Re: [PATCH v2] drm/i915/cdclk: Don't force cd2x pipe select on Xe3 in bxt_sanitize_cdclk()

[Ville Syrjälä]

On Wed, Aug 19, 2026 at 12:33:56PM +0300, Jani Nikula wrote:
> On Tue, 18 Aug 2026, Jani Nikula <jani.nikula@xxxxxxxxxxxxxxx> wrote:
> > On Sun, 16 Aug 2026, Chang Jiong(常炯) <xspeed1989@xxxxxxxxx> wrote:
> >> Since commit 2ee8dbd880b1 ("drm/i915/cdclk: Fix up CDCLK_FREQ_DECIMAL
> >> without a full PLL re-enable") bxt_sanitize_cdclk() normalizes the
> >> CD2X pipe select field of the CDCLK_CTL readback to PIPE_NONE before
> >> comparing against the expected value.
> >>
> >> However bxt_cdclk_ctl() does not include the CD2X pipe select field
> >> at all on DISPLAY_VER() >= 30, so on Xe3 (PTL) the normalized
> >> readback (pipe field forced to the PIPE_NONE encoding, 0b111) can
> >> never match the expected value (pipe field 0). The comparison now
> >> fails on every boot and we always take the full CDCLK PLL
> >> disable+re-enable path with the BIOS framebuffer actively scanning
> >> out - exactly what this code was trying to avoid. On a Panther Lake
> >> laptop (Arc B390, 8086:b080, eDP 3120x2080@120) this causes
> >> "CPU pipe A FIFO underrun" during probe and persistent full-screen
> >> scanout corruption for the first KMS client (the SDDM greeter);
> >> the corruption lasts until the next full modeset.
> >>
> >> Only normalize the CD2X pipe select field on platforms where
> >> bxt_cdclk_ctl() actually programs it, mirroring its DISPLAY_VER()
> >> check.
> >>
> >> Fixes: 2ee8dbd880b1 ("drm/i915/cdclk: Fix up CDCLK_FREQ_DECIMAL without a full PLL re-enable")
> >> Cc: stable@xxxxxxxxxxxxxxx
> >> Signed-off-by: Chang Jiong(常炯) <xspeed1989@xxxxxxxxx>
> >
> > Reviewed-by: Jani Nikula <jani.nikula@xxxxxxxxx>
> > Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/8550
> > Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/8569
> > Closes: https://gitlab.freedesktop.org/drm/xe/kernel/-/work_items/8901
> >
> > The first one has a patch from Ville that is identical to this one.
> >
> > I'll try to kick CI to get this tested before merging.
> 
> Okay, the patch is against some stable kernel, not against the current
> development tree. Please rebase on top of the drm-tip branch of the
> drm-tip repository.

I pushed my original fix and closed the other bugs as duplicates.

-- 
Ville Syrjälä
Intel