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

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

On Tue, Sep 08, 2020 at 02:04:56PM -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.
> > +		  */
> > +		if ((dpcd[DP_DOWNSTREAMPORT_PRESENT] &
> > DP_DETAILED_CAP_INFO_AVAILABLE) == 0)
> > +			return 300000;
> > +		return port_cap[1] * 2500;
> > +	case DP_DS_PORT_TYPE_DVI:
> > +		if ((dpcd[DP_DOWNSTREAMPORT_PRESENT] &
> > DP_DETAILED_CAP_INFO_AVAILABLE) == 0)
> > +			return 165000;
> > +		/* FIXME what to do about DVI dual link? */
> > +		return port_cap[1] * 2500;
> > +	default:
> > +		return 0;
> > +	}
> > +}
> > +EXPORT_SYMBOL(drm_dp_downstream_max_tmds_clock);
> > +
> > +/**
> > + * drm_dp_downstream_min_tmds_clock() - extract downstream facing port min
> > TMDS clock
> > + * @dpcd: DisplayPort configuration data
> > + * @port_cap: port capabilities
> > + * @edid: EDID
> > + *
> > + * Returns HDMI/DVI downstream facing port min TMDS clock in kHz on
> > success,
> > + * or 0 if max TMDS clock not defined
> 
> s/max/min/
> 
> Also, I would assume if callers are interested in min they're also interested
> in max and vice versa, would it maybe make sense to combine the min/max
> functions here?

Returning multiple things in C requires ugly stuff, so I try to avoid
it.

-- 
Ville Syrjälä
Intel
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