drm/i915/skl: Work around incorrect BIOS WRPLL PDIV programming

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.

v3: Return 0 freq for incorrect divider values. (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201006013555.1488262-1-imre.deak@intel.com

drivers/gpu/drm/i915/display/intel_dpll_mgr.c

@@ -1602,9 +1602,19 @@ static int skl_ddi_wrpll_get_freq(struct drm_i915_private *i915,
 	case DPLL_CFGCR2_PDIV_3:
 		p0 = 3;
 		break;
+	case 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");
+		fallthrough;
 	case DPLL_CFGCR2_PDIV_7:
 		p0 = 7;
 		break;
+	default:
+		MISSING_CASE(p0);
+		return 0;
 	}
 
 	switch (p2) {
@@ -1620,6 +1630,9 @@ static int skl_ddi_wrpll_get_freq(struct drm_i915_private *i915,
 	case DPLL_CFGCR2_KDIV_1:
 		p2 = 1;
 		break;
+	default:
+		MISSING_CASE(p2);
+		return 0;
 	}
 
 	dco_freq = (pll_state->cfgcr1 & DPLL_CFGCR1_DCO_INTEGER_MASK) *

drivers/gpu/drm/i915/i915_reg.h

@@ -10264,6 +10264,7 @@ 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)