface:
- DEFINE_DMA_BUF_EXPORT_INFO(exp_info)
- struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info)
-
- If this succeeds, dma_buf_export allocates a dma_buf structure, and
- returns a pointer to the same. It also associates an anonymous file with this
- buffer, so it can be exported. On failure to allocate the dma_buf object,
- it returns NULL.
-
- 'exp_name' in struct dma_buf_export_info is the name of exporter - to
- facilitate information while debugging. It is set to KBUILD_MODNAME by
- default, so exporters don't have to provide a specific name, if they don't
- wish to.
-
- DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct dma_buf_export_info,
- zeroes it out and pre-populates exp_name in it.
-
-
-2. Userspace gets a handle to pass around to potential buffer-users
-
- Userspace entity requests for a file-descriptor (fd) which is a handle to the
- anonymous file associated with the buffer. It can then share the fd with other
- drivers and/or processes.
-
- Interface:
- int dma_buf_fd(struct dma_buf *dmabuf, int flags)
-
- This API installs an fd for the anonymous file associated with this buffer;
- returns either 'fd', or error.
-
-3. Each buffer-user 'connects' itself to the buffer
-
- Each buffer-user now gets a reference to the buffer, using the fd passed to
- it.
-
- Interface:
- struct dma_buf *dma_buf_get(int fd)
-
- This API will return a reference to the dma_buf, and increment refcount for
- it.
-
- After this, the buffer-user needs to attach its device with the buffer, which
- helps the exporter to know of device buffer constraints.
-
- Interface:
- struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
- struct device *dev)
-
- This API returns reference to an attachment structure, which is then used
- for scatterlist operations. It will optionally call the 'attach' dma_buf
- operation, if provided by the exporter.
-
- The dma-buf sharing framework does the bookkeeping bits related to managing
- the list of all attachments to a buffer.
-
-Until this stage, the buffer-exporter has the option to choose not to actually
-allocate the backing storage for this buffer, but wait for the first buffer-user
-to request use of buffer for allocation.
-
-
-4. When needed, buffer-user requests access to the buffer
-
- Whenever a buffer-user wants to use the buffer for any DMA, it asks for
- access to the buffer using dma_buf_map_attachment API. At least one attach to
- the buffer must have happened before map_dma_buf can be called.
-
- Interface:
- struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *,
- enum dma_data_direction);
-
- This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
- "dma_buf->ops->" indirection from the users of this interface.
-
- In struct dma_buf_ops, map_dma_buf is defined as
- struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *,
- enum dma_data_direction);
-
- It is one of the buffer operations that must be implemented by the exporter.
- It should return the sg_table containing scatterlist for this buffer, mapped
- into caller's address space.
-
- If this is being called for the first time, the exporter can now choose to
- scan through the list of attachments for this buffer, collate the requirements
- of the attached devices, and choose an appropriate backing storage for the
- buffer.
-
- Based on enum dma_data_direction, it might be possible to have multiple users
- accessing at the same time (for reading, maybe), or any other kind of sharing
- that the exporter might wish to make available to buffer-users.
-
- map_dma_buf() operation can return -EINTR if it is interrupted by a signal.
-
-
-5. When finished, the buffer-user notifies end-of-DMA to exporter
-
- Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
- the exporter using the dma_buf_unmap_attachment API.
-
- Interface:
- void dma_buf_unmap_attachment(struct dma_buf_attachment *,
- struct sg_table *);
-
- This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
- "dma_buf->ops->" indirection from the users of this interface.
-
- In struct dma_buf_ops, unmap_dma_buf is defined as
- void (*unmap_dma_buf)(struct dma_buf_attachment *,
- struct sg_table *,
- enum dma_data_direction);
-
- unmap_dma_buf signifies the end-of-DMA for the attachment provided. Like
- map_dma_buf, this API also must be implemented by the exporter.
-
-
-6. when buffer-user is done using this buffer, it 'disconnects' itself from the
- buffer.
-
- After the buffer-user has no more interest in using this buffer, it should
- disconnect itself from the buffer:
-
- - it first detaches itself from the buffer.
-
- Interface:
- void dma_buf_detach(struct dma_buf *dmabuf,
- struct dma_buf_attachment *dmabuf_attach);
-
- This API removes the attachment from the list in dmabuf, and optionally calls
- dma_buf->ops->detach(), if provided by exporter, for any housekeeping bits.
-
- - Then, the buffer-user returns the buffer reference to exporter.
-
- Interface:
- void dma_buf_put(struct dma_buf *dmabuf);
-
- This API then reduces the refcount for this buffer.
-
- If, as a result of this call, the refcount becomes 0, the 'release' file
- operation related to this fd is called. It calls the dmabuf->ops->release()
- operation in turn, and frees the memory allocated for dmabuf when exported.
-
-NOTES:
-- Importance of attach-detach and {map,unmap}_dma_buf operation pairs
- The attach-detach calls allow the exporter to figure out backing-storage
- constraints for the currently-interested devices. This allows preferential
- allocation, and/or migration of pages across different types of storage
- available, if possible.
-
- Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
- to allow just-in-time backing of storage, and migration mid-way through a
- use-case.
