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

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

On Fri, Mar 19, 2021 at 11:07:21PM +0200, Imre Deak wrote:
> On Fri, Mar 19, 2021 at 04:44:26PM -0400, Lyude Paul wrote:
> > > > > [...]
> > > > > I think it would work if we can make the retries configurable and set it
> > > > > to
> > > > >         retries = total_timeout / platform_specific_timeout_per_retry
> > > > > 
> > > > > where total_timeout would be something reasonable like 1 sec.
> > > > 
> > > > I actually think I'm more open to the idea of configurable retries after
> > > > learning that apparently this is a thing that the i2c subsystem does - so
> > > > there's more precedence for it in the rest of the kernel than I originally
> > > > thought.
> > > > 
> > > > I'm still curious if we need these extra retries in here though - there seems
> > > > to
> > > > be one set of retries that is actually platform specific, and then just a
> > > > random
> > > > set of 5 retries that don't seem to have anything to do with platform specific
> > > > behavior - so I think it'd still be worth giving a shot at getting rid of that
> > > 
> > > The platform specific part of the timeout is the one desctibed in the
> > > maximum timeout values comments.
> > 
> > You mean the
> > 
> > 		/* Must try at least 3 times according to DP spec */
> > 		for (try = 0; try < 5; try++) {
> > 
> > bit? I thought that wasn't related to platform specific retries at all, since
> > the code in that loop seems to only reference parts of the DP spec, and that the
> > 
> > 	while ((aux_clock_divider = intel_dp->get_aux_clock_divider(intel_dp, clock++))) {
> > 
> > Loop was the portion that was platform specific, since it prompts the driver to
> > retry the transaction with different aux clock divider rates depending on the
> > platform in use. Feel free to correct me if I'm wrong though.
> 
> Nope. I meant every HW transaction will have a platform specific
> timeout. For instance it's 1.6ms on SKL, but 4ms on ICL. So now since
> the overall retry count is 32 * 5 = 160, on SKL we'll retry for ~2.6
> seconds, on ICL we'll retry for ~6.4 seconds (disregarding now the extra
> 400usec delay inserted by drm_dp_dpcd_access(), which adds a fixed
> ~1.3ms delay).

Err, looks like I missed some coffee. Max total timeouts atm, which we
would need to make the same on all platforms:

	g4x-glk: 5 * 32 * 1.6ms + 32 * 400us = 268.8ms
	cnl    : 5 * 32 * 3.2ms + 32 * 400us = 524.8ms
	icl+   : 5 * 32 * 4ms   + 32 * 400us = 652.8ms

> This is what I think should be normalized, so that we have the same
> amount of overall maximum timeout period on all platforms.
> 
> > Also - with the timeouts we're seeing, does the LTTPR return NAKs at all? That's
> > still another thing I had suggested alternate workarounds for so that we could
> > terminate transactions immediately on NAKs, so I wonder if that could save time
> > here as well.
> 
> There's not much LTTPR specific in that wrt. what sinks would do
> normally (no NAKs for read, only for writes) except LTTPRs may rewrite
> NAKs to ACKs to account for buggy monitors returning NAKs when reading
> the 0xf0000 -> range. But I'd suggest not dealing with this aspect now,
> just sanitize the above retry thing, as you suggested, remove the i915
> retry loop and make the drm retry loop configurable.
> 
> (In any case I also had the idea to stop transactions early when HPD
>  gets deasserted, but not sure if that's completely robust.)
> 
> > > > > > Thanks
> > > > > > Khaled
> > > > > > 
> > > > > > > 
> > > > > > > > > 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
> > > > > 
> > > > 
> > > > -- 
> > > > Sincerely,
> > > >    Lyude Paul (she/her)
> > > >    Software Engineer at Red Hat
> > > >    
> > > > Note: I deal with a lot of emails and have a lot of bugs on my plate. If
> > > > you've
> > > > asked me a question, are waiting for a review/merge on a patch, etc. and I
> > > > haven't responded in a while, please feel free to send me another email to
> > > > check
> > > > on my status. I don't bite!
> > > > 
> > > 
> > 
> > -- 
> > Sincerely,
> >    Lyude Paul (she/her)
> >    Software Engineer at Red Hat
> >    
> > Note: I deal with a lot of emails and have a lot of bugs on my plate. If you've
> > asked me a question, are waiting for a review/merge on a patch, etc. and I
> > haven't responded in a while, please feel free to send me another email to check
> > on my status. I don't bite!
> >