Re: [PATCH] drm/atomic: Fix the early return in drm_atomic_set_mode_for_crtc()

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Nov 21, 2018 at 10:59:56AM +0100, Daniel Vetter wrote:
> On Tue, Nov 20, 2018 at 07:55:42PM +0200, Ville Syrjala wrote:
> > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > 
> > The early return in drm_atomic_set_mode_for_crtc() isn't quite
> > right. It would mistakenly return and fail to update
> > crtc_state->enable if someone actually tried to set a zeroed
> > mode on a currently disabled crtc. I suppose that should never
> > happen but better safe than sorry.
> > 
> > Additionally the early return will not be taken if we're trying to
> > disable an already disable crtc. While that is not actually harmful
> > it is inconsistent, so let's handle that case as well.
> > 
> > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> Do we have an igt for this? I.e. trying to set a all-0 mode for a disabled
> CRTC and seeing what happens ...

It should get rejected by drm_mode_convert_umode()->drm_mode_validate_basic()
so presumably you shouldn't be able to do this from userspace. In kernel
users could bypass that check and sneak something like this in however.

But I guess having an igt to verify that we do indeed reject bad modes
would a decent idea. Looks like currently we only have
kms_invalid_dotclock.

> 
> Patch itself looks fine, has my r-b if the igt materializes.
> -Daniel
> 
> > ---
> >  drivers/gpu/drm/drm_atomic_uapi.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_atomic_uapi.c b/drivers/gpu/drm/drm_atomic_uapi.c
> > index 86ac33922b09..ed0ea82e8a1d 100644
> > --- a/drivers/gpu/drm/drm_atomic_uapi.c
> > +++ b/drivers/gpu/drm/drm_atomic_uapi.c
> > @@ -68,8 +68,13 @@ int drm_atomic_set_mode_for_crtc(struct drm_crtc_state *state,
> >  	struct drm_mode_modeinfo umode;
> >  
> >  	/* Early return for no change. */
> > -	if (mode && memcmp(&state->mode, mode, sizeof(*mode)) == 0)
> > -		return 0;
> > +	if (state->enable) {
> > +		if (mode && memcmp(&state->mode, mode, sizeof(*mode)) == 0)
> > +			return 0;
> > +	} else {
> > +		if (!mode)
> > +			return 0;
> > +	}
> >  
> >  	drm_property_blob_put(state->mode_blob);
> >  	state->mode_blob = NULL;
> > -- 
> > 2.18.1
> > 
> > _______________________________________________
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> > Intel-gfx@lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/intel-gfx
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Ville Syrjälä
Intel
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