-
-- Migration of backing storage if needed
- If after
- - at least one map_dma_buf has happened,
- - and the backing storage has been allocated for this buffer,
- another new buffer-user intends to attach itself to this buffer, it might
- be allowed, if possible for the exporter.
-
- In case it is allowed by the exporter:
- if the new buffer-user has stricter 'backing-storage constraints', and the
- exporter can handle these constraints, the exporter can just stall on the
- map_dma_buf until all outstanding access is completed (as signalled by
- unmap_dma_buf).
- Once all users have finished accessing and have unmapped this buffer, the
- exporter could potentially move the buffer to the stricter backing-storage,
- and then allow further {map,unmap}_dma_buf operations from any buffer-user
- from the migrated backing-storage.
-
- If the exporter cannot fulfill the backing-storage constraints of the new
- buffer-user device as requested, dma_buf_attach() would return an error to
- denote non-compatibility of the new buffer-sharing request with the current
- buffer.
-
- If the exporter chooses not to allow an attach() operation once a
- map_dma_buf() API has been called, it simply returns an error.
-
-Kernel cpu access to a dma-buf buffer object
---------------------------------------------
-
-The motivation to allow cpu access from the kernel to a dma-buf object from the
-importers side are:
-- fallback operations, e.g. if the devices is connected to a usb bus and the
- kernel needs to shuffle the data around first before sending it away.
-- full transparency for existing users on the importer side, i.e. userspace
- should not notice the difference between a normal object from that subsystem
- and an imported one backed by a dma-buf. This is really important for drm
- opengl drivers that expect to still use all the existing upload/download
- paths.
-
-Access to a dma_buf from the kernel context involves three steps:
-
-1. Prepare access, which invalidate any necessary caches and make the object
- available for cpu access.
-2. Access the object page-by-page with the dma_buf map apis
-3. Finish access, which will flush any necessary cpu caches and free reserved
- resources.
-
-1. Prepare access
-
- Before an importer can access a dma_buf object with the cpu from the kernel
- context, it needs to notify the exporter of the access that is about to
- happen.
-
- Interface:
- int dma_buf_begin_cpu_access(struct dma_buf *dmabuf,
- enum dma_data_direction direction)
-
- This allows the exporter to ensure that the memory is actually available for
- cpu access - the exporter might need to allocate or swap-in and pin the
- backing storage. The exporter also needs to ensure that cpu access is
- coherent for the access direction. The direction can be used by the exporter
- to optimize the cache flushing, i.e. access with a different direction (read
- instead of write) might return stale or even bogus data (e.g. when the
- exporter needs to copy the data to temporary storage).
-
- This step might fail, e.g. in oom conditions.
-
-2. Accessing the buffer
-
- To support dma_buf objects residing in highmem cpu access is page-based using
- an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
- PAGE_SIZE size. Before accessing a chunk it needs to be mapped, which returns
- a pointer in kernel virtual address space. Afterwards the chunk needs to be
- unmapped again. There is no limit on how often a given chunk can be mapped
- and unmapped, i.e. the importer does not need to call begin_cpu_access again
- before mapping the same chunk again.
-
- Interfaces:
- void *dma_buf_kmap(struct dma_buf *, unsigned long);
- void dma_buf_kunmap(struct dma_buf *, unsigned long, void *);
-
- There are also atomic variants of these interfaces. Like for kmap they
- facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
- the callback) is allowed to block when using these.
-
- Interfaces:
- void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long);
- void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *);
-
- For importers all the restrictions of using kmap apply, like the limited
- supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
- atomic dma_buf kmaps at the same time (in any given process context).
-
- dma_buf kmap calls outside of the range specified in begin_cpu_access are
- undefined. If the range is not PAGE_SIZE aligned, kmap needs to succeed on
- the partial chunks at the beginning and end but may return stale or bogus
- data outside of the range (in these partial chunks).
-
- Note that these calls need to always succeed. The exporter needs to complete
- any preparations that might fail in begin_cpu_access.
-
- For some cases the overhead of kmap can be too high, a vmap interface
- is introduced. This interface should be used very carefully, as vmalloc
- space is a limited resources on many architectures.
-
- Interfaces:
- void *dma_buf_vmap(struct dma_buf *dmabuf)
- void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr)
-
- The vmap call can fail if there is no vmap support in the exporter, or if it
- runs out of vmalloc space. Fallback to kmap should be implemented. Note that
- the dma-buf layer keeps a reference count for all vmap access and calls down
- into the exporter's vmap function only when no vmapping exists, and only
- unmaps it once. Protection against concurrent vmap/vunmap calls is provided
- by taking the dma_buf->lock mutex.
-
-3. Finish access
-
- When the importer is done accessing the CPU, it needs to announce this to
- the exporter (to facilitate cache flushing and unpinning of any pinned
- resources). The result of any dma_buf kmap calls after end_cpu_access is
- undefined.
-
- Interface:
- void dma_buf_end_cpu_access(struct dma_buf *dma_buf,
- enum dma_data_direction dir);
-
-
-Direct Userspace Access/mmap Support
-------------------------------------
-
-Being able to mmap an export dma-buf buffer object has 2 main use-cases:
-- CPU fallback processing in a pipeline and
-- supporting existing mmap interfaces in importers.
