Re: [PATCH v2 2/3] drm/i915: Disable LTTPR support when the DPCD rev < 1.4

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

On Thu, Mar 18, 2021 at 07:57:22PM +0200, Ville Syrjälä wrote:
> On Wed, Mar 17, 2021 at 09:01:49PM +0200, Imre Deak wrote:
> > By the specification the 0xF0000-0xF02FF range is only valid when the
> > DPCD revision is 1.4 or higher. Disable LTTPR support if this isn't so.
> > 
> > Trying to detect LTTPRs returned corrupted values for the above DPCD
> > range at least on a Skylake host with an LG 43UD79-B monitor with a DPCD
> > revision 1.2 connected.
> > 
> > v2: Add the actual version check.
> > 
> > Fixes: 7b2a4ab8b0ef ("drm/i915: Switch to LTTPR transparent mode link training")
> > Cc: <stable@vger.kernel.org> # v5.11
> > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > ---
> >  drivers/gpu/drm/i915/display/intel_dp.c       |  4 +-
> >  .../drm/i915/display/intel_dp_link_training.c | 48 ++++++++++++++-----
> >  .../drm/i915/display/intel_dp_link_training.h |  2 +-
> >  3 files changed, 39 insertions(+), 15 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp.c b/drivers/gpu/drm/i915/display/intel_dp.c
> > index b6b5776f5a66..873684da0cd4 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp.c
> > @@ -3711,9 +3711,7 @@ intel_dp_get_dpcd(struct intel_dp *intel_dp)
> >  {
> >  	int ret;
> >  
> > -	intel_dp_lttpr_init(intel_dp);
> > -
> > -	if (drm_dp_read_dpcd_caps(&intel_dp->aux, intel_dp->dpcd))
> > +	if (intel_dp_init_lttpr_and_dprx_caps(intel_dp) < 0)
> >  		return false;
> >  
> >  	/*
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.c b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > index c0e25c75c105..5a821d644e9c 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > @@ -35,6 +35,11 @@ intel_dp_dump_link_status(struct drm_device *drm,
> >  		    link_status[3], link_status[4], link_status[5]);
> >  }
> >  
> > +static void intel_dp_reset_lttpr_common_caps(struct intel_dp *intel_dp)
> > +{
> > +	memset(&intel_dp->lttpr_common_caps, 0, sizeof(intel_dp->lttpr_common_caps));
> > +}
> > +
> >  static void intel_dp_reset_lttpr_count(struct intel_dp *intel_dp)
> >  {
> >  	intel_dp->lttpr_common_caps[DP_PHY_REPEATER_CNT -
> > @@ -96,8 +101,7 @@ static bool intel_dp_read_lttpr_common_caps(struct intel_dp *intel_dp)
> >  
> >  	if (drm_dp_read_lttpr_common_caps(&intel_dp->aux,
> >  					  intel_dp->lttpr_common_caps) < 0) {
> > -		memset(intel_dp->lttpr_common_caps, 0,
> > -		       sizeof(intel_dp->lttpr_common_caps));
> > +		intel_dp_reset_lttpr_common_caps(intel_dp);
> >  		return false;
> >  	}
> >  
> > @@ -119,30 +123,49 @@ intel_dp_set_lttpr_transparent_mode(struct intel_dp *intel_dp, bool enable)
> >  }
> >  
> >  /**
> > - * intel_dp_lttpr_init - detect LTTPRs and init the LTTPR link training mode
> > + * intel_dp_init_lttpr_and_dprx_caps - detect LTTPR and DPRX caps, init the LTTPR link training mode
> >   * @intel_dp: Intel DP struct
> >   *
> > - * Read the LTTPR common capabilities, switch to non-transparent link training
> > - * mode if any is detected and read the PHY capabilities for all detected
> > - * LTTPRs. In case of an LTTPR detection error or if the number of
> > + * Read the LTTPR common and DPRX capabilities and switch to non-transparent
> > + * link training mode if any is detected and read the PHY capabilities for all
> > + * detected LTTPRs. In case of an LTTPR detection error or if the number of
> >   * LTTPRs is more than is supported (8), fall back to the no-LTTPR,
> >   * transparent mode link training mode.
> >   *
> >   * Returns:
> > - *   >0  if LTTPRs were detected and the non-transparent LT mode was set
> > + *   >0  if LTTPRs were detected and the non-transparent LT mode was set. The
> > + *       DPRX capabilities are read out.
> >   *    0  if no LTTPRs or more than 8 LTTPRs were detected or in case of a
> > - *       detection failure and the transparent LT mode was set
> > + *       detection failure and the transparent LT mode was set. The DPRX
> > + *       capabilities are read out.
> > + *   <0  Reading out the DPRX capabilities failed.
> >   */
> > -int intel_dp_lttpr_init(struct intel_dp *intel_dp)
> > +int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
> >  {
> >  	int lttpr_count;
> >  	bool ret;
> >  	int i;
> >  
> >  	ret = intel_dp_read_lttpr_common_caps(intel_dp);
> > +
> > +	/* The DPTX shall read the DRPX caps after LTTPR detection. */
> > +	if (drm_dp_read_dpcd_caps(&intel_dp->aux, intel_dp->dpcd)) {
> > +		intel_dp_reset_lttpr_common_caps(intel_dp);
> > +		return -EIO;
> > +	}
> > +
> >  	if (!ret)
> >  		return 0;
> >  
> > +	/*
> > +	 * The 0xF0000-0xF02FF range is only valid if the DPCD revision is
> > +	 * at least 1.4.
> > +	 */
> > +	if (intel_dp->dpcd[DP_DPCD_REV] < 0x14) {
> > +		intel_dp_reset_lttpr_common_caps(intel_dp);
> > +		return 0;
> > +	}
> 
> Slight chicken vs. egg I guess. Seems ok

