drm/i915: Sanitize BIOS debugging bits from PIPECONF

Quoting the BSpec from time immemorial:

  PIPEACONF, bits 28:27: Frame Start Delay (Debug)

  Used to delay the frame start signal that is sent to the display planes.
  Care must be taken to insure that there are enough lines during VBLANK
  to support this setting.

An instance of the BIOS leaving these bits set was found in the wild,
where it caused our modesetting to go all squiffy and skewiff.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=47271
Reported-and-tested-by: Eva Wang <evawang@linpus.com>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=43012
Reported-and-tested-by: Carl Richell <carl@system76.com>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 3886cf051bac24cf276d16afacd5201b2d159ec0..2abf4eb9403928be8cfb237910090cca0e8a4840 100644 (file)
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -2385,6 +2385,7 @@
 #define   PIPECONF_DISABLE     0
 #define   PIPECONF_DOUBLE_WIDE (1<<30)
 #define   I965_PIPECONF_ACTIVE (1<<30)
+#define   PIPECONF_FRAME_START_DELAY_MASK (3<<27)
 #define   PIPECONF_SINGLE_WIDE 0
 #define   PIPECONF_PIPE_UNLOCKED 0
 #define   PIPECONF_PIPE_LOCKED (1<<25)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index d514719f65e26516a823577a8ef3e5c161ffb5a8..078477306975351e52565027b8f8a370b5a0f256 100644 (file)
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -7580,6 +7580,12 @@ static void intel_sanitize_modesetting(struct drm_device *dev,
        struct drm_i915_private *dev_priv = dev->dev_private;
        u32 reg, val;
 
+       /* Clear any frame start delays used for debugging left by the BIOS */
+       for_each_pipe(pipe) {
+               reg = PIPECONF(pipe);
+               I915_WRITE(reg, I915_READ(reg) & ~PIPECONF_FRAME_START_DELAY_MASK);
+       }
+
        if (HAS_PCH_SPLIT(dev))
                return;