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Date:	Wed, 18 Jun 2014 21:25:17 -0400
From:	Rob Clark <>
To:	Greg KH <>
Cc:	Maarten Lankhorst <>,,
	Thomas Hellstrom <>,
	Linux Kernel Mailing List <>,
	"" <>,
	"" <>,
	Thierry Reding <>,
	Colin Cross <>,
	Daniel Vetter <>,
	Sumit Semwal <>,
	"" <>
Subject: Re: [REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)

On Wed, Jun 18, 2014 at 9:16 PM, Greg KH <> wrote:
> On Wed, Jun 18, 2014 at 12:36:54PM +0200, Maarten Lankhorst wrote:
>> A fence can be attached to a buffer which is being filled or consumed
>> by hw, to allow userspace to pass the buffer without waiting to another
>> device.  For example, userspace can call page_flip ioctl to display the
>> next frame of graphics after kicking the GPU but while the GPU is still
>> rendering.  The display device sharing the buffer with the GPU would
>> attach a callback to get notified when the GPU's rendering-complete IRQ
>> fires, to update the scan-out address of the display, without having to
>> wake up userspace.
>> A driver must allocate a fence context for each execution ring that can
>> run in parallel. The function for this takes an argument with how many
>> contexts to allocate:
>>   + fence_context_alloc()
>> A fence is transient, one-shot deal.  It is allocated and attached
>> to one or more dma-buf's.  When the one that attached it is done, with
>> the pending operation, it can signal the fence:
>>   + fence_signal()
>> To have a rough approximation whether a fence is fired, call:
>>   + fence_is_signaled()
>> The dma-buf-mgr handles tracking, and waiting on, the fences associated
>> with a dma-buf.
>> The one pending on the fence can add an async callback:
>>   + fence_add_callback()
>> The callback can optionally be cancelled with:
>>   + fence_remove_callback()
>> To wait synchronously, optionally with a timeout:
>>   + fence_wait()
>>   + fence_wait_timeout()
>> When emitting a fence, call:
>>   + trace_fence_emit()
>> To annotate that a fence is blocking on another fence, call:
>>   + trace_fence_annotate_wait_on(fence, on_fence)
>> A default software-only implementation is provided, which can be used
>> by drivers attaching a fence to a buffer when they have no other means
>> for hw sync.  But a memory backed fence is also envisioned, because it
>> is common that GPU's can write to, or poll on some memory location for
>> synchronization.  For example:
>>   fence = custom_get_fence(...);
>>   if ((seqno_fence = to_seqno_fence(fence)) != NULL) {
>>     dma_buf *fence_buf = seqno_fence->sync_buf;
>>     get_dma_buf(fence_buf);
>>     ... tell the hw the memory location to wait ...
>>     custom_wait_on(fence_buf, seqno_fence->seqno_ofs, fence->seqno);
>>   } else {
>>     /* fall-back to sw sync * /
>>     fence_add_callback(fence, my_cb);
>>   }
>> On SoC platforms, if some other hw mechanism is provided for synchronizing
>> between IP blocks, it could be supported as an alternate implementation
>> with it's own fence ops in a similar way.
>> enable_signaling callback is used to provide sw signaling in case a cpu
>> waiter is requested or no compatible hardware signaling could be used.
>> The intention is to provide a userspace interface (presumably via eventfd)
>> later, to be used in conjunction with dma-buf's mmap support for sw access
>> to buffers (or for userspace apps that would prefer to do their own
>> synchronization).
>> v1: Original
>> v2: After discussion w/ danvet and mlankhorst on #dri-devel, we decided
>>     that dma-fence didn't need to care about the sw->hw signaling path
>>     (it can be handled same as sw->sw case), and therefore the fence->ops
>>     can be simplified and more handled in the core.  So remove the signal,
>>     add_callback, cancel_callback, and wait ops, and replace with a simple
>>     enable_signaling() op which can be used to inform a fence supporting
>>     hw->hw signaling that one or more devices which do not support hw
>>     signaling are waiting (and therefore it should enable an irq or do
>>     whatever is necessary in order that the CPU is notified when the
>>     fence is passed).
>> v3: Fix locking fail in attach_fence() and get_fence()
>> v4: Remove tie-in w/ dma-buf..  after discussion w/ danvet and mlankorst
