linux-kernel - Re: [PATCH] drm/core: Fail atomic IOCTL with no CRTC state but with signaling.

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Date:   Thu, 15 Jun 2017 16:46:49 -0400
From:   Andrey Grodzovsky <Andrey.Grodzovsky@....com>
To:     dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, maarten.lankhorst@...ux.intel.com
Cc:     amd-gfx@...ts.freedesktop.org, harry.wentland@....com
Subject: Re: [PATCH] drm/core: Fail atomic IOCTL with no CRTC state but with
 signaling.

Just a reminder.

Thanks.

On 06/09/2017 05:30 PM, Andrey Grodzovsky wrote:
> Problem:
> While running IGT kms_atomic_transition test suite i encountered
> a hang in drmHandleEvent immidietly follwoing an atomic_commit.
> After dumping the atomic state I relized that in this case there was
> not even one CRTC attached to the state and only disabled
> planes. This probably due to a commit which hadn't changed any property
> which would require attaching crtc state. This means drmHandleEvent
> will never wake up from read since without CRTC in atomic state
> the event fd will not be singnaled.
> This point to a bug in IGT but also DRM should gracefully
> fail  such scenario so no hang on user side will happen.
>
> Fix:
> Explicitly fail by failing atomic_commit early in
> drm_mode_atomic_commit where such problem can be identified.
>
> Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@....com>
> ---
>   drivers/gpu/drm/drm_atomic.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c
> index a567310..32eae1c 100644
> --- a/drivers/gpu/drm/drm_atomic.c
> +++ b/drivers/gpu/drm/drm_atomic.c
> @@ -1933,7 +1933,7 @@ static int prepare_crtc_signaling(struct drm_device *dev,
>   {
>   	struct drm_crtc *crtc;
>   	struct drm_crtc_state *crtc_state;
> -	int i, ret;
> +	int i, c = 0, ret;
>   
>   	if (arg->flags & DRM_MODE_ATOMIC_TEST_ONLY)
>   		return 0;
> @@ -1994,8 +1994,17 @@ static int prepare_crtc_signaling(struct drm_device *dev,
>   
>   			crtc_state->event->base.fence = fence;
>   		}
> +
> +		c++;
>   	}
>   
> +	/*
> +	 * Having this flag means user mode pends on event which will never
> +	 * reach due to lack of at least one CRTC for signaling
> +	 */
> +	if (c == 0 && (arg->flags & DRM_MODE_PAGE_FLIP_EVENT))
> +		return -EINVAL;
> +
>   	return 0;
>   }
>   
> @@ -2179,6 +2188,8 @@ int drm_mode_atomic_ioctl(struct drm_device *dev,
>   		drm_mode_object_unreference(obj);
>   	}
>   
> +
> +
>   	ret = prepare_crtc_signaling(dev, state, arg, file_priv, &fence_state,
>   				     &num_fences);
>   	if (ret)