Re: [PATCH 01/13] drm/connector: Add define for HDMI 1.4 Maximum Pixel Rate

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

On Wed, Nov 03, 2021 at 01:02:11PM +0200, Ville Syrjälä wrote:
> On Tue, Nov 02, 2021 at 03:59:32PM +0100, Maxime Ripard wrote:
> > --- a/drivers/gpu/drm/drm_edid.c
> > +++ b/drivers/gpu/drm/drm_edid.c
> > @@ -4966,7 +4966,7 @@ static void drm_parse_hdmi_forum_vsdb(struct drm_connector *connector,
> >  		u32 max_tmds_clock = hf_vsdb[5] * 5000;
> >  		struct drm_scdc *scdc = &hdmi->scdc;
> >  
> > -		if (max_tmds_clock > 340000) {
> > +		if (max_tmds_clock > DRM_HDMI_14_MAX_TMDS_CLK_KHZ) {
> >  			display->max_tmds_clock = max_tmds_clock;
> >  			DRM_DEBUG_KMS("HF-VSDB: max TMDS clock %d kHz\n",
> >  				display->max_tmds_clock);
> > diff --git a/drivers/gpu/drm/i915/display/intel_hdmi.c b/drivers/gpu/drm/i915/display/intel_hdmi.c
> > index d2e61f6c6e08..0666203d52b7 100644
> > --- a/drivers/gpu/drm/i915/display/intel_hdmi.c
> > +++ b/drivers/gpu/drm/i915/display/intel_hdmi.c
> > @@ -2226,7 +2226,7 @@ int intel_hdmi_compute_config(struct intel_encoder *encoder,
> >  		if (scdc->scrambling.low_rates)
> >  			pipe_config->hdmi_scrambling = true;
> >  
> > -		if (pipe_config->port_clock > 340000) {
> > +		if (pipe_config->port_clock > DRM_HDMI_14_MAX_TMDS_CLK_KHZ) {
> >  			pipe_config->hdmi_scrambling = true;
> >  			pipe_config->hdmi_high_tmds_clock_ratio = true;
> >  		}
> 
> All of that is HDMI 2.0 stuff. So this just makes it all super
> confusing IMO. Nak.

So reading throgh HDMI 1.4 again it does specify 340 MHz as some kind
of upper limit for the physical cable. But nowhere else is that number
really mentioned AFAICS. HDMI 2.0 does talk quite a bit about the 340
Mcsc limit in various places.

I wonder what people would think of a couple of helpers like:
- drm_hdmi_{can,must}_use_scrambling()
- drm_hdmi_is_high_tmds_clock_ratio()
or something along those lines? At least with those the code would
read decently and I wouldn't have to wonder what this HDMI 1.4 TMDS
clock limit really is.

-- 
Ville Syrjälä
Intel