-
-1. CPU fallback processing in a pipeline
-
- In many processing pipelines it is sometimes required that the cpu can access
- the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
- the need to handle this specially in userspace frameworks for buffer sharing
- it's ideal if the dma_buf fd itself can be used to access the backing storage
- from userspace using mmap.
-
- Furthermore Android's ION framework already supports this (and is otherwise
- rather similar to dma-buf from a userspace consumer side with using fds as
- handles, too). So it's beneficial to support this in a similar fashion on
- dma-buf to have a good transition path for existing Android userspace.
-
- No special interfaces, userspace simply calls mmap on the dma-buf fd, making
- sure that the cache synchronization ioctl (DMA_BUF_IOCTL_SYNC) is *always*
- used when the access happens. Note that DMA_BUF_IOCTL_SYNC can fail with
- -EAGAIN or -EINTR, in which case it must be restarted.
-
- Some systems might need some sort of cache coherency management e.g. when
- CPU and GPU domains are being accessed through dma-buf at the same time. To
- circumvent this problem there are begin/end coherency markers, that forward
- directly to existing dma-buf device drivers vfunc hooks. Userspace can make
- use of those markers through the DMA_BUF_IOCTL_SYNC ioctl. The sequence
- would be used like following:
- - mmap dma-buf fd
- - for each drawing/upload cycle in CPU 1. SYNC_START ioctl, 2. read/write
- to mmap area 3. SYNC_END ioctl. This can be repeated as often as you
- want (with the new data being consumed by the GPU or say scanout device)
- - munmap once you don't need the buffer any more
-
- For correctness and optimal performance, it is always required to use
- SYNC_START and SYNC_END before and after, respectively, when accessing the
- mapped address. Userspace cannot rely on coherent access, even when there
- are systems where it just works without calling these ioctls.
-
-2. Supporting existing mmap interfaces in importers
-
- Similar to the motivation for kernel cpu access it is again important that
- the userspace code of a given importing subsystem can use the same interfaces
- with a imported dma-buf buffer object as with a native buffer object. This is
- especially important for drm where the userspace part of contemporary OpenGL,
- X, and other drivers is huge, and reworking them to use a different way to
- mmap a buffer rather invasive.
-
- The assumption in the current dma-buf interfaces is that redirecting the
- initial mmap is all that's needed. A survey of some of the existing
- subsystems shows that no driver seems to do any nefarious thing like syncing
- up with outstanding asynchronous processing on the device or allocating
- special resources at fault time. So hopefully this is good enough, since
- adding interfaces to intercept pagefaults and allow pte shootdowns would
- increase the complexity quite a bit.
-
- Interface:
- int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *,
- unsigned long);
-
- If the importing subsystem simply provides a special-purpose mmap call to set
- up a mapping in userspace, calling do_mmap with dma_buf->file will equally
- achieve that for a dma-buf object.
-
-3. Implementation notes for exporters
-
- Because dma-buf buffers have invariant size over their lifetime, the dma-buf
- core checks whether a vma is too large and rejects such mappings. The
- exporter hence does not need to duplicate this check.
-
- Because existing importing subsystems might presume coherent mappings for
- userspace, the exporter needs to set up a coherent mapping. If that's not
- possible, it needs to fake coherency by manually shooting down ptes when
- leaving the cpu domain and flushing caches at fault time. Note that all the
- dma_buf files share the same anon inode, hence the exporter needs to replace
- the dma_buf file stored in vma->vm_file with it's own if pte shootdown is
- required. This is because the kernel uses the underlying inode's address_space
- for vma tracking (and hence pte tracking at shootdown time with
- unmap_mapping_range).
-
- If the above shootdown dance turns out to be too expensive in certain
- scenarios, we can extend dma-buf with a more explicit cache tracking scheme
- for userspace mappings. But the current assumption is that using mmap is
- always a slower path, so some inefficiencies should be acceptable.
-
- Exporters that shoot down mappings (for any reasons) shall not do any
- synchronization at fault time with outstanding device operations.
- Synchronization is an orthogonal issue to sharing the backing storage of a
- buffer and hence should not be handled by dma-buf itself. This is explicitly
- mentioned here because many people seem to want something like this, but if
- different exporters handle this differently, buffer sharing can fail in
- interesting ways depending upong the exporter (if userspace starts depending
- upon this implicit synchronization).
-
-Other Interfaces Exposed to Userspace on the dma-buf FD
-------------------------------------------------------
-
-- Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
- with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
- the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
- llseek operation will report -EINVAL.
-
- If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
- cases. Userspace can use this to detect support for discovering the dma-buf
- size using llseek.
-
-Miscellaneous notes
--------------------
-
-- Any exporters or users of the dma-buf buffer sharing framework must have
- a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
-
-- In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
- on the file descriptor. This is not just a resource leak, but a
- potential security hole. It could give the newly exec'd application
- access to buffers, via the leaked fd, to which it should otherwise
- not be permitted access.
-
- The problem with doing this via a separate fcntl() call, versus doing it
- atomically when the fd is created, is that this is inherently racy in a
- multi-threaded app[3]. The issue is made worse when it is library code
- opening/creating the file descriptor, as the application may not even be
- aware of the fd's.
-
- To avoid this problem, userspace must have a way to request O_CLOEXEC
- flag be set when the dma-buf fd is created. So any API provided by
- the exporting driver to create a dmabuf fd must provide a way to let
- userspace control setting of O_CLOEXEC flag passed in to dma_buf_fd().
