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

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jun 19, 2017 at 04:11:35PM -0400, Andrey Grodzovsky wrote:
> 
> 
> On 06/19/2017 03:24 PM, Sean Paul wrote:
> > On Mon, Jun 19, 2017 at 11:35:28AM -0400, Harry Wentland wrote:
> > > 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.
> > I wonder if there's ever a case where it might be desirable to generate an event
> > from a commit without a crtc. I don't know if anyone has explicitly said that an
> > event can only be generated from state with crtc.
> For a generic event i agree, bit seems to me without active CRTC no way you
> can  expect PAGE_FLIP_EVENT to fire.
> > 
> > I usually don't mind letting userspace shoot itself in the foot, so keep that in
> > mind :)
> > 
> > Sean
> 
> Seems to me you still would try to avoid a bad configuration, returning
> error
> will help debugging for user who made a mistake. I also see something
> similar
> in drm_mode_atomic_ioctl around line 2122 -
> 
> /* can't test and expect an event at the same time. */
> if ((arg->flags & DRM_MODE_ATOMIC_TEST_ONLY) &&
> (arg->flags & DRM_MODE_PAGE_FLIP_EVENT))
> return -EINVAL;

Asking for an event without anything changed is a bug imo. We need to fix
the testcases, and even better would be to have an explicit testcase which
does such a no-op atomic ioctl + event and checks that we get the -EINVAL.

Same applies for asking for events when the crtc is off and stays off btw,
but I think we catch that already somewhere. But again would be good to
make sure we have an igt for it.

Thanks, Daniel

> 
> Thanks,
> Andrey
> > > > 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)
> > > > 
> 

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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch