[PATCH] drm/syncobj: Add documentation for timeline syncobj
Lionel Landwerlin
lionel.g.landwerlin at intel.com
Mon Aug 26 04:30:08 UTC 2019
On 26/08/2019 00:01, Daniel Vetter wrote:
> On Fri, Aug 23, 2019 at 8:53 PM Jason Ekstrand <jason at jlekstrand.net> wrote:
>>
>> On Thu, Aug 22, 2019 at 5:28 PM Lionel Landwerlin <lionel.g.landwerlin at intel.com> wrote:
>>> On 22/08/2019 21:24, Jason Ekstrand wrote:
>>>
>>> On Thu, Aug 22, 2019 at 9:55 AM Lionel Landwerlin <lionel.g.landwerlin at intel.com> wrote:
>>>> We've added a set of new APIs to manipulate syncobjs holding timelines
>>>> of dma_fence. This adds a bit of documentation about how this works.
>>>>
>>>> Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
>>>> Cc: Christian Koenig <Christian.Koenig at amd.com>
>>>> Cc: Jason Ekstrand <jason at jlekstrand.net>
>>>> Cc: David(ChunMing) Zhou <David1.Zhou at amd.com>
>>>> ---
>>>> drivers/gpu/drm/drm_syncobj.c | 87 +++++++++++++++++++++++++++++------
>>>> 1 file changed, 74 insertions(+), 13 deletions(-)
>>>>
>>>> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
>>>> index b5ad73330a48..32ffded6d2c0 100644
>>>> --- a/drivers/gpu/drm/drm_syncobj.c
>>>> +++ b/drivers/gpu/drm/drm_syncobj.c
>>>> @@ -43,27 +43,66 @@
>>>> * - Signal a syncobj (set a trivially signaled fence)
>>>> * - Wait for a syncobj's fence to appear and be signaled
>>>> *
>>>> + * The syncobj userspace API also provides operations to manipulate a syncobj
>>>> + * in terms of a timeline of struct &dma_fence rather than a single struct
>>>> + * &dma_fence, through the following operations:
>>>> + *
>>>> + * - Signal a given point on the timeline
>>>> + * - Wait for a given point to appear and/or be signaled
>>>> + * - Import and export from/to a given point of a timeline
>>>> + *
>>>> * At it's core, a syncobj is simply a wrapper around a pointer to a struct
>>>> * &dma_fence which may be NULL.
>>>> * When a syncobj is first created, its pointer is either NULL or a pointer
>>>> * to an already signaled fence depending on whether the
>>>> * &DRM_SYNCOBJ_CREATE_SIGNALED flag is passed to
>>>> * &DRM_IOCTL_SYNCOBJ_CREATE.
>>>> - * When GPU work which signals a syncobj is enqueued in a DRM driver,
>>>> - * the syncobj fence is replaced with a fence which will be signaled by the
>>>> - * completion of that work.
>>>> - * When GPU work which waits on a syncobj is enqueued in a DRM driver, the
>>>> - * driver retrieves syncobj's current fence at the time the work is enqueued
>>>> - * waits on that fence before submitting the work to hardware.
>>>> - * If the syncobj's fence is NULL, the enqueue operation is expected to fail.
>>>> - * All manipulation of the syncobjs's fence happens in terms of the current
>>>> - * fence at the time the ioctl is called by userspace regardless of whether
>>>> - * that operation is an immediate host-side operation (signal or reset) or
>>>> - * or an operation which is enqueued in some driver queue.
>>>> - * &DRM_IOCTL_SYNCOBJ_RESET and &DRM_IOCTL_SYNCOBJ_SIGNAL can be used to
>>>> - * manipulate a syncobj from the host by resetting its pointer to NULL or
>>>> + *
>>>> + * If the syncobj is considered as a binary (signal/unsignaled) primitive,
>>>
>>> What does "considered as a binary" mean? Is it an inherent property of the syncobj given at create time? Is it a state the syncobj can be in? Or is it a property of how the submit ioctl in the DRM driver references it? I'm really hoping it's either 1 or 3....
>>>
>>>
>>> 3: you either use it binary/legacy apis, or timeline apis. timeline apis also provide some binary compatibility with the point 0 (in particular for wait).
>>
>> Right. Maybe we should say something like "When GPU work is enqueued which signals a non-zero time point" or something like that? I guess that implies a certain unification across drivers that maybe we don't want....
> [Just jumping in on this comment here]
>
> I thought the point of syncobj is that you can share them across
> drivers (not just within drivers)? Otherwise not much sense in the
> common infrastructure. Hence I'd say we should spec all these things.
> Concern from someone who's seen way too many cross-driver apis that
> turned out the be decidedly cross-driver than planned ...
The sharing of a timeline semaphore/syncobj between 2 apps/drivers
implies that they both know they're dealing with a timeline semaphore.
I see that at the same level as sharing a file descriptor and knowing it
represents a syncfd or a syncobj.
There has to be some kind of understanding, otherwise nothing works.
If the shared semantic between the 2 clients is a binary
(signal/unsignaled) semaphore, then both drivers should share the
existing syncobj type, that is a syncobj that will only ever contain a
single dma-fence.
You can build that out of the timeline by exporting a particular point
into another syncobj (transfer ioctl).
-Lionel
>
> Cheers, Daniel
>
>
>>>
>>>> + * when GPU work is enqueued in a DRM driver to signal the syncobj, the fence
>>>> + * is replaced with a fence which will be signaled by the completion of that
>>>> + * work.
