Re: [PATCH 1/2] Revert "drm/i915/dp: Reject HBR3 when sink doesn't support TPS4"

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Wed, Jun 25, 2025 at 11:18:03AM +0300, Jani Nikula wrote:
> On Tue, 24 Jun 2025, Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> > On Tue, Jun 24, 2025 at 10:10:53AM +0530, Nautiyal, Ankit K wrote:
> >> 
> >> On 6/23/2025 8:12 PM, Ville Syrjälä wrote:
> >> > On Fri, Jun 20, 2025 at 06:14:16PM +0530, Ankit Nautiyal wrote:
> >> >> This reverts commit 584cf613c24a4250d9be4819efc841aa2624d5b6.
> >> >> Commit 584cf613c24a ("drm/i915/dp: Reject HBR3 when sink doesn't support
> >> >> TPS4") introduced a blanket rejection of HBR3 link rate when the sink does
> >> >> not support TPS4. While this was intended to address instability observed
> >> >> on certain eDP panels [1], the TPS4 requirement is only mandated for DPRX
> >> >> and not for eDPRX.
> >> > I see no exception given for eDP regarding this rule. The only exception
> >> > allowed is that eDP can say DPCD_REV=1.4 + TPS4_SUPPORTED=0. So I still
> >> > claim that these eDP sinks are violating the spec.
> >> 
> >> Hmm.. Yes the spec allows eDP sinks to report DPCD_REV=1.4 and 
> >> TPS4_SUPPORTED, so perhaps eDPs claiming HBR3 with DPCD rev other than 
> >> rev 1.4 and not supporting TPS4 are indeed violating the spec.
> >> 
> >> Would it make sense to add a condition that checks for DPCD_REV=1.4.
> >> 
> >> Specifically:
> >> 
> >> if DPCD_REV=1.4 and TPS4_SUPPORTED = 0, then do not prune the HBR3 rate?
> >> 
> >> Or otherway if DPCD_REV!=1.4 and TPS4_SUPPORTER = 0, prune the HBR3 rate
> >> 
> >> This way the patch need not be reverted, but modified to address 
> >> instability issues for eDP panels that are not aligned with the spec.
> >> 
> >> That said, the gitlab issue#5969 [1] will still need another solution 
> >> since it seems to have DPCD rev 14 as per logs:
> >> 
> >> DPCD: 14 1e 44 41 00 00 01 80 02 00 02 00 00 0b 80
> >
> > I think we do need the quirk. But the commit message should IMO be
> > adjusted so that it doesn't claim that these panels are 100% legal.
> 
> Right. But are you okay with reverting 584cf613c24a ("drm/i915/dp:
> Reject HBR3 when sink doesn't support TPS4") i.e. allowing HBR3
> *without* TPS4 by default, and quirking the panel that can't handle
> HBR3?

Yeah, I think the revert+quirk for the one known bad system is the way
to go.

> 
> No matter what the spec says, this seems to be common. More regressing
> systems are cropping up, e.g. [1].
> 
> 
> BR,
> Jani.
> 
> 
> [1] https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/14517
> 
> 
> 
> 
> -- 
> Jani Nikula, Intel

-- 
Ville Syrjälä
Intel