-
-- If an exporter needs to manually flush caches and hence needs to fake
- coherency for mmap support, it needs to be able to zap all the ptes pointing
- at the backing storage. Now linux mm needs a struct address_space associated
- with the struct file stored in vma->vm_file to do that with the function
- unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
- with the anon_file struct file, i.e. all dma_bufs share the same file.
-
- Hence exporters need to setup their own file (and address_space) association
- by setting vma->vm_file and adjusting vma->vm_pgoff in the dma_buf mmap
- callback. In the specific case of a gem driver the exporter could use the
- shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
- zap ptes by unmapping the corresponding range of the struct address_space
- associated with their own file.
-
-References:
-[1] struct dma_buf_ops in include/linux/dma-buf.h
-[2] All interfaces mentioned above defined in include/linux/dma-buf.h
-[3] https://lwn.net/Articles/236486/
diff --git a/Documentation/dma-buf/guide.rst b/Documentation/dma-buf/guide.rst
new file mode 100644
index 000000000000..7407d255e9da
--- /dev/null
+++ b/Documentation/dma-buf/guide.rst
@@ -0,0 +1,507 @@
+
+.. _dma-buf-guide:
+
+============================
+DMA Buffer Sharing API Guide
+============================
+
+This document serves as a guide to device-driver writers on what is the dma-buf
+buffer sharing API, how to use it for exporting and using shared buffers.
+
+Any device driver which wishes to be a part of DMA buffer sharing, can do so as
+either the 'exporter' of buffers, or the 'user' of buffers.
+
+Say a driver A wants to use buffers created by driver B, then we call B as the
+exporter, and A as buffer-user.
+
+The exporter
+
+* implements and manages operations for the buffer via
+ :c:type:`struct dma_buf_ops <dma_buf_ops>`
+* allows other users to share the buffer by using dma_buf sharing APIs,
+* manages the details of buffer allocation,
+* decides about the actual backing storage where this allocation happens,
+* takes care of any migration of scatterlist - for all (shared) users of this
+ buffer,
+
+The buffer-user
+
+* is one of (many) sharing users of the buffer.
+* doesn't need to worry about how the buffer is allocated, or where.
+* needs a mechanism to get access to the scatterlist that makes up this buffer
+ in memory, mapped into its own address space, so it can access the same area
+ of memory.
+
+dma-buf operations for device dma only
+======================================
+
+The dma_buf buffer sharing API usage contains the following steps:
+
+1. Exporter announces that it wishes to export a buffer
+2. Userspace gets the file descriptor associated with the exported buffer, and
+ passes it around to potential buffer-users based on use case
+3. Each buffer-user 'connects' itself to the buffer
+4. When needed, buffer-user requests access to the buffer from exporter
+5. When finished with its use, the buffer-user notifies end-of-DMA to exporter
+6. when buffer-user is done using this buffer completely, it 'disconnects'
+ itself from the buffer.
+
+
+Exporter's announcement of buffer export
+----------------------------------------
+
+The buffer exporter announces its wish to export a buffer. In this, it
+connects its own private buffer data, provides implementation for operations
+that can be performed on the exported :c:type:`dma_buf`, and flags for the file
+associated with this buffer. All these fields are filled in struct
+:c:type:`dma_buf_export_info`, defined via the DEFINE_DMA_BUF_EXPORT_INFO macro.
+
+Interface:
+ :c:type:`DEFINE_DMA_BUF_EXPORT_INFO(exp_info) <DEFINE_DMA_BUF_EXPORT_INFO>`
+
+ :c:func:`struct dma_buf *dma_buf_export(struct dma_buf_export_info *exp_info)<dma_buf_export>`
+
+If this succeeds, dma_buf_export allocates a dma_buf structure, and
+returns a pointer to the same. It also associates an anonymous file with this
+buffer, so it can be exported. On failure to allocate the dma_buf object,
+it returns NULL.
+
+``exp_name`` in struct dma_buf_export_info is the name of exporter - to
+facilitate information while debugging. It is set to ``KBUILD_MODNAME`` by
+default, so exporters don't have to provide a specific name, if they don't
+wish to.
+
+DEFINE_DMA_BUF_EXPORT_INFO macro defines the struct dma_buf_export_info,
+zeroes it out and pre-populates exp_name in it.
+
+Userspace gets a handle to pass around to potential buffer-users
+----------------------------------------------------------------
+
+Userspace entity requests for a file-descriptor (fd) which is a handle to the
+anonymous file associated with the buffer. It can then share the fd with other
+drivers and/or processes.
+
+Interface:
+ :c:func:`int dma_buf_fd(struct dma_buf *dmabuf, int flags) <dma_buf_fd>`
+
+This API installs an fd for the anonymous file associated with this buffer;
+returns either ``fd``, or error.
+
+Each buffer-user 'connects' itself to the buffer
+------------------------------------------------
+
+Each buffer-user now gets a reference to the buffer, using the fd passed to it.
+
+Interface:
+ :c:func:`struct dma_buf *dma_buf_get(int fd) <dma_buf_get>`
+
+This API will return a reference to the dma_buf, and increment refcount forit.
