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Date:	Wed, 18 Jun 2014 18:16:53 -0700
From:	Greg KH <>
To:	Maarten Lankhorst <>
Subject: Re: [REPOST PATCH 1/8] fence: dma-buf cross-device synchronization

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...)


greg k-h
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