linux-kernel - Re: [PATCH] drm/syncobj: ensure progress for syncobj queries

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Message-ID: <20241022123052.3e0f3f17@collabora.com>
Date: Tue, 22 Oct 2024 12:30:52 +0200
From: Boris Brezillon <boris.brezillon@...labora.com>
To: Chia-I Wu <olvaffe@...il.com>
Cc: Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>, Maxime Ripard
 <mripard@...nel.org>, Thomas Zimmermann <tzimmermann@...e.de>, David Airlie
 <airlied@...il.com>, Daniel Vetter <daniel@...ll.ch>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
 faith.ekstrand@...labora.com
Subject: Re: [PATCH] drm/syncobj: ensure progress for syncobj queries

On Thu, 17 Oct 2024 09:20:53 -0700
Chia-I Wu <olvaffe@...il.com> wrote:

> Userspace might poll a syncobj with the query ioctl.  Call
> dma_fence_enable_sw_signaling to ensure dma_fence_is_signaled returns
> true in finite time.
> 
> ---
> 
> panvk hits this issue when timeline semaphore is enabled.  It uses the
> transfer ioctl to propagate fences.  dma_fence_unwrap_merge converts the
> dma_fence_chain to a dma_fence_array.  dma_fence_array_signaled never
> return true unless signaling is enabled.

Looks like a bugfix to me. Should we add Fixes+Cc-stable tags so it
gets backported to stable branches.

> ---
>  drivers/gpu/drm/drm_syncobj.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> index 4fcfc0b9b386c..58c5593c897a2 100644
> --- a/drivers/gpu/drm/drm_syncobj.c
> +++ b/drivers/gpu/drm/drm_syncobj.c
> @@ -1689,6 +1689,9 @@ int drm_syncobj_query_ioctl(struct drm_device *dev, void *data,
>  			    DRM_SYNCOBJ_QUERY_FLAGS_LAST_SUBMITTED) {
>  				point = fence->seqno;
>  			} else {
> +				/* ensure forward progress */
> +				dma_fence_enable_sw_signaling(fence);
> +
>  				dma_fence_chain_for_each(iter, fence) {
>  					if (iter->context != fence->context) {
>  						dma_fence_put(iter);