+
+After this, the buffer-user needs to attach its device with the buffer, which
+helps the exporter to know of device buffer constraints.
+
+Interface:
+ :c:func:`struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf, struct device *dev) <dma_buf_attach>`
+
+This API returns reference to an attachment structure, which is then used
+for scatterlist operations. It will optionally call the 'attach' dma_buf
+operation, if provided by the exporter.
+
+The dma-buf sharing framework does the bookkeeping bits related to managing
+the list of all attachments to a buffer.
+
+.. note::
+
+ Until this stage, the buffer-exporter has the option to choose not to actually
+ allocate the backing storage for this buffer, but wait for the first
+ buffer-user to request use of buffer for allocation.
+
+When needed, buffer-user requests access to the buffer
+------------------------------------------------------
+
+Whenever a buffer-user wants to use the buffer for any DMA, it asks for
+access to the buffer using dma_buf_map_attachment API. At least one attach to
+the buffer must have happened before map_dma_buf can be called.
+
+Interface:
+ :c:func:`struct sg_table * dma_buf_map_attachment(struct dma_buf_attachment *, enum dma_data_direction) <dma_buf_map_attachment>`
+
+This is a wrapper to dma_buf->ops->map_dma_buf operation, which hides the
+``dma_buf->ops->`` indirection from the users of this interface.
+
+In struct :c:type:`dma_buf_ops`, ``map_dma_buf`` is defined as
+ ``struct sg_table * (*map_dma_buf)(struct dma_buf_attachment *, enum dma_data_direction);``
+
+It is one of the buffer operations that must be implemented by the exporter.
+It should return the sg_table containing scatterlist for this buffer, mapped
+into caller's address space.
+
+If this is being called for the first time, the exporter can now choose to
+scan through the list of attachments for this buffer, collate the requirements
+of the attached devices, and choose an appropriate backing storage for the
+buffer.
+
+Based on enum :c:type:`dma_data_direction`, it might be possible to have multiple users
+accessing at the same time (for reading, maybe), or any other kind of sharing
+that the exporter might wish to make available to buffer-users.
+
+``map_dma_buf()`` operation can return -EINTR if it is interrupted by a signal.
+
+
+When finished, the buffer-user notifies end-of-DMA to exporter
+--------------------------------------------------------------
+
+Once the DMA for the current buffer-user is over, it signals 'end-of-DMA' to
+the exporter using the dma_buf_unmap_attachment API.
+
+Interface:
+ :c:func:`void dma_buf_unmap_attachment(struct dma_buf_attachment *, struct sg_table *) <dma_buf_unmap_attachment>`
+
+This is a wrapper to dma_buf->ops->unmap_dma_buf() operation, which hides the
+"dma_buf->ops->" indirection from the users of this interface.
+
+In struct dma_buf_ops, unmap_dma_buf is defined as
+ ``void (*unmap_dma_buf)(struct dma_buf_attachment *, struct sg_table *, enum dma_data_direction)``
+
+``unmap_dma_buf()`` signifies the end-of-DMA for the attachment provided. Like
+``map_dma_buf``, this API also must be implemented by the exporter.
+
+
+when buffer-user is done using this buffer, it 'disconnects' itself from the buffer.
+------------------------------------------------------------------------------------
+
+After the buffer-user has no more interest in using this buffer, it should
+disconnect itself from the buffer:
+
+ * it first detaches itself from the buffer.
+
+ Interface:
+ :c:func:`void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *dmabuf_attach) <dma_buf_detach>`
+
+ This API removes the attachment from the list in dmabuf, and optionally calls
+ ``dma_buf->ops->detach()``, if provided by exporter, for any housekeeping bits.
+
+ * Then, the buffer-user returns the buffer reference to exporter.
+
+ Interface:
+ :c:func:`void dma_buf_put(struct dma_buf *dmabuf) <dma_buf_put>`
+
+ This API then reduces the refcount for this buffer.
+
+ If, as a result of this call, the refcount becomes 0, the 'release' file
+ operation related to this fd is called. It calls the
+ ``dmabuf->ops->release()`` operation in turn, and frees the memory allocated
+ for dmabuf when exported.
+
+NOTES
+-----
+
+Importance of attach-detach and {map,unmap}_dma_buf operation pairs
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+ The attach-detach calls allow the exporter to figure out backing-storage
+ constraints for the currently-interested devices. This allows preferential
+ allocation, and/or migration of pages across different types of storage
+ available, if possible.
+
+ Bracketing of DMA access with {map,unmap}_dma_buf operations is essential
+ to allow just-in-time backing of storage, and migration mid-way through a
+ use-case.
+
+Migration of backing storage if needed
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+ If after
+
+ * at least one map_dma_buf has happened,
+ * and the backing storage has been allocated for this buffer,
+
+ another new buffer-user intends to attach itself to this buffer, it might
+ be allowed, if possible for the exporter.
+
+ In case it is allowed by the exporter:
+
+ * if the new buffer-user has stricter 'backing-storage constraints', and the
+ exporter can handle these constraints, the exporter can just stall on the
+ map_dma_buf until all outstanding access is completed (as signalled by
+ unmap_dma_buf).
+
+ * Once all users have finished accessing and have unmapped this buffer, the
+ exporter could potentially move the buffer to the stricter backing-storage,
+ and then allow further {map,unmap}_dma_buf operations from any buffer-user
+ from the migrated backing-storage.
