Re: [PATCH v2 10/18] drm/dp: Add drm_dp_downstream_{min,max}_tmds_clock()

[Lyude Paul <lyude@redhat.com>]

On Thu, 2020-09-10 at 16:55 +0300, Ville Syrjälä wrote:
> On Tue, Sep 08, 2020 at 02:08:17PM -0400, Lyude Paul wrote:
> > On Fri, 2020-09-04 at 14:53 +0300, Ville Syrjala wrote:
> > > From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > 
> > > Add helpers to get the TMDS clock limits for HDMI/DVI downstream
> > > facing ports.
> > > 
> > > Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > ---
> > >  drivers/gpu/drm/drm_dp_helper.c | 116 ++++++++++++++++++++++++++++++++
> > >  include/drm/drm_dp_helper.h     |   6 ++
> > >  2 files changed, 122 insertions(+)
> > > 
> > > diff --git a/drivers/gpu/drm/drm_dp_helper.c
> > > b/drivers/gpu/drm/drm_dp_helper.c
> > > index 822a30e609ef..f567428f2aef 100644
> > > --- a/drivers/gpu/drm/drm_dp_helper.c
> > > +++ b/drivers/gpu/drm/drm_dp_helper.c
> > > @@ -643,6 +643,114 @@ int drm_dp_downstream_max_dotclock(const u8
> > > dpcd[DP_RECEIVER_CAP_SIZE],
> > >  }
> > >  EXPORT_SYMBOL(drm_dp_downstream_max_dotclock);
> > >  
> > > +/**
> > > + * drm_dp_downstream_max_tmds_clock() - extract downstream facing port
> > > max
> > > TMDS clock
> > > + * @dpcd: DisplayPort configuration data
> > > + * @port_cap: port capabilities
> > > + * @edid: EDID
> > > + *
> > > + * Returns HDMI/DVI downstream facing port max TMDS clock in kHz on
> > > success,
> > > + * or 0 if max TMDS clock not defined
> > > + */
> > > +int drm_dp_downstream_max_tmds_clock(const u8 dpcd[DP_RECEIVER_CAP_SIZE],
> > > +				     const u8 port_cap[4],
> > > +				     const struct edid *edid)
> > > +{
> > > +	if (!drm_dp_is_branch(dpcd))
> > > +		return 0;
> > > +
> > > +	if (dpcd[DP_DPCD_REV] < 0x11) {
> > > +		switch (dpcd[DP_DOWNSTREAMPORT_PRESENT] &
> > > DP_DWN_STRM_PORT_TYPE_MASK) {
> > > +		case DP_DWN_STRM_PORT_TYPE_TMDS:
> > > +			return 165000;
> > > +		default:
> > > +			return 0;
> > > +		}
> > > +	}
> > > +
> > > +	switch (port_cap[0] & DP_DS_PORT_TYPE_MASK) {
> > > +	case DP_DS_PORT_TYPE_DP_DUALMODE:
> > > +		if (is_edid_digital_input_dp(edid))
> > > +			return 0;
> > > +		/*
> > > +		 * It's left up to the driver to check the
> > > +		 * DP dual mode adapter's max TMDS clock.
> > > +		 *
> > > +		 * Unfortunatley it looks like branch devices
> > > +		 * may not fordward that the DP dual mode i2c
> > > +		 * access so we just usually get i2c nak :(
> > > +		 */
> > > +		fallthrough;
> > > +	case DP_DS_PORT_TYPE_HDMI:
> > > +		 /*
> > > +		  * We should perhaps assume 165 MHz when detailed cap
> > > +		  * info is not available. But looks like many typical
> > > +		  * branch devices fall into that category and so we'd
> > > +		  * probably end up with users complaining that they can't
> > > +		  * get high resolution modes with their favorite dongle.
> > > +		  *
> > > +		  * So let's limit to 300 MHz instead since DPCD 1.4
> > > +		  * HDMI 2.0 DFPs are required to have the detailed cap
> > > +		  * info. So it's more likely we're dealing with a HDMI 1.4
> > > +		  * compatible* device here.
> > 
> > Forgot to mention - not directly related to this series, there's some hidden
> > i2c bits that I think can also be probed for this sort of information on
> > passive adapters, I know amdgpu actually supports this. I wonder how many of
> > them also apply to older active adapters...
> 
> Something other than the normal DP dual mode stuff?
Actually that -may- have been what I was thinking of but I'm not sure, I'd
probably need to look through DAL to find out
> 
-- 
Sincerely,
      Lyude Paul (she/her)
      Software Engineer at Red Hat

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