Re: [PATCH] drm/i915: Improve DP downstream HPD handling

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

On Tue, Jul 07, 2015 at 02:37:46PM +0300, Ville Syrjälä wrote:
> On Tue, Jul 07, 2015 at 04:45:11PM +0530, Sivakumar Thulasimani wrote:
> > 
> > 
> > On 7/7/2015 4:40 PM, Ville Syrjälä wrote:
> > > On Tue, Jul 07, 2015 at 03:26:36PM +0530, Sivakumar Thulasimani wrote:
> > >>
> > >> On 7/6/2015 5:42 PM, ville.syrjala@linux.intel.com wrote:
> > >>> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > >>>
> > >>> DP dongles may signal downstream HPD via short HPD pulses. If we know
> > >>> the device has a HPD capable downstream port, make sure we kick off the
> > >>> full hotplug processing even for short HPDs.
> > >>>
> > >>> Additonally setting the sink to DPMS off kills the downstream HPD (at
> > >>> least on my DP->VGA dongle), so skip the DPMS off for such dongles
> > >>> when we turn off the port.
> > >>>
> > >>> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > >>> ---
> > >>>    drivers/gpu/drm/i915/intel_dp.c | 18 +++++++++++++++++-
> > >>>    1 file changed, 17 insertions(+), 1 deletion(-)
> > >>>
> > >>> diff --git a/drivers/gpu/drm/i915/intel_dp.c b/drivers/gpu/drm/i915/intel_dp.c
> > >>> index e88cec2..f424833 100644
> > >>> --- a/drivers/gpu/drm/i915/intel_dp.c
> > >>> +++ b/drivers/gpu/drm/i915/intel_dp.c
> > >>> @@ -2324,6 +2324,13 @@ static void intel_dp_get_config(struct intel_encoder *encoder,
> > >>>    	}
> > >>>    }
> > >>>    
> > >>> +static bool intel_dp_has_downstream_hpd(struct intel_dp *intel_dp)
> > >>> +{
> > >>> +	return intel_dp->dpcd[DP_DOWNSTREAMPORT_PRESENT] & DP_DWN_STRM_PORT_PRESENT &&
> > >>> +		intel_dp->dpcd[DP_DPCD_REV] >= 0x11 &&
> > >>> +		intel_dp->downstream_ports[0] & DP_DS_PORT_HPD;
> > >>> +}
> > >>> +
> > >>>    static void intel_disable_dp(struct intel_encoder *encoder)
> > >>>    {
> > >>>    	struct intel_dp *intel_dp = enc_to_intel_dp(&encoder->base);
> > >>> @@ -2340,7 +2347,9 @@ static void intel_disable_dp(struct intel_encoder *encoder)
> > >>>    	 * ensure that we have vdd while we switch off the panel. */
> > >>>    	intel_edp_panel_vdd_on(intel_dp);
> > >>>    	intel_edp_backlight_off(intel_dp);
> > >>> -	intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_OFF);
> > >>> +	/* Skip power down to keep downstream HPD working */
> > >>> +	if (!intel_dp_has_downstream_hpd(intel_dp))
> > >>> +		intel_dp_sink_dpms(intel_dp, DRM_MODE_DPMS_OFF);
> > >>>    	intel_edp_panel_off(intel_dp);
> > >>>    
> > >>>    	/* disable the port before the pipe on g4x */
> > >>> @@ -4944,6 +4953,13 @@ intel_dp_hpd_pulse(struct intel_digital_port *intel_dig_port, bool long_hpd)
> > >>>    			drm_modeset_lock(&dev->mode_config.connection_mutex, NULL);
> > >>>    			intel_dp_check_link_status(intel_dp);
> > >>>    			drm_modeset_unlock(&dev->mode_config.connection_mutex);
> > >>> +
> > >>> +			/*
> > >>> +			 * Downstream HPD will generate a short HPD,
> > >>> +			 * so we want full hotplug processing here.
> > >>> +			 */
> > >>> +			if (intel_dp_has_downstream_hpd(intel_dp))
> > >>> +				goto put_power;
> > >>>    		}
> > >>>    	}
> > >>>    
> > >> I am looking into compliance changes for DP and this seems a relevant
> > >> change for compliance as well. but as per Link CTS 1.2 section 4.2.2.8,
> > >> we are supposed to read the sink_count and do full detection if
> > >> sink_count is >1.  So instead of checking for DP_DS_PORT_HPD can we just
> > >> check SINK_COUNT and do full detect ?
> > > ->detect() will be called from the hotplug work and that will
> > > check SINK_COUNT.
> > >
> > No, the Compliance Sink tool, will not set the DP_DS_PORT_HPD resulting 
> > in detect not getting executed for
> > the short pulse generated. The spec requires the sink to set only the 
> > sink count so it is not a must for
> > the sink to update the DP_DOWNSTREAM_PORT_0. so only a check for 
> > SINK_COUNT will pass the
> > compliance test.
> 
> That seems stupid. If the downstream port isn't HPD capable then we have
> no reason to check SINK_COUNT after a short HPD as the short HPD
> coudln't have been caused by a downstram HPD. Obviuously we still
> check SINK_COUNT after a long HPD to figure out if anything is connected
> when the branch device itself gets connected to the source.

Actually that's not correct. We don't check SINK_COUNT unless the downstream
port is HPD capable.

The spec says:
"If the DFP does not provide for means for plug/unplug detection, the
adaptor must set the SINK_COUNT field bits, as if those Sink devices were
all permanently plugged."

So according to the there can't be any changes in SINK_COUNT if the
downstream port is not HPD capable.

-- 
Ville Syrjälä
Intel OTC
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