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

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

[Chang Jiong(常炯)]

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@vger.kernel.org
Signed-off-by: Chang Jiong(常炯) <304750714@qq.com>
---
 drivers/gpu/drm/i915/display/intel_cdclk.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
--- a/drivers/gpu/drm/i915/display/intel_cdclk.c
+++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
@@ -2364,8 +2364,10 @@ static void bxt_sanitize_cdclk(struct intel_display *display)
 	 * dividers both syncing to an active pipe, or asynchronously
 	 * (PIPE_NONE).
 	 */
-	cdctl &= ~bxt_cdclk_cd2x_pipe(display, INVALID_PIPE);
-	cdctl |= bxt_cdclk_cd2x_pipe(display, INVALID_PIPE);
+	if (DISPLAY_VER(display) < 30) {
+		cdctl &= ~bxt_cdclk_cd2x_pipe(display, INVALID_PIPE);
+		cdctl |= bxt_cdclk_cd2x_pipe(display, INVALID_PIPE);
+	}

 	if (cdctl != expected) {
 		if (DISPLAY_VER(display) < 20) {

✗ LGCI.VerificationFailed: failure for drm/i915/cdclk: Don't force cd2x pipe select on Xe3 in bxt_sanitize_cdclk()

[Patchwork]

== Series Details ==

Series: drm/i915/cdclk: Don't force cd2x pipe select on Xe3 in bxt_sanitize_cdclk()
URL   : https://patchwork.freedesktop.org/series/172268/
State : failure

== Summary ==

Series author address '304750714@qq.com' is not on the allowlist, which prevents CI from being automatically triggered.
If you want CI to run for this series, ask Patchwork project owners to click 'retest' on the series in Patchwork.
Exception occurred during validation, bailing out!
Build URL: http://intel-gfx-ci-public.igk.intel.com:8080/job/xe_pw_trigger/1249384/ (on master)

✗ LGCI.VerificationFailed: failure for drm/i915/cdclk: Don't force cd2x pipe select on Xe3 in bxt_sanitize_cdclk()

[Patchwork]

== Series Details ==

Series: drm/i915/cdclk: Don't force cd2x pipe select on Xe3 in bxt_sanitize_cdclk()
URL   : https://patchwork.freedesktop.org/series/172269/
State : failure

== Summary ==

Series author address '304750714@qq.com' is not on the allowlist, which prevents CI from being automatically triggered.
If you want CI to run for this series, ask Patchwork project owners to click 'retest' on the series in Patchwork.
Exception occurred during validation, bailing out!
Build URL: http://gfx-ci.igk.intel.com:8080/job/CI_PW_kernel/184338/ (on built-in)

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

[Chang Jiong(常炯)]

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@vger.kernel.org
Signed-off-by: Chang Jiong(常炯) <xspeed1989@gmail.com>
---
v2: No code changes. Resent with From matching Signed-off-by
    (v1 was sent from 304750714@qq.com, which is not on the
    Intel CI allowlist).

 drivers/gpu/drm/i915/display/intel_cdclk.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/display/intel_cdclk.c b/drivers/gpu/drm/i915/display/intel_cdclk.c
index 7bc9b956554b..0438f5ac78c0 100644
--- a/drivers/gpu/drm/i915/display/intel_cdclk.c
+++ b/drivers/gpu/drm/i915/display/intel_cdclk.c
@@ -2366,8 +2366,10 @@ static void bxt_sanitize_cdclk(struct intel_display *display)
 	 * dividers both syncing to an active pipe, or asynchronously
 	 * (PIPE_NONE).
 	 */
-	cdctl &= ~bxt_cdclk_cd2x_pipe(display, INVALID_PIPE);
-	cdctl |= bxt_cdclk_cd2x_pipe(display, INVALID_PIPE);
+	if (DISPLAY_VER(display) < 30) {
+		cdctl &= ~bxt_cdclk_cd2x_pipe(display, INVALID_PIPE);
+		cdctl |= bxt_cdclk_cd2x_pipe(display, INVALID_PIPE);
+	}
 
 	if (cdctl != expected) {
 		if (DISPLAY_VER(display) < 20) {
-- 
2.55.0

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

[Zhefu Zhang]

I hit the same bug independently on a Panther Lake laptop
(Xiaomi Book Pro 14 2026, 8086:b080, Xe3_LPD display 30 stepping B0).

Arch linux 7.1.8 stock reproduces the corruption with two
"CPU pipe A FIFO underrun" errors. With the same change applied on 7.1.8,
the panel is clean and the boot log has no FIFO underrun. Also verified
the same change on current drm-tip (b801f94c49ffdb0f65e37baf31d68570356d4983):
normal panel, no FIFO underrun.

Tested-by: Zhefu Zhang <a723356@gmail.com>