Re: [PATCH v2] drm/tidss: dispc: Rewrite naive plane positioning code

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

On Wed, Feb 12, 2020 at 04:08:11PM +0200, Jyri Sarha wrote:
> On 12/02/2020 15:59, Jyri Sarha wrote:
> > The old implementation of placing planes on the CRTC while configuring
> > the planes was naive and relied on the order in which the planes were
> > configured, enabled, and disabled. The situation where a plane's zpos
> > was changed on the fly was completely broken. The usual symptoms of
> > this problem was scrambled display and a flood of sync lost errors,
> > when a plane was active in two layers at the same time, or a missing
> > plane, in case when a layer was accidentally disabled.
> > 
> > The rewrite takes a more straight forward approach when HW is
> > concerned. The plane positioning registers are in the CRTC (actually
> > OVR) register space and it is more natural to configure them in one go
> > while configuring the CRTC. To do this we need to make sure we have
> > all the planes on updated CRTCs in the new atomic state to be
> > committed. This is done by calling drm_atomic_add_affected_planes() in
> > crtc_atomic_check().
> > 
> > Signed-off-by: Jyri Sarha <jsarha@ti.com>
> > ---
> >  drivers/gpu/drm/tidss/tidss_crtc.c  | 55 ++++++++++++++++++++++++++++-
> >  drivers/gpu/drm/tidss/tidss_dispc.c | 55 +++++++++++------------------
> >  drivers/gpu/drm/tidss/tidss_dispc.h |  5 +++
> >  3 files changed, 79 insertions(+), 36 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/tidss/tidss_crtc.c b/drivers/gpu/drm/tidss/tidss_crtc.c
> > index 032c31ee2820..f7c5fd1094a8 100644
> > --- a/drivers/gpu/drm/tidss/tidss_crtc.c
> > +++ b/drivers/gpu/drm/tidss/tidss_crtc.c
> ...
> > @@ -108,7 +110,54 @@ static int tidss_crtc_atomic_check(struct drm_crtc *crtc,
> >  		return -EINVAL;
> >  	}
> >  
> > -	return dispc_vp_bus_check(dispc, hw_videoport, state);
> > +	ret = dispc_vp_bus_check(dispc, hw_videoport, state);
> > +	if (ret)
> > +		return ret;
> > +
> > +	/* Add unchanged planes on this crtc to state for zpos update. */
> > +	return drm_atomic_add_affected_planes(state->state, crtc);
> 
> Is this a correct way to use drm_atomic_add_affected_planes()?
> 
> I saw that some other drivers implement their own mode_config
> atomic_check() and have this call there in
> for_each_new_crtc_in_state()-loop, but I thought it should be fine to
> call it in crtc_atomic_check().

You seem to be using drm_atomic_helper_check_planes(), which means
crtc.atomic_check() gets called after plane.atomic_check(). So this
might be good or bad depending on whether you'd like the planes you
add here to go through their .atomic_check() or not.

-- 
Ville Syrjälä
Intel
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