Yes, reading 0xF0000-0xF0007 has a side effect and I suppose the LTTPRs
could change something in the returned DPRX caps, depending on whether
the read happened or not.

> Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> 
> > +
> >  	lttpr_count = drm_dp_lttpr_count(intel_dp->lttpr_common_caps);
> >  	/*
> >  	 * Prevent setting LTTPR transparent mode explicitly if no LTTPRs are
> > @@ -182,7 +205,7 @@ int intel_dp_lttpr_init(struct intel_dp *intel_dp)
> >  
> >  	return lttpr_count;
> >  }
> > -EXPORT_SYMBOL(intel_dp_lttpr_init);
> > +EXPORT_SYMBOL(intel_dp_init_lttpr_and_dprx_caps);
> >  
> >  static u8 dp_voltage_max(u8 preemph)
> >  {
> > @@ -817,7 +840,10 @@ void intel_dp_start_link_train(struct intel_dp *intel_dp,
> >  	 * TODO: Reiniting LTTPRs here won't be needed once proper connector
> >  	 * HW state readout is added.
> >  	 */
> > -	int lttpr_count = intel_dp_lttpr_init(intel_dp);
> > +	int lttpr_count = intel_dp_init_lttpr_and_dprx_caps(intel_dp);
> > +
> > +	if (lttpr_count < 0)
> > +		return;
> >  
> >  	if (!intel_dp_link_train_all_phys(intel_dp, crtc_state, lttpr_count))
> >  		intel_dp_schedule_fallback_link_training(intel_dp, crtc_state);
> > diff --git a/drivers/gpu/drm/i915/display/intel_dp_link_training.h b/drivers/gpu/drm/i915/display/intel_dp_link_training.h
> > index 6a1f76bd8c75..9cb7c28027f0 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.h
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.h
> > @@ -11,7 +11,7 @@
> >  struct intel_crtc_state;
> >  struct intel_dp;
> >  
> > -int intel_dp_lttpr_init(struct intel_dp *intel_dp);
> > +int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp);
> >  
> >  void intel_dp_get_adjust_train(struct intel_dp *intel_dp,
> >  			       const struct intel_crtc_state *crtc_state,
> > -- 
> > 2.25.1
> 
> -- 
> Ville Syrjälä
> Intel