Re: [PATCH v3 5/5] drm/i915/dp: Disable the AUX DPCD probe quirk if it's not required

[Imre Deak <imre.deak@intel.com>]

On Fri, Jun 06, 2025 at 04:50:03PM +0300, Imre Deak wrote:
> On Fri, Jun 06, 2025 at 04:44:37PM +0300, Jani Nikula wrote:
> > On Thu, 05 Jun 2025, Imre Deak <imre.deak@intel.com> wrote:
> > > Reading DPCD registers has side-effects and some of these can cause a
> > > problem for instance during link training. Based on this it's better to
> > > avoid the probing quirk done before each DPCD register read, limiting
> > > this to the monitor which requires it. The only known problematic
> > > monitor is an external SST sink, so keep the quirk disabled always for
> > > eDP and MST sinks. Reenable the quirk after a hotplug event and after
> > > resuming from a power state without hotplug support, until the
> > > subsequent EDID based detection.
> > >
> > > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > Cc: Jani Nikula <jani.nikula@linux.intel.com>
> > > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/display/intel_dp.c      | 11 +++++++++--
> > >  drivers/gpu/drm/i915/display/intel_dp_aux.c  |  2 ++
> > >  drivers/gpu/drm/i915/display/intel_hotplug.c | 10 ++++++++++
> > >  3 files changed, 21 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> > > index 208a953b04a2f..d65a18fc1aeb9 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > > @@ -5747,6 +5747,11 @@ intel_dp_set_edid(struct intel_dp *intel_dp)
> > >  	/* Below we depend on display info having been updated */
> > >  	drm_edid_connector_update(&connector->base, drm_edid);
> > >  
> > > +	if (!intel_dp_is_edp(intel_dp))
> > 
> > Why does this depend on !edp?
> > 
> > Feels like unnecessary optimization based on your knowledge of that one
> > specific display.
> 
> The detection itself requires probing before each DPCD access. I want to
> avoid it whenever possible and since the quirk is relevant only the
> particular HP external display, we can avoid the probing on eDP
> completely.

Ok, the eDP check here can be removed, as the panel's EDID panel ID
should not match the quirk's panel ID. Will remove it.

> > > +		drm_dp_dpcd_set_probe_quirk(&intel_dp->aux,
> > > +					    drm_edid_has_quirk(&connector->base,
> > > +							       DRM_EDID_QUIRK_DP_DPCD_PROBE));
> > > +
> > >  	vrr_capable = intel_vrr_is_capable(connector);
> > >  	drm_dbg_kms(display->drm, "[CONNECTOR:%d:%s] VRR capable: %s\n",
> > >  		    connector->base.base.id, connector->base.name, str_yes_no(vrr_capable));
> > > @@ -5881,6 +5886,7 @@ intel_dp_detect(struct drm_connector *_connector,
> > >  	intel_dp_print_rates(intel_dp);
> > >  
> > >  	if (intel_dp->is_mst) {
> > > +		drm_dp_dpcd_set_probe_quirk(&intel_dp->aux, false);
> > 
> > Isn't this excessive? Haven't we already set the quirk state?
> 
> No, this is the MST root connector's detection and we don't read the EDID
> for it (we read it for MST non-root connectors, but those are not
> relavant in any case). So this should be set here explicitly, with the
> same justification as above for eDP (on MST the probing is never needed,
> so we can avoid it on such links completely).
> 
> > 
> > >  		/*
> > >  		 * If we are in MST mode then this connector
> > >  		 * won't appear connected or have anything
> > > @@ -6321,10 +6327,11 @@ intel_dp_hpd_pulse(struct intel_digital_port *dig_port, bool long_hpd)
> > >  	 * complete the DP tunnel BW request for the latter connector/encoder
> > >  	 * waiting for this encoder's DPRX read, perform a dummy read here.
> > >  	 */
> > > -	if (long_hpd)
> > > +	if (long_hpd) {
> > > +		drm_dp_dpcd_set_probe_quirk(&intel_dp->aux, true);
> > > +
> > >  		intel_dp_read_dprx_caps(intel_dp, dpcd);
> > >  
> > > -	if (long_hpd) {
> > >  		intel_dp->reset_link_params = true;
> > >  		intel_dp_invalidate_source_oui(intel_dp);
> > >  
> > > diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux.c b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> > > index bf8e8e0cc19c9..410252ba6fd16 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_dp_aux.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> > > @@ -835,6 +835,8 @@ void intel_dp_aux_init(struct intel_dp *intel_dp)
> > >  
> > >  	intel_dp->aux.transfer = intel_dp_aux_transfer;
> > >  	cpu_latency_qos_add_request(&intel_dp->pm_qos, PM_QOS_DEFAULT_VALUE);
> > > +
> > > +	drm_dp_dpcd_set_probe_quirk(&intel_dp->aux, !intel_dp_is_edp(intel_dp));
> > >  }
> > >  
> > >  static enum aux_ch default_aux_ch(struct intel_encoder *encoder)
> > > diff --git a/drivers/gpu/drm/i915/display/intel_hotplug.c b/drivers/gpu/drm/i915/display/intel_hotplug.c
> > > index 74fe398663d63..1093c6c3714c0 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_hotplug.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_hotplug.c
> > > @@ -33,6 +33,7 @@
> > >  #include "intel_display_core.h"
> > >  #include "intel_display_rpm.h"
> > >  #include "intel_display_types.h"
> > > +#include "intel_dp.h"
> > >  #include "intel_hdcp.h"
> > >  #include "intel_hotplug.h"
> > >  #include "intel_hotplug_irq.h"
> > > @@ -906,9 +907,18 @@ void intel_hpd_poll_enable(struct intel_display *display)
> > >   */
> > >  void intel_hpd_poll_disable(struct intel_display *display)
> > >  {
> > > +	struct intel_encoder *encoder;
> > > +
> > >  	if (!HAS_DISPLAY(display))
> > >  		return;
> > >  
> > > +	for_each_intel_dp(display->drm, encoder) {
> > > +		struct intel_dp *intel_dp = enc_to_intel_dp(encoder);
> > > +
> > > +		if (!intel_dp_is_edp(intel_dp))
> > > +			drm_dp_dpcd_set_probe_quirk(&intel_dp->aux, true);
> > > +	}
> > > +
> > >  	WRITE_ONCE(display->hotplug.poll_enabled, false);
> > >  
> > >  	spin_lock_irq(&display->irq.lock);
> > 
> > -- 
> > Jani Nikula, Intel