linux-kernel - Re: [PATCH v4 06/14] dma-buf/sync

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Date:   Mon, 20 Feb 2023 10:53:45 +0200
From:   Pekka Paalanen <ppaalanen@...il.com>
To:     Rob Clark <robdclark@...il.com>
Cc:     dri-devel@...ts.freedesktop.org, freedreno@...ts.freedesktop.org,
        Daniel Vetter <daniel@...ll.ch>,
        Christian König <ckoenig.leichtzumerken@...il.com>,
        Michel Dänzer <michel@...nzer.net>,
        Tvrtko Ursulin <tvrtko.ursulin@...el.com>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        Alex Deucher <alexander.deucher@....com>,
        Simon Ser <contact@...rsion.fr>,
        Rob Clark <robdclark@...omium.org>,
        Sumit Semwal <sumit.semwal@...aro.org>,
        Gustavo Padovan <gustavo@...ovan.org>,
        Christian König <christian.koenig@....com>,
        linux-media@...r.kernel.org (open list:SYNC FILE FRAMEWORK),
        linaro-mm-sig@...ts.linaro.org (moderated list:DMA BUFFER SHARING
        FRAMEWORK), linux-kernel@...r.kernel.org (open list)
Subject: Re: [PATCH v4 06/14] dma-buf/sync_file: Support (E)POLLPRI

On Sat, 18 Feb 2023 13:15:49 -0800
Rob Clark <robdclark@...il.com> wrote:

> From: Rob Clark <robdclark@...omium.org>
> 
> Allow userspace to use the EPOLLPRI/POLLPRI flag to indicate an urgent
> wait (as opposed to a "housekeeping" wait to know when to cleanup after
> some work has completed).  Usermode components of GPU driver stacks
> often poll() on fence fd's to know when it is safe to do things like
> free or reuse a buffer, but they can also poll() on a fence fd when
> waiting to read back results from the GPU.  The EPOLLPRI/POLLPRI flag
> lets the kernel differentiate these two cases.
> 
> Signed-off-by: Rob Clark <robdclark@...omium.org>

Hi,

where would the UAPI documentation of this go?
It seems to be missing.

If a Wayland compositor is polling application fences to know which
client buffer to use in its rendering, should the compositor poll with
PRI or not? If a compositor polls with PRI, then all fences from all
applications would always be PRI. Would that be harmful somehow or
would it be beneficial?


Thanks,
pq

> ---
>  drivers/dma-buf/sync_file.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
> index fb6ca1032885..c30b2085ee0a 100644
> --- a/drivers/dma-buf/sync_file.c
> +++ b/drivers/dma-buf/sync_file.c
> @@ -192,6 +192,14 @@ static __poll_t sync_file_poll(struct file *file, poll_table *wait)
>  {
>  	struct sync_file *sync_file = file->private_data;
>  
> +	/*
> +	 * The POLLPRI/EPOLLPRI flag can be used to signal that
> +	 * userspace wants the fence to signal ASAP, express this
> +	 * as an immediate deadline.
> +	 */
> +	if (poll_requested_events(wait) & EPOLLPRI)
> +		dma_fence_set_deadline(sync_file->fence, ktime_get());
> +
>  	poll_wait(file, &sync_file->wq, wait);
>  
>  	if (list_empty(&sync_file->cb.node) &&


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