+
+ * If the exporter cannot fulfill the backing-storage constraints of the new
+ buffer-user device as requested, dma_buf_attach() would return an error to
+ denote non-compatibility of the new buffer-sharing request with the current
+ buffer.
+
+
+ If the exporter chooses not to allow an attach() operation once a
+ map_dma_buf() API has been called, it simply returns an error.
+
+Kernel cpu access to a dma-buf buffer object
+============================================
+
+The motivation to allow cpu access from the kernel to a dma-buf object from the
+importers side are:
+
+* fallback operations, e.g. if the devices is connected to a usb bus and the
+ kernel needs to shuffle the data around first before sending it away.
+
+* full transparency for existing users on the importer side, i.e. userspace
+ should not notice the difference between a normal object from that subsystem
+ and an imported one backed by a dma-buf. This is really important for drm
+ opengl drivers that expect to still use all the existing upload/download
+ paths.
+
+Access to a dma_buf from the kernel context involves three steps:
+
+1. Prepare access, which invalidate any necessary caches and make the object
+ available for cpu access.
+
+2. Access the object page-by-page with the dma_buf map apis
+
+3. Finish access, which will flush any necessary cpu caches and free reserved
+ resources.
+
+Prepare access
+--------------
+
+Before an importer can access a dma_buf object with the cpu from the kernel
+context, it needs to notify the exporter of the access that is about to
+happen.
+
+Interface:
+ :c:func:`int dma_buf_begin_cpu_access(struct dma_buf *dmabuf, enum dma_data_direction direction) <dma_buf_begin_cpu_access>`
+
+This allows the exporter to ensure that the memory is actually available for
+cpu access - the exporter might need to allocate or swap-in and pin the
+backing storage. The exporter also needs to ensure that cpu access is
+coherent for the access direction. The direction can be used by the exporter
+to optimize the cache flushing, i.e. access with a different direction (read
+instead of write) might return stale or even bogus data (e.g. when the
+exporter needs to copy the data to temporary storage).
+
+This step might fail, e.g. in oom conditions.
+
+Access the buffer
+-----------------
+
+To support dma_buf objects residing in highmem cpu access is page-based using
+an api similar to kmap. Accessing a dma_buf is done in aligned chunks of
+``PAGE_SIZE`` size. Before accessing a chunk it needs to be mapped, which returns
+a pointer in kernel virtual address space. Afterwards the chunk needs to be
+unmapped again. There is no limit on how often a given chunk can be mapped
+and unmapped, i.e. the importer does not need to call ``begin_cpu_access()`` again
+before mapping the same chunk again.
+
+Interfaces:
+ :c:func:`void *dma_buf_kmap(struct dma_buf *, unsigned long) <dma_buf_kmap>`
+
+ :c:func:`void dma_buf_kunmap(struct dma_buf *, unsigned long, void *) <dma_buf_kunmap>`
+
+There are also atomic variants of these interfaces. Like for kmap they
+facilitate non-blocking fast-paths. Neither the importer nor the exporter (in
+the callback) is allowed to block when using these.
+
+Interfaces:
+ :c:func:`void *dma_buf_kmap_atomic(struct dma_buf *, unsigned long) <dma_buf_kmap_atomic>`
+
+ :c:func:`void dma_buf_kunmap_atomic(struct dma_buf *, unsigned long, void *) <dma_buf_kunmap_atomic>`
+
+For importers all the restrictions of using kmap apply, like the limited
+supply of kmap_atomic slots. Hence an importer shall only hold onto at most 2
+atomic dma_buf kmaps at the same time (in any given process context).
+
+``dma_buf kmap`` calls outside of the range specified in ``begin_cpu_access`` are
+undefined. If the range is not ``PAGE_SIZE`` aligned, kmap needs to succeed on
+the partial chunks at the beginning and end but may return stale or bogus
+data outside of the range (in these partial chunks).
+
+Note that these calls need to always succeed. The exporter needs to complete
+any preparations that might fail in ``begin_cpu_access()``.
+
+For some cases the overhead of kmap can be too high, a vmap interface
+is introduced. This interface should be used very carefully, as vmalloc
+space is a limited resources on many architectures.
+
+Interfaces:
+ :c:func:`void *dma_buf_vmap(struct dma_buf *dmabuf) <dma_buf_vmap>`
+
+ :c:func:`void dma_buf_vunmap(struct dma_buf *dmabuf, void *vaddr) <dma_buf_vunmap>`
+
+The vmap call can fail if there is no vmap support in the exporter, or if it
+runs out of vmalloc space. Fallback to kmap should be implemented. Note that
+the dma-buf layer keeps a reference count for all vmap access and calls down
+into the exporter's vmap function only when no vmapping exists, and only
+unmaps it once. Protection against concurrent vmap/vunmap calls is provided
+by taking the ``dma_buf->lock`` mutex.
+
+Finish access
+-------------
+
+When the importer is done accessing the CPU, it needs to announce this to
+the exporter (to facilitate cache flushing and unpinning of any pinned
+resources). The result of any dma_buf kmap calls after end_cpu_access is
+undefined.
