Re: [PATCH] drm/nouveau/kms/nv50-: Fix clock checking algorithm in nv50_dp_mode_valid()

[Lyude Paul <lyude@redhat.com>]

On Mon, 2020-09-28 at 16:01 +0300, Ville Syrjälä wrote:
> On Tue, Sep 22, 2020 at 05:05:10PM -0400, Lyude Paul wrote:
> > While I thought I had this correct (since it actually did reject modes
> > like I expected during testing), Ville Syrjala from Intel pointed out
> > that the logic here isn't correct. max_clock refers to the max symbol
> > rate supported by the encoder, so limiting clock to ds_clock using max()
> > doesn't make sense. Additionally, we want to check against 6bpc for the
> > time being since that's the minimum possible bpc here, not the reported
> > bpc from the connector. See:
> > 
> > https://lists.freedesktop.org/archives/dri-devel/2020-September/280276.html
> > 
> > For more info.
> > 
> > So, let's rewrite this using Ville's advice.
> > 
> > Signed-off-by: Lyude Paul <lyude@redhat.com>
> > Fixes: 409d38139b42 ("drm/nouveau/kms/nv50-: Use downstream DP clock
> > limits for mode validation")
> > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > Cc: Lyude Paul <lyude@redhat.com>
> > Cc: Ben Skeggs <bskeggs@redhat.com>
> > ---
> >  drivers/gpu/drm/nouveau/nouveau_dp.c | 23 +++++++++++++----------
> >  1 file changed, 13 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/nouveau/nouveau_dp.c
> > b/drivers/gpu/drm/nouveau/nouveau_dp.c
> > index 7b640e05bd4cd..24c81e423d349 100644
> > --- a/drivers/gpu/drm/nouveau/nouveau_dp.c
> > +++ b/drivers/gpu/drm/nouveau/nouveau_dp.c
> > @@ -231,23 +231,26 @@ nv50_dp_mode_valid(struct drm_connector *connector,
> >  		   const struct drm_display_mode *mode,
> >  		   unsigned *out_clock)
> >  {
> > -	const unsigned min_clock = 25000;
> > -	unsigned max_clock, ds_clock, clock;
> > +	const unsigned int min_clock = 25000;
> > +	unsigned int max_clock, ds_clock, clock;
> > +	const u8 bpp = 18; /* 6 bpc */
> 
> AFAICS nv50_outp_atomic_check() and nv50_msto_atomic_check()
> just blindly use connector->display_info.bpc without any fallback
> logic to lower the bpc. So Ilia's concerns seem well founded.
> Without that logic I guess you should just use
> connector->display_info.bpc here as well.
> 
> >  	enum drm_mode_status ret;
> >  
> >  	if (mode->flags & DRM_MODE_FLAG_INTERLACE && !outp->caps.dp_interlace)
> >  		return MODE_NO_INTERLACE;
> >  
> >  	max_clock = outp->dp.link_nr * outp->dp.link_bw;
> > -	ds_clock = drm_dp_downstream_max_dotclock(outp->dp.dpcd,
> > -						  outp->dp.downstream_ports);
> > -	if (ds_clock)
> > -		max_clock = min(max_clock, ds_clock);
> > -
> > -	clock = mode->clock * (connector->display_info.bpc * 3) / 10;
> > -	ret = nouveau_conn_mode_clock_valid(mode, min_clock, max_clock,
> > -					    &clock);
> > +	clock = mode->clock * bpp / 8;
> > +	if (clock > max_clock)
> > +		return MODE_CLOCK_HIGH;
> 
> This stuff vs. nouveau_conn_mode_clock_valid() still seems a bit messy.
> The max_clock you pass to nouveau_conn_mode_clock_valid() is the max
> symbol clock, but nouveau_conn_mode_clock_valid() checks it against the
> dotclock. Also only nouveau_conn_mode_clock_valid() has any kind of
> stereo 3D handling, but AFAICS stereo_allowed is also set for DP?

...not sure I'm following you here, it's set to true for DP so don't we want
to check it and adjust the pixel clock we output accordingly?

> 
> > +
> > +	ds_clock = drm_dp_downstream_max_dotclock(outp->dp.dpcd, outp-
> > >dp.downstream_ports);
> > +	if (ds_clock && mode->clock > ds_clock)
> > +		return MODE_CLOCK_HIGH;
> > +
> > +	ret = nouveau_conn_mode_clock_valid(mode, min_clock, max_clock,
> > &clock);
> >  	if (out_clock)
> >  		*out_clock = clock;
> > +
> >  	return ret;
> >  }
> > -- 
> > 2.26.2
-- 
Cheers,
	Lyude Paul (she/her)
	Software Engineer at Red Hat