Subject: Re: [PATCH] drm/core: Fail atomic IOCTL with no CRTC state but with
 signaling.
To: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>,
        dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
        intel-gfx@lists.freedesktop.org, maarten.lankhorst@linux.intel.com
Cc: amd-gfx@lists.freedesktop.org
References: <1497043819-28591-1-git-send-email-Andrey.Grodzovsky@amd.com>
From: Harry Wentland <harry.wentland@amd.com>
Message-ID: <0d603149-3fb0-3d48-89e5-77e3f7686415@amd.com>
Date: Mon, 19 Jun 2017 11:35:28 -0400
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On 2017-06-09 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.

s/immidietly/immediately/g
s/follwoing/following/g

> 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.

s/singnaled/signaled/g

> This point to a bug in IGT but also DRM should gracefully
> fail  such scenario so no hang on user side will happen.
> 

Can we create an IGT fix for this to make sure this won't happen?

> Fix:
> Explicitly fail by failing atomic_commit early in
> drm_mode_atomic_commit where such problem can be identified.
> 

The change seems reasonable to me but I would like to see some input
from someone who's more familiar with the usermode side of things.

> Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.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++;

Not sure if intentional, but I like it.

>  	}
>  
> +	/*
> +	 * 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);
>  	}
>  
> +
> +

Remove these extraneous newlines.

Harry

>  	ret = prepare_crtc_signaling(dev, state, arg, file_priv, &fence_state,
>  				     &num_fences);
>  	if (ret)
>