linux-kernel - Re: [PATCH v2] drm/syncobj: ensure progress for syncobj queries

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Message-ID: <900f8658-726c-4034-90ff-398e6c57ec47@amd.com>
Date: Tue, 22 Oct 2024 18:53:07 +0200
From: Christian König <christian.koenig@....com>
To: Chia-I Wu <olvaffe@...il.com>, boris.brezillon@...labora.com,
 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>,
 Chunming Zhou <david1.zhou@....com>,
 Lionel Landwerlin <lionel.g.landwerlin@...el.com>,
 dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Cc: faith.ekstrand@...labora.com
Subject: Re: [PATCH v2] drm/syncobj: ensure progress for syncobj queries

Am 22.10.24 um 18:18 schrieb Chia-I Wu:
> 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.

Wait a second, just querying the fence status is absolutely not 
guaranteed to return true in finite time. That is well documented on the 
dma_fence() object.

When you want to poll on signaling from userspace you really need to 
call poll or the wait IOCTL with a zero timeout. That will also return 
immediately but should enable signaling while doing that.

So just querying the status should absolutely *not* enable signaling. 
That's an intentional separation.

Regards,
Christian.

>
> Fixes: 27b575a9aa2f ("drm/syncobj: add timeline payload query ioctl v6")
> Signed-off-by: Chia-I Wu <olvaffe@...il.com>
>
> ---
>
> v2: add Signed-off-by and Fixes tags
> ---
>   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);