[PATCH] drm/dp: Change AUX DPCD probe address from LANE0_1_STATUS to TRAINING_PATTERN_SET

[Imre Deak]

Hi,

On Thu, Jul 10, 2025 at 02:28:28PM +0300, Imre Deak wrote:
> Hi Thomas, Maxime, Maarten,
> 
> the patch this change fixes (commit a40c5d727b81) was merged via
> drm-intel and is also part of v6.16-rc4 (there cherry-picked in commit
> a3ef3c2da675).
> 
> Are you ok with merging this fix via drm-intel as well, so that it could
> be still merged to v6.16 before that's released?

any objections to merging this patch to drm-intel? If not, could someone
ack it?

Thanks,
Imre

> 
> Thanks,
> Imre
> 
> On Wed, Jul 09, 2025 at 12:23:31AM +0300, Imre Deak wrote:
> > Commit a40c5d727b81 ("drm/dp: Change AUX DPCD probe address from
> > DPCD_REV to LANE0_1_STATUS") stopped using the DPCD_REV register for
> > DPCD probing, since this results in link training failures at least when
> > using an Intel Barlow Ridge TBT hub at UHBR link rates (the
> > DP_INTRA_HOP_AUX_REPLY_INDICATION never getting cleared after the failed
> > link training). Since accessing DPCD_REV during link training is
> > prohibited by the DP Standard, LANE0_1_STATUS (0x202) was used instead,
> > as it falls within the Standard's valid register address range
> > (0x102-0x106, 0x202-0x207, 0x200c-0x200f, 0x2216) and it fixed the link
> > training on the above TBT hub.
> > 
> > However, reading the LANE0_1_STATUS register also has a side-effect at
> > least on a Novatek eDP panel, as reported on the Closes: link below,
> > resulting in screen flickering on that panel. One clear side-effect when
> > doing the 1-byte probe reads from LANE0_1_STATUS during link training
> > before reading out the full 6 byte link status starting at the same
> > address is that the panel will report the link training as completed
> > with voltage swing 0. This is different from the normal, flicker-free
> > scenario when no DPCD probing is done, the panel reporting the link
> > training complete with voltage swing 2.
> > 
> > Using the TRAINING_PATTERN_SET register for DPCD probing doesn't have
> > the above side-effect, the panel will link train with voltage swing 2 as
> > expected and it will stay flicker-free. This register is also in the
> > above valid register range and is unlikely to have a side-effect as that
> > of LANE0_1_STATUS: Reading LANE0_1_STATUS is part of the link training
> > CR/EQ sequences and so it may cause a state change in the sink - even if
> > inadvertently as I suspect in the case of the above Novatek panel. As
> > opposed to this, reading TRAINING_PATTERN_SET is not part of the link
> > training sequence (it must be only written once at the beginning of the
> > CR/EQ sequences), so it's unlikely to cause any state change in the
> > sink.
> > 
> > As a side-note, this Novatek panel also lacks support for TPS3, while
> > claiming support for HBR2, which violates the DP Standard (the Standard
> > mandating TPS3 for HBR2).
> > 
> > Besides the Novatek panel (PSR 1), which this change fixes, I also
> > verified the change on a Samsung (PSR 1) and an Analogix (PSR 2) eDP
> > panel as well as on the Intel Barlow Ridge TBT hub.
> > 
> > Note that in the drm-tip tree (targeting the v6.17 kernel version) the
> > i915 and xe drivers keep DPCD probing enabled only for the panel known
> > to require this (HP ZR24w), hence those drivers in drm-tip are not
> > affected by the above problem.
> > 
> > Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> > Cc: Jani Nikula <jani.nikula at linux.intel.com>
> > Fixes: a40c5d727b81 ("drm/dp: Change AUX DPCD probe address from DPCD_REV to LANE0_1_STATUS")
> > Reported-and-tested-by: Paul Menzel <pmenzel at molgen.mpg.de>
> > Closes: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/14558
> > Signed-off-by: Imre Deak <imre.deak at intel.com>
> > ---
> >  drivers/gpu/drm/display/drm_dp_helper.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/gpu/drm/display/drm_dp_helper.c b/drivers/gpu/drm/display/drm_dp_helper.c
> > index 1c3920297906b..1ecc3df7e3167 100644
> > --- a/drivers/gpu/drm/display/drm_dp_helper.c
> > +++ b/drivers/gpu/drm/display/drm_dp_helper.c
> > @@ -742,7 +742,7 @@ ssize_t drm_dp_dpcd_read(struct drm_dp_aux *aux, unsigned int offset,
> >  	int ret;
> >  
> >  	if (dpcd_access_needs_probe(aux)) {
> > -		ret = drm_dp_dpcd_probe(aux, DP_LANE0_1_STATUS);
> > +		ret = drm_dp_dpcd_probe(aux, DP_TRAINING_PATTERN_SET);
> >  		if (ret < 0)
> >  			return ret;
> >  	}
> > -- 
> > 2.44.2
> >