+
+Interface:
+ :c:func:`void dma_buf_end_cpu_access(struct dma_buf *dma_buf, enum dma_data_direction dir) <dma_buf_end_cpu_access>`
+
+
+Direct Userspace Access/mmap Support
+====================================
+
+Being able to mmap an export dma-buf buffer object has 2 main use-cases:
+
+* CPU fallback processing in a pipeline and
+* supporting existing mmap interfaces in importers.
+
+CPU fallback processing in a pipeline
+-------------------------------------
+
+In many processing pipelines it is sometimes required that the cpu can access
+the data in a dma-buf (e.g. for thumbnail creation, snapshots, ...). To avoid
+the need to handle this specially in userspace frameworks for buffer sharing
+it's ideal if the dma_buf ``fd`` itself can be used to access the backing storage
+from userspace using mmap.
+
+Furthermore Android's ION framework already supports this (and is otherwise
+rather similar to dma-buf from a userspace consumer side with using fds as
+handles, too). So it's beneficial to support this in a similar fashion on
+dma-buf to have a good transition path for existing Android userspace.
+
+No special interfaces, userspace simply calls mmap on the dma-buf fd, making
+sure that the cache synchronization ioctl (``DMA_BUF_IOCTL_SYNC``) is *always*
+used when the access happens. Note that ``DMA_BUF_IOCTL_SYNC`` can fail with
+-EAGAIN or -EINTR, in which case it must be restarted.
+
+Some systems might need some sort of cache coherency management e.g. when
+CPU and GPU domains are being accessed through dma-buf at the same time. To
+circumvent this problem there are begin/end coherency markers, that forward
+directly to existing dma-buf device drivers vfunc hooks. Userspace can make
+use of those markers through the ``DMA_BUF_IOCTL_SYNC`` ioctl. The sequence
+would be used like following:
+
+ * mmap dma-buf fd
+ * for each drawing/upload cycle in CPU
+
+ 1. SYNC_START ioctl,
+ 2. read/write to mmap area
+ 3. SYNC_END ioctl.
+
+ This can be repeated as often as you want (with the new data being
+ consumed by the GPU or say scanout device)
+
+ * munmap once you don't need the buffer any more
+
+For correctness and optimal performance, it is always required to use
+``SYNC_START`` and ``SYNC_END`` before and after, respectively, when accessing the
+mapped address. Userspace cannot rely on coherent access, even when there
+are systems where it just works without calling these ioctls.
+
+Supporting existing mmap interfaces in importers
+------------------------------------------------
+
+Similar to the motivation for kernel cpu access it is again important that
+the userspace code of a given importing subsystem can use the same interfaces
+with a imported dma-buf buffer object as with a native buffer object. This is
+especially important for drm where the userspace part of contemporary OpenGL,
+X, and other drivers is huge, and reworking them to use a different way to
+mmap a buffer rather invasive.
+
+The assumption in the current dma-buf interfaces is that redirecting the
+initial mmap is all that's needed. A survey of some of the existing
+subsystems shows that no driver seems to do any nefarious thing like syncing
+up with outstanding asynchronous processing on the device or allocating
+special resources at fault time. So hopefully this is good enough, since
+adding interfaces to intercept pagefaults and allow pte shootdowns would
+increase the complexity quite a bit.
+
+Interface:
+ :c:func:`int dma_buf_mmap(struct dma_buf *, struct vm_area_struct *, unsigned long) <dma_buf_mmap>`
+
+If the importing subsystem simply provides a special-purpose mmap call to set
+up a mapping in userspace, calling ``do_mmap`` with ``dma_buf->file`` will equally
+achieve that for a dma-buf object.
+
+Implementation notes for exporters
+----------------------------------
+
+Because dma-buf buffers have invariant size over their lifetime, the dma-buf
+core checks whether a vma is too large and rejects such mappings. The
+exporter hence does not need to duplicate this check.
+
+Because existing importing subsystems might presume coherent mappings for
+userspace, the exporter needs to set up a coherent mapping. If that's not
+possible, it needs to fake coherency by manually shooting down ptes when
+leaving the cpu domain and flushing caches at fault time. Note that all the
+``dma_buf`` files share the same anon inode, hence the exporter needs to replace
+the ``dma_buf`` file stored in ``vma->vm_file`` with it's own if pte shootdown is
+required. This is because the kernel uses the underlying inode's address_space
+for vma tracking (and hence pte tracking at shootdown time with
+``unmap_mapping_range``).
+
+If the above shootdown dance turns out to be too expensive in certain
+scenarios, we can extend dma-buf with a more explicit cache tracking scheme
+for userspace mappings. But the current assumption is that using mmap is
+always a slower path, so some inefficiencies should be acceptable.
+
+Exporters that shoot down mappings (for any reasons) shall not do any
+synchronization at fault time with outstanding device operations.
+Synchronization is an orthogonal issue to sharing the backing storage of a
+buffer and hence should not be handled by dma-buf itself. This is explicitly
+mentioned here because many people seem to want something like this, but if
+different exporters handle this differently, buffer sharing can fail in
+interesting ways depending upong the exporter (if userspace starts depending
+upon this implicit synchronization).
