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Date:	Wed, 18 Jun 2014 21:54:38 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	Sumit Semwal <sumit.semwal@...aro.org>
Cc:	Rob Clark <robdclark@...il.com>,
	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	linux-arch@...r.kernel.org,
	Thomas Hellstrom <thellstrom@...are.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"dri-devel@...ts.freedesktop.org" <dri-devel@...ts.freedesktop.org>,
	"linaro-mm-sig@...ts.linaro.org" <linaro-mm-sig@...ts.linaro.org>,
	Thierry Reding <thierry.reding@...il.com>,
	Colin Cross <ccross@...gle.com>,
	Daniel Vetter <daniel@...ll.ch>,
	"linux-media@...r.kernel.org" <linux-media@...r.kernel.org>
Subject: Re: [REPOST PATCH 1/8] fence: dma-buf cross-device synchronization
 (v17)

On Thu, Jun 19, 2014 at 09:57:35AM +0530, Sumit Semwal wrote:
> Hi Greg,
> 
> On 19 June 2014 06:55, Rob Clark <robdclark@...il.com> wrote:
> > On Wed, Jun 18, 2014 at 9:16 PM, Greg KH <gregkh@...uxfoundation.org> 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 <maarten.lankhorst@...onical.com>
> >>> Signed-off-by: Thierry Reding <thierry.reding@...il.com> #use smp_mb__before_atomic()
> >>> Reviewed-by: Rob Clark <robdclark@...il.com>
> >>> ---
> >>>  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)
> Yes, that would be me - it might be better to create a new directory
> as suggested above (drivers/dma-buf).

That's fine with me, there is going to be more than just one file in
there, right?  :)

greg k-h
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