[REPOST PATCH 1/8] fence: dma-buf cross-device synchronization (v17)
Rob Clark
robdclark at gmail.com
Wed Jun 18 18:25:17 PDT 2014
On Wed, Jun 18, 2014 at 9:16 PM, Greg KH <gregkh at linuxfoundation.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 at canonical.com>
>> Signed-off-by: Thierry Reding <thierry.reding at gmail.com> #use smp_mb__before_atomic()
>> Reviewed-by: Rob Clark <robdclark at gmail.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)
BR,
-R
> thanks,
>
> greg k-h
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