[PATCH 2/2] drm/i915/bxt: Sanitize CDCLK to fix breakage during S4 resume

[Imre Deak <imre.deak@intel.com>]

I noticed that during S4 resume BIOS incorrectly sets bits 18, 19 which
are reserved/MBZ and sets the decimal frequency fields to all 0xff in
the CDCLK register. The result is a hard lockup as display register
accesses are attempted later. Work around this by sanitizing the CDCLK
PLL/dividers the same way it's done on SKL.

While this is clearly a BIOS bug which should be fixed separately, it
doesn't hurt to check/sanitize this regardless.

Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 drivers/gpu/drm/i915/intel_display.c | 51 ++++++++++++++++++++++++++++++++++--
 1 file changed, 49 insertions(+), 2 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index b8e5995..479d2e4 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -5412,11 +5412,58 @@ static void broxton_set_cdclk(struct drm_i915_private *dev_priv, int cdclk)
 	intel_update_cdclk(dev_priv->dev);
 }
 
+static void bxt_sanitize_cdclk(struct drm_i915_private *dev_priv)
+{
+	u32 cdctl, expected;
+
+	intel_update_cdclk(dev_priv->dev);
+
+	if (!dev_priv->cdclk_pll.vco ||
+	    dev_priv->cdclk_freq == dev_priv->cdclk_pll.ref)
+		goto sanitize;
+
+	/* DPLL okay; verify the cdclock
+	 *
+	 * Some BIOS versions leave an incorrect decimal frequency value and
+	 * set reserved MBZ bits in CDCLK_CTL at least during exiting from S4,
+	 * so sanitize this register.
+	 */
+	cdctl = I915_READ(CDCLK_CTL);
+	/*
+	 * Let's ignore the pipe field, since BIOS could have configured the
+	 * dividers both synching to an active pipe, or asynchronously
+	 * (PIPE_NONE).
+	 */
+	cdctl &= ~BXT_CDCLK_CD2X_PIPE_NONE;
+
+	expected = (cdctl & BXT_CDCLK_CD2X_DIV_SEL_MASK) |
+		   skl_cdclk_decimal(dev_priv->cdclk_freq);
+	/*
+	 * Disable SSA Precharge when CD clock frequency < 500 MHz,
+	 * enable otherwise.
+	 */
+	if (dev_priv->cdclk_freq >= 500000)
+		expected |= BXT_CDCLK_SSA_PRECHARGE_ENABLE;
+
+	if (cdctl == expected)
+		/* All well; nothing to sanitize */
+		return;
+
+sanitize:
+	DRM_DEBUG_KMS("Sanitizing cdclk programmed by pre-os\n");
+
+	/* force cdclk programming */
+	dev_priv->cdclk_freq = 0;
+
+	/* force full PLL disable + enable */
+	dev_priv->cdclk_pll.vco = -1;
+}
+
 void broxton_init_cdclk(struct drm_i915_private *dev_priv)
 {
-	intel_update_cdclk(dev_priv->dev);
+	bxt_sanitize_cdclk(dev_priv);
 
-	if (dev_priv->cdclk_pll.vco != 0)
+	if (dev_priv->cdclk_freq > 0 && dev_priv->cdclk_pll.vco > 0)
 		return;
 
 	/*
-- 
2.5.0

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