>>>> + * If the syncobj is considered as a timeline primitive, when GPU work is
>>>> + * enqueued in a DRM driver to signal the a given point of the syncobj, a new
>>>> + * struct &dma_fence_chain pointing to the DRM driver's fence and also
>>>> + * pointing to the previous fence that was in the syncobj. The new struct
>>>> + * &dma_fence_chain fence put into the syncobj will be signaled by completion
>>>> + * of the DRM driver's work and also any work associated with the fence
>>>> + * previously in the syncobj.
>>>> + *
>>>> + * When GPU work which waits on a syncobj is enqueued in a DRM driver, at the
>>>> + * time the work is enqueued, it waits on the fence coming from the syncobj
>>>> + * before submitting the work to hardware. That fence is either :
>>>> + *
>>>> + * - The syncobj's current fence if the syncobj is considered as a binary
>>>> + * primitive.
>>>> + * - The struct &dma_fence associated with a given point if the syncobj is
>>>> + * considered as a timeline primitive.
>>>> + *
>>>> + * If the syncobj's fence is NULL or not present in the syncobj's timeline,
>>>> + * the enqueue operation is expected to fail.
>>>> + *
>>>> + * With binary syncobj, all manipulation of the syncobjs's fence happens in
>>>> + * terms of the current fence at the time the ioctl is called by userspace
>>>> + * regardless of whether that operation is an immediate host-side operation
>>>> + * (signal or reset) or or an operation which is enqueued in some driver
>>>> + * queue. &DRM_IOCTL_SYNCOBJ_RESET and &DRM_IOCTL_SYNCOBJ_SIGNAL can be used
>>>> + * to manipulate a syncobj from the host by resetting its pointer to NULL or
>>>> * setting its pointer to a fence which is already signaled.
>>>> *
>>>> + * With timeline syncobj, all manipulation of the timeline fences happens in
>>>> + * terms of the fence referred to in the timeline. See
>>>> + * dma_fence_chain_find_seqno() to see how a given point is found in the
>>>> + * timeline.
>>>> + *
>>>> + * Note that applications should be careful to always use timeline set of
>>>> + * ioctl() when dealing with syncobj considered as timeline. Using a binary
>>>> + * set of ioctl() with a syncobj considered as timeline could result incorrect
>>>> + * synchronization. The use of binary syncobj is supported through the
>>>> + * timeline set of ioctl() by using a point value of 0, this will reproduce
>>>> + * the behavior of the binary set of ioctl() (for example replace the
>>>> + * syncobj's fence when signaling).
>>>
>>> I know I've asked this before but I feel compelled to ask it again. Why do we allow them to mix and match? Why not just have a create flag and enforce meaningful behavior? I'm a bit concerned that userspace is going to start relying on the subtlties of the interaction between timeline and binary syncobjs which are neither documented nor properly tested in IGT.
>>>
>>>
>>> For one, you might have to mix both types of syncobjs in a given wait/signal operation. So 0 ensures we can do that.
>>
>> Right, that sounds like a useful feature.
>>
>>> Second, drm-syncobj is a container and its payload is an interface (dma_fence) which has several implementations.
>>>
>>> The kernel primitive is just less restrictive than the Vulkan API here.
>>>
>>> I guess we could add a flag at creation to ensure the replacement of the fence in a timeline syncobj cannot happen.
>>
>> I would be in favor of that but I'd be interested to hear what Christian or David think.
>>
>>> I haven't thought of all the implications that might have though... Should we allow reset on a timeline syncobj?
>>
>> Good question. I'm inclined to say "yes" because it's pretty well-defined what such a reset means. However, it's not really needed.
>>
>>> -Lionel
>>>
>>>
>>>
>>>> + *
>>>> *
>>>> * Host-side wait on syncobjs
>>>> * --------------------------
>>>> @@ -87,6 +126,16 @@
>>>> * synchronize between the two.
>>>> * This requirement is inherited from the Vulkan fence API.
>>>> *
>>>> + * Similarly, &DRM_IOCTL_SYNCOBJ_TIMELINE_WAIT takes an array of syncobj
>>>> + * handles as well as an array of u64 points and does a host-side wait on all
>>>> + * of syncobj fences at the given points simultaneously.
>>>> + *
>>>> + * &DRM_IOCTL_SYNCOBJ_TIMELINE_WAIT also adds the ability to wait for a given
>>>> + * fence to materialize on the timeline without waiting for the fence to be
>>>> + * signaled by using the &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_AVAILABLE flag. This
>>>> + * requirement is inherited from the wait-before-signal behavior required by
>>>> + * the Vulkan timeline semaphore API.
>>>> + *
>>>> *
>>>> * Import/export of syncobjs
>>>> * -------------------------
>>>> @@ -120,6 +169,18 @@
>>>> * Because sync files are immutable, resetting or signaling the syncobj
>>>> * will not affect any sync files whose fences have been imported into the
>>>> * syncobj.
>>>> + *
>>>> + *
>>>> + * Import/export of timeline points in timeline syncobjs
>>>> + * -----------------------------------------------------
>>>> + *
>>>> + * &DRM_IOCTL_SYNCOBJ_TRANSFER provides a mechanism to transfer a struct
>>>> + * &dma_fence of at a given point from a timeline syncobj to another point
>>>> + * into another timeline syncobj.
>>>> + *
>>>> + * Note that if you want to transfer a struct &dma_fence from a given point on
>>>> + * a timeline syncobj from/into a binary syncobj, you can use the point 0 to
>>>> + * mean take/replace the fence in the syncobj.
>>>> */
>>>>
>>>> #include <linux/anon_inodes.h>
>>>> --
>>>> 2.23.0
>>>>
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>
>
> --
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch
>
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