>>     we decided that we need to be able to attach one fence to N dma-buf's,
>>     so using the list_head in dma-fence struct would be problematic.
>> v5: [ Maarten Lankhorst ] Updated for dma-bikeshed-fence and dma-buf-manager.
>> v6: [ Maarten Lankhorst ] I removed dma_fence_cancel_callback and some comments
>>     about checking if fence fired or not. This is broken by design.
>>     waitqueue_active during destruction is now fatal, since the signaller
>>     should be holding a reference in enable_signalling until it signalled
>>     the fence. Pass the original dma_fence_cb along, and call __remove_wait
>>     in the dma_fence_callback handler, so that no cleanup needs to be
>>     performed.
>> v7: [ Maarten Lankhorst ] Set cb->func and only enable sw signaling if
>>     fence wasn't signaled yet, for example for hardware fences that may
>>     choose to signal blindly.
>> v8: [ Maarten Lankhorst ] Tons of tiny fixes, moved __dma_fence_init to
>>     header and fixed include mess. dma-fence.h now includes dma-buf.h
>>     All members are now initialized, so kmalloc can be used for
>>     allocating a dma-fence. More documentation added.
>> v9: Change compiler bitfields to flags, change return type of
>>     enable_signaling to bool. Rework dma_fence_wait. Added
>>     dma_fence_is_signaled and dma_fence_wait_timeout.
>>     s/dma// and change exports to non GPL. Added fence_is_signaled and
>>     fence_enable_sw_signaling calls, add ability to override default
>>     wait operation.
>> v10: remove event_queue, use a custom list, export try_to_wake_up from
>>     scheduler. Remove fence lock and use a global spinlock instead,
>>     this should hopefully remove all the locking headaches I was having
>>     on trying to implement this. enable_signaling is called with this
>>     lock held.
>> v11:
>>     Use atomic ops for flags, lifting the need for some spin_lock_irqsaves.
>>     However I kept the guarantee that after fence_signal returns, it is
>>     guaranteed that enable_signaling has either been called to completion,
>>     or will not be called any more.
>>     Add contexts and seqno to base fence implementation. This allows you
>>     to wait for less fences, by testing for seqno + signaled, and then only
>>     wait on the later fence.
>>     Add FENCE_TRACE, FENCE_WARN, and FENCE_ERR. This makes debugging easier.
>>     An CONFIG_DEBUG_FENCE will be added to turn off the FENCE_TRACE
>>     spam, and another runtime option can turn it off at runtime.
>> v12:
>>     Add CONFIG_FENCE_TRACE. Add missing documentation for the fence->context
>>     and fence->seqno members.
>> v13:
>>     Fixup CONFIG_FENCE_TRACE kconfig description.
>>     Move fence_context_alloc to fence.
>>     Simplify fence_later.
>>     Kill priv member to fence_cb.
>> v14:
>>     Remove priv argument from fence_add_callback, oops!
>> v15:
>>     Remove priv from documentation.
>>     Explicitly include linux/atomic.h.
>> v16:
>>     Add trace events.
>>     Import changes required by android syncpoints.
>> v17:
>>     Use wake_up_state instead of try_to_wake_up. (Colin Cross)
>>     Fix up commit description for seqno_fence. (Rob Clark)
>> Signed-off-by: Maarten Lankhorst <>
>> Signed-off-by: Thierry Reding <> #use smp_mb__before_atomic()
>> Reviewed-by: Rob Clark <>
>> ---
>>  Documentation/DocBook/device-drivers.tmpl |    2
>>  drivers/base/Kconfig                      |    9 +
>>  drivers/base/Makefile                     |    2
>>  drivers/base/fence.c                      |  416 +++++++++++++++++++++++++++++
>>  include/linux/fence.h                     |  333 +++++++++++++++++++++++
>>  include/trace/events/fence.h              |  128 +++++++++
>>  6 files changed, 889 insertions(+), 1 deletion(-)
>>  create mode 100644 drivers/base/fence.c
>>  create mode 100644 include/linux/fence.h
>>  create mode 100644 include/trace/events/fence.h
> Who is going to sign up to maintain this code?  (hint, it's not me...)

that would be Sumit (dma-buf tree)..

probably we should move fence/reservation/dma-buf into drivers/dma-buf
(or something approximately like that)


> thanks,
> greg k-h
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