[Intel-gfx] [PATCH 1/5] drm/i915/skl: Work around incorrect BIOS WRPLL PDIV programming

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

The BIOS of at least one ASUS-Z170M system with an SKL I have programs
the 101b WRPLL PDIV divider value, which is the encoding for PDIV=7 with
bit#0 incorrectly set.

This happens with the

"3840x2160": 30 262750 3840 3888 3920 4000 2160 2163 2168 2191 0x48 0x9

HDMI mode (scaled from a 1024x768 src fb) set by BIOS and the

ref_clock=24000, dco_integer=383, dco_fraction=5802, pdiv=7, qdiv=1, kdiv=1

WRPLL parameters (assuming PDIV=7 was the intended setting). This
corresponds to 262749 PLL frequency/port clock.

Later the driver sets the same mode for which it calculates the same
dco_int/dco_frac/div WRPLL parameters (with the correct PDIV=7 encoding).

Based on the above, let's assume that PDIV=7 was intended and the HW
just ignores bit#0 in the PDIV register field for this setting, treating
100b and 101b encodings the same way.

While at it add the MISSING_CASE() for the p0,p2 divider decodings.

v2: (Ville)
- Add a define for the incorrect divider value.
- Emit only a debug message when detecting the incorrect divider value.
- Use fallthrough from the incorrect divider value case.
- Add the MISSING_CASE()s.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
---
 drivers/gpu/drm/i915/display/intel_dpll_mgr.c | 14 ++++++++++++++
 drivers/gpu/drm/i915/i915_reg.h               |  1 +
 2 files changed, 15 insertions(+)

diff --git a/drivers/gpu/drm/i915/display/intel_dpll_mgr.c b/drivers/gpu/drm/i915/display/intel_dpll_mgr.c
index e08684e34078..61cb558c60d1 100644
--- a/drivers/gpu/drm/i915/display/intel_dpll_mgr.c
+++ b/drivers/gpu/drm/i915/display/intel_dpll_mgr.c
@@ -1602,12 +1602,26 @@ static int skl_ddi_wrpll_get_freq(struct drm_i915_private *i915,
 	case DPLL_CFGCR2_PDIV_3:
 		p0 = 3;
 		break;
+	default:
+		if (p0 == DPLL_CFGCR2_PDIV_7_INVALID)
+			/*
+			 * Incorrect ASUS-Z170M BIOS setting, the HW seems to ignore bit#0,
+			 * handling it the same way as PDIV_7.
+			 */
+			drm_dbg_kms(&i915->drm, "Invalid WRPLL PDIV divider value, fixing it.\n");
+		else
+			MISSING_CASE(p0);
+
+		fallthrough;
 	case DPLL_CFGCR2_PDIV_7:
 		p0 = 7;
 		break;
 	}
 
 	switch (p2) {
+	default:
+		MISSING_CASE(p2);
+		fallthrough;
 	case DPLL_CFGCR2_KDIV_5:
 		p2 = 5;
 		break;
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 88c215cf97d4..d911583526db 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -10261,6 +10261,7 @@ enum skl_power_gate {
 #define  DPLL_CFGCR2_PDIV_2 (1 << 2)
 #define  DPLL_CFGCR2_PDIV_3 (2 << 2)
 #define  DPLL_CFGCR2_PDIV_7 (4 << 2)
+#define  DPLL_CFGCR2_PDIV_7_INVALID	(5 << 2)
 #define  DPLL_CFGCR2_CENTRAL_FREQ_MASK	(3)
 
 #define DPLL_CFGCR1(id)	_MMIO_PIPE((id) - SKL_DPLL1, _DPLL1_CFGCR1, _DPLL2_CFGCR1)
-- 
2.25.1

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