Hi Greg,
On 19 June 2014 06:55, Rob Clark <robdclark@xxxxxxxxx> wrote:
> On Wed, Jun 18, 2014 at 9:16 PM, Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>>> Signed-off-by: Thierry Reding <thierry.reding@xxxxxxxxx> #use smp_mb__before_atomic()
>>> Reviewed-by: Rob Clark <robdclark@xxxxxxxxx>
>>> ---
>>> 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).
>
> BR,
> -R
>
>
>> thanks,
>>
>> greg k-h
Best regards,
~Sumit.
--
Thanks and regards,
Sumit Semwal
Graphics Engineer - Graphics working group
Linaro.org │ Open source software for ARM SoCs
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