+
+Other Interfaces Exposed to Userspace on the dma-buf FD
+-------------------------------------------------------
+
+* Since kernel 3.12 the dma-buf FD supports the llseek system call, but only
+ with offset=0 and whence=SEEK_END|SEEK_SET. SEEK_SET is supported to allow
+ the usual size discover pattern size = SEEK_END(0); SEEK_SET(0). Every other
+ llseek operation will report -EINVAL.
+
+ If llseek on dma-buf FDs isn't support the kernel will report -ESPIPE for all
+ cases. Userspace can use this to detect support for discovering the dma-buf
+ size using llseek.
+
+Miscellaneous notes
+-------------------
+
+* Any exporters or users of the dma-buf buffer sharing framework must have
+ a 'select DMA_SHARED_BUFFER' in their respective Kconfigs.
+
+* In order to avoid fd leaks on exec, the FD_CLOEXEC flag must be set
+ on the file descriptor. This is not just a resource leak, but a
+ potential security hole. It could give the newly exec'd application
+ access to buffers, via the leaked fd, to which it should otherwise
+ not be permitted access.
+
+The problem with doing this via a separate fcntl() call, versus doing it
+atomically when the fd is created, is that this is inherently racy in a
+multi-threaded app (See `here <https://lwn.net/Articles/236486/>`_). The issue
+is made worse when it is library code opening/creating the file descriptor,
+as the application may not even be aware of the fd's.
+
+To avoid this problem, userspace must have a way to request ``O_CLOEXEC``
+flag be set when the dma-buf fd is created. So any API provided by
+the exporting driver to create a dmabuf fd must provide a way to let
+userspace control setting of ``O_CLOEXEC`` flag passed in to ``dma_buf_fd()``.
+
+* If an exporter needs to manually flush caches and hence needs to fake
+ coherency for mmap support, it needs to be able to zap all the ptes pointing
+ at the backing storage. Now linux mm needs a struct address_space associated
+ with the struct file stored in ``vma->vm_file`` to do that with the function
+ unmap_mapping_range. But the dma_buf framework only backs every dma_buf fd
+ with the ``anon_file`` struct file, i.e. all dma_bufs share the same file.
+
+Hence exporters need to setup their own file (and address_space) association
+by setting ``vma->vm_file`` and adjusting ``vma->vm_pgoff`` in the ``dma_buf`` mmap
+callback. In the specific case of a gem driver the exporter could use the
+shmem file already provided by gem (and set vm_pgoff = 0). Exporters can then
+zap ptes by unmapping the corresponding range of the struct address_space
+associated with their own file.
diff --git a/Documentation/dma-buf/intro.rst b/Documentation/dma-buf/intro.rst
new file mode 100644
index 000000000000..4ecdbea50e75
--- /dev/null
+++ b/Documentation/dma-buf/intro.rst
@@ -0,0 +1,82 @@
+==================================
+Buffer Sharing and Synchronization
+==================================
+
+
+Introduction
+------------
+
+The dma-buf subsystem provides the framework for sharing buffers for
+hardware (DMA) access across multiple device drivers and subsystems, and
+for synchronizing asynchronous hardware access.
+
+This is used, for example, by drm "prime" multi-GPU support, but is of
+course not limited to GPU use cases.
+
+The three main components of this are:
+
+* dma-buf_: represents an sg_table, and is exposed to userspace as a file
+ descriptor to allow passing between devices,
+
+* fence_: which provides a mechanism to signal when one device has finished
+ access, and
+
+* reservation_: manages the shared or exclusive fence(s) associated with the
+ buffer.
+
+Please refer to :ref:`dma-buf-guide` for more details.
+
+.. _dma-buf:
+
+dma-buf
+~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/dma-buf.c
+ :export:
+
+.. kernel-doc:: include/linux/dma-buf.h
+ :internal:
+
+.. _fence:
+
+fence
+~~~~~
+
+.. kernel-doc:: drivers/dma-buf/fence.c
+ :export:
+
+.. kernel-doc:: include/linux/fence.h
+ :internal:
+
+.. kernel-doc:: drivers/dma-buf/fence-array.c
+ :export:
+
+.. kernel-doc:: include/linux/fence-array.h
+ :internal:
+
+.. kernel-doc:: drivers/dma-buf/seqno-fence.c
+ :export:
+
+.. kernel-doc:: include/linux/seqno-fence.h
+ :internal:
+
+.. kernel-doc:: drivers/dma-buf/sync_file.c
+ :export:
+
+.. kernel-doc:: include/linux/sync_file.h
+ :internal:
+
+.. _reservation:
+
+reservation
+~~~~~~~~~~~
+
+.. kernel-doc:: drivers/dma-buf/reservation.c
+ :doc: Reservation Object Overview
+
+.. kernel-doc:: drivers/dma-buf/reservation.c
+ :export:
+
+.. kernel-doc:: include/linux/reservation.h
+ :internal:
+
diff --git a/MAINTAINERS b/MAINTAINERS
index 0bbe4b105c34..dd422c6e54fe 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -3838,7 +3838,7 @@ F: drivers/dma-buf/
F: include/linux/dma-buf*
F: include/linux/reservation.h
F: include/linux/*fence.h
-F: Documentation/dma-buf-sharing.txt
+F: Documentation/dma-buf/
T: git git://git.linaro.org/people/sumitsemwal/linux-dma-buf.git
SYNC FILE FRAMEWORK
--
2.7.4
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