[PATCH v2] drm/syncobj: add IOCTL to register an eventfd

[Pekka Paalanen]

On Wed, 12 Oct 2022 12:32:53 +0000
Simon Ser <contact at emersion.fr> wrote:

> Introduce a new DRM_IOCTL_SYNCOBJ_EVENTFD IOCTL which signals an
> eventfd from a syncobj.
> 
> This is useful for Wayland compositors to handle wait-before-submit.
> Wayland clients can send a timeline point to the compositor
> before the point has materialized yet, then compositors can wait
> for the point to materialize via this new IOCTL.
> 
> The existing DRM_IOCTL_SYNCOBJ_TIMELINE_WAIT IOCTL is not suitable
> because it blocks. Compositors want to integrate the wait with
> their poll(2)-based event loop.
> 
> v2:
> - Wait for fence when flags is zero
> - Improve documentation (Pekka)
> - Rename IOCTL (Christian)
> - Fix typo in drm_syncobj_add_eventfd() (Christian)
> 
> Signed-off-by: Simon Ser <contact at emersion.fr>
> Cc: Jason Ekstrand <jason at jlekstrand.net>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Christian König <christian.koenig at amd.com>
> Cc: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
> Cc: Daniel Stone <daniel at fooishbar.org>
> Cc: Pekka Paalanen <ppaalanen at gmail.com>
> Cc: James Jones <jajones at nvidia.com>
> ---
> 
> Toy user-space available at:
> https://paste.sr.ht/~emersion/674bd4fc614570f262b5bb2213be5c77d68944ac
> 
>  drivers/gpu/drm/drm_internal.h |   2 +
>  drivers/gpu/drm/drm_ioctl.c    |   2 +
>  drivers/gpu/drm/drm_syncobj.c  | 143 +++++++++++++++++++++++++++++++--
>  include/drm/drm_syncobj.h      |   6 +-
>  include/uapi/drm/drm.h         |  22 +++++
>  5 files changed, 168 insertions(+), 7 deletions(-)
> 

...

> diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
> index 642808520d92..5ac0a48b0169 100644
> --- a/include/uapi/drm/drm.h
> +++ b/include/uapi/drm/drm.h
> @@ -909,6 +909,27 @@ struct drm_syncobj_timeline_wait {
>  	__u32 pad;
>  };
>  
> +/**
> + * struct drm_syncobj_eventfd
> + * @handle: syncobj handle.
> + * @flags: Zero to wait for the point to be signalled, or
> + *         &DRM_SYNCOBJ_WAIT_FLAGS_WAIT_AVAILABLE to wait for a fence to be
> + *         available for the point.
> + * @point: syncobj timeline point (set to zero for binary syncobjs).
> + * @fd: Existing eventfd to sent events to.
> + * @pad: Must be zero.
> + *
> + * Register an eventfd to be signalled by a syncobj. The eventfd counter will
> + * be incremented by one.
> + */
> +struct drm_syncobj_eventfd {
> +	__u32 handle;
> +	__u32 flags;
> +	__u64 point;
> +	__s32 fd;
> +	__u32 pad;
> +};
> +
>  
>  struct drm_syncobj_array {
>  	__u64 handles;
> @@ -1095,6 +1116,7 @@ extern "C" {
>  #define DRM_IOCTL_SYNCOBJ_QUERY		DRM_IOWR(0xCB, struct drm_syncobj_timeline_array)
>  #define DRM_IOCTL_SYNCOBJ_TRANSFER	DRM_IOWR(0xCC, struct drm_syncobj_transfer)
>  #define DRM_IOCTL_SYNCOBJ_TIMELINE_SIGNAL	DRM_IOWR(0xCD, struct drm_syncobj_timeline_array)
> +#define DRM_IOCTL_SYNCOBJ_EVENTFD	DRM_IOWR(0xCE, struct drm_syncobj_eventfd)
>  
>  /**
>   * DRM_IOCTL_MODE_GETFB2 - Get framebuffer metadata.

Hi,

UAPI

Acked-by: Pekka Paalanen <pekka.paalanen at collabora.com>

with the disclaimer that I know nothing about syncobjs. The doc sounds
good though, and I read through eventfd man page to see that the
increment makes sense.

I think there is also a way initialize eventfd with 0xffffffffffffffff - N
where N is the number of syncobjs you want to wait, and then poll for
readable. Once the Nth syncobj fires, the eventfd overflows and polls
readable (and produces POLLERR). Maybe that's abuse, and maybe you
usually have no need to poll for all of a set of syncobjs, so fine.

I suspect the Wayland compositor use case wants to know about each
syncobj individually, so either you have one eventfd per syncobj or
poll for "any" in an eventfd with multiple syncobjs and check them all
each time it polls readable.

Seems fine to me.


Thanks,
pq
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