[PATCH] drm/syncobj: Add documentation for timeline syncobj

Thu Aug 22 19:24:52 UTC 2019

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....

> + * 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.

+ *
>   *
>   * 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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