Re: [PATCH v2 1/3] drm/i915/ilk-glk: Fix link training on links with LTTPRs

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

On Thu, Mar 18, 2021 at 07:49:13PM +0200, Imre Deak wrote:
> On Thu, Mar 18, 2021 at 07:33:20PM +0200, Ville Syrjälä wrote:
> > On Wed, Mar 17, 2021 at 08:48:59PM +0200, Imre Deak wrote:
> > > The spec requires to use at least 3.2ms for the AUX timeout period if
> > > there are LT-tunable PHY Repeaters on the link (2.11.2). An upcoming
> > > spec update makes this more specific, by requiring a 3.2ms minimum
> > > timeout period for the LTTPR detection reading the 0xF0000-0xF0007
> > > range (3.6.5.1).
> > 
> > I'm pondering if we could reduce the timeout after having determined
> > wherther LTTPRs are present or not? But maybe that wouldn't really speed
> > up anything since we can't reduce the timeout until after detecting
> > *something*. And once there is something there we shouldn't really get
> > any more timeouts I guess. So probably a totally stupid idea.
> 
> Right, if something is connected it would take anyway as much time as it
> takes for the sink to reply whether or not we decreased the timeout.
> 
> However if nothing is connected, we have the excessive timeout Khaled
> already noticed (160 * 4ms = 6.4 sec on ICL+). I think to improve that
> we could scale the total number of retries by making it
> total_timeout/platform_specific_timeout (letting total_timeout=2sec for
> instance) or just changing the drm retry logic to be time based instead
> of the number of retries we use atm. 

Doh, reducing simply the HW timeouts would be enough to fix this.

> > Anyways, this seems about the only thing we can do given the limited
> > hw capabilities.
> > Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
> >
> > > Accordingly disable LTTPR detection until GLK, where the maximum timeout
> > > we can set is only 1.6ms.
> > > 
> > > Link training in the non-transparent mode is known to fail at least on
> > > some SKL systems with a WD19 dock on the link, which exposes an LTTPR
> > > (see the References below). While this could have different reasons
> > > besides the too short AUX timeout used, not detecting LTTPRs (and so not
> > > using the non-transparent LT mode) fixes link training on these systems.
> > > 
> > > While at it add a code comment about the platform specific maximum
> > > timeout values.
> > > 
> > > v2: Add a comment about the g4x maximum timeout as well. (Ville)
> > > 
> > > Reported-by: Takashi Iwai <tiwai@suse.de>
> > > Reported-and-tested-by: Santiago Zarate <santiago.zarate@suse.com>
> > > Reported-and-tested-by: Bodo Graumann <mail@bodograumann.de>
> > > References: https://gitlab.freedesktop.org/drm/intel/-/issues/3166
> > > Fixes: b30edfd8d0b4 ("drm/i915: Switch to LTTPR non-transparent mode link training")
> > > Cc: <stable@vger.kernel.org> # v5.11
> > > Cc: Takashi Iwai <tiwai@suse.de>
> > > Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
> > > Signed-off-by: Imre Deak <imre.deak@intel.com>
> > > ---
> > >  drivers/gpu/drm/i915/display/intel_dp_aux.c       |  7 +++++++
> > >  .../gpu/drm/i915/display/intel_dp_link_training.c | 15 ++++++++++++---
> > >  2 files changed, 19 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/drivers/gpu/drm/i915/display/intel_dp_aux.c b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> > > index eaebf123310a..10fe17b7280d 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_dp_aux.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_dp_aux.c
> > > @@ -133,6 +133,7 @@ static u32 g4x_get_aux_send_ctl(struct intel_dp *intel_dp,
> > >  	else
> > >  		precharge = 5;
> > >  
> > > +	/* Max timeout value on G4x-BDW: 1.6ms */
> > >  	if (IS_BROADWELL(dev_priv))
> > >  		timeout = DP_AUX_CH_CTL_TIME_OUT_600us;
> > >  	else
> > > @@ -159,6 +160,12 @@ static u32 skl_get_aux_send_ctl(struct intel_dp *intel_dp,
> > >  	enum phy phy = intel_port_to_phy(i915, dig_port->base.port);
> > >  	u32 ret;
> > >  
> > > +	/*
> > > +	 * Max timeout values:
> > > +	 * SKL-GLK: 1.6ms
> > > +	 * CNL: 3.2ms
> > > +	 * ICL+: 4ms
> > > +	 */
> > >  	ret = DP_AUX_CH_CTL_SEND_BUSY |
> > >  	      DP_AUX_CH_CTL_DONE |
> > >  	      DP_AUX_CH_CTL_INTERRUPT |
> > > 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 19ba7c7cbaab..c0e25c75c105 100644
> > > --- a/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > > +++ b/drivers/gpu/drm/i915/display/intel_dp_link_training.c
> > > @@ -82,6 +82,18 @@ static void intel_dp_read_lttpr_phy_caps(struct intel_dp *intel_dp,
> > >  
> > >  static bool intel_dp_read_lttpr_common_caps(struct intel_dp *intel_dp)
> > >  {
> > > +	struct drm_i915_private *i915 = dp_to_i915(intel_dp);
> > > +
> > > +	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 (INTEL_GEN(i915) < 10)
> > > +		return false;
> > > +
> > >  	if (drm_dp_read_lttpr_common_caps(&intel_dp->aux,
> > >  					  intel_dp->lttpr_common_caps) < 0) {
> > >  		memset(intel_dp->lttpr_common_caps, 0,
> > > @@ -127,9 +139,6 @@ int intel_dp_lttpr_init(struct intel_dp *intel_dp)
> > >  	bool ret;
> > >  	int i;
> > >  
> > > -	if (intel_dp_is_edp(intel_dp))
> > > -		return 0;
> > > -
> > >  	ret = intel_dp_read_lttpr_common_caps(intel_dp);
> > >  	if (!ret)
> > >  		return 0;
> > > -- 
> > > 2.25.1
> > 
> > -- 
> > Ville Syrjälä
> > Intel