[Intel-gfx] [PATCH v3 2/2] drm/i915/dp: Add workaround for spurious AUX timeouts/hotplugs on LTTPR links

[Imre Deak]

On Thu, Apr 14, 2022 at 01:49:54PM +0300, Jani Nikula wrote:
> On Sat, 09 Apr 2022, Imre Deak <imre.deak at intel.com> wrote:
> > Some ADLP DP link configuration at least with multiple LTTPRs expects
> > the first DPCD access during the LTTPR/DPCD detection after hotplug to
> > be a read from the LTTPR range starting with
> > DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV. The side effect of
> > this read is to put each LTTPR into the LTTPR transparent or LTTPR
> > non-transparent mode.
> >
> > The lack of the above read may leave some of the LTTPRs in non-LTTPR
> > mode, while other LTTPRs in LTTPR transparent or LTTPR non-transparent
> > mode (for instance LTTPRs after system suspend/resume that kept their
> > mode from before suspend). Due to the different AUX timeouts the
> > different modes imply, the DPCD access from a non-LTTPR range will
> > timeout and lead to an LTTPR generated hotplug towards the source (which
> > the LTTPR firmware uses to account for buggy TypeC adapters with a long
> > wake-up delay).
> >
> > To avoid the above AUX timeout and subsequent hotplug interrupt, make
> > sure that the first DPCD access during detection is a read from an
> > LTTPR register.
> >
> > VLK: SYSCROS-72939
> >
> > v2: Keep DPCD read-out working on non-LTTPR platforms.
> >
> > Signed-off-by: Imre Deak <imre.deak at intel.com>
> 
> I kinda dislike how complicated this looks for a diff so small. *shrug*.
> Seems to do what it says on the box,

I can have a better summary of what the patch does and why at the
beginning of the commit message. Imo the details for the related LTTPR
behavior are still useful for later reference; it's not too intuitive
with the read side-effects for instance, neither it is well documented
by the DP standard.

> Reviewed-by: Jani Nikula <jani.nikula at intel.com>

Thanks.

I pushed the dependent
d8bb92e70a43 ("drm/dp: Factor out a function to probe a DPCD address")
patch to drm-misc-next. Could you help back merging it to
drm-intel-next?

> > ---
> >  .../drm/i915/display/intel_dp_link_training.c | 33 ++++++++++---------
> >  1 file changed, 17 insertions(+), 16 deletions(-)
> >
> > 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 26f9e2b748e40..9feaf1a589f38 100644
> > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > @@ -82,19 +82,8 @@ static bool intel_dp_read_lttpr_common_caps(struct intel_dp *intel_dp,
> >  					    const u8 dpcd[DP_RECEIVER_CAP_SIZE])
> >  {
> >  	struct intel_encoder *encoder = &dp_to_dig_port(intel_dp)->base;
> > -	struct drm_i915_private *i915 = to_i915(encoder->base.dev);
> >  	int ret;
> >  
> > -	if (intel_dp_is_edp(intel_dp))
> > -		return false;
> > -
> > -	/*
> > -	 * Detecting LTTPRs must be avoided on platforms with an AUX timeout
> > -	 * period < 3.2ms. (see DP Standard v2.0, 2.11.2, 3.6.6.1).
> > -	 */
> > -	if (DISPLAY_VER(i915) < 10 || IS_GEMINILAKE(i915))
> > -		return false;
> > -
> >  	ret = drm_dp_read_lttpr_common_caps(&intel_dp->aux, dpcd,
> >  					    intel_dp->lttpr_common_caps);
> >  	if (ret < 0)
> > @@ -197,13 +186,25 @@ static int intel_dp_init_lttpr(struct intel_dp *intel_dp, const u8 dpcd[DP_RECEI
> >   */
> >  int intel_dp_init_lttpr_and_dprx_caps(struct intel_dp *intel_dp)
> >  {
> > -	u8 dpcd[DP_RECEIVER_CAP_SIZE];
> > -	int lttpr_count;
> > +	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> > +	int lttpr_count = 0;
> >  
> > -	if (drm_dp_read_dpcd_caps(&intel_dp->aux, dpcd))
> > -		return -EIO;
> > +	/*
> > +	 * Detecting LTTPRs must be avoided on platforms with an AUX timeout
> > +	 * period < 3.2ms. (see DP Standard v2.0, 2.11.2, 3.6.6.1).
> > +	 */
> > +	if (!intel_dp_is_edp(intel_dp) &&
> > +	    (DISPLAY_VER(i915) >= 10 && !IS_GEMINILAKE(i915))) {
> > +		u8 dpcd[DP_RECEIVER_CAP_SIZE];
> >  
> > -	lttpr_count = intel_dp_init_lttpr(intel_dp, dpcd);
> > +		if (drm_dp_dpcd_probe(&intel_dp->aux, DP_LT_TUNABLE_PHY_REPEATER_FIELD_DATA_STRUCTURE_REV))
> > +			return -EIO;
> > +
> > +		if (drm_dp_read_dpcd_caps(&intel_dp->aux, dpcd))
> > +			return -EIO;
> > +
> > +		lttpr_count = intel_dp_init_lttpr(intel_dp, dpcd);
> > +	}
> >  
> >  	/*
> >  	 * The DPTX shall read the DPRX caps after LTTPR detection, so re-read
> 
> -- 
> Jani Nikula, Intel Open Source Graphics Center