drm/i915/adlp: Add workaround to disable CMTG clock gating

The driver doesn't depend atm on the common mode timing generator
functionality (it would be used for some power saving feature and panel
timing synchronization), however DMC will corrupt the CMTG registers
across DC5 entry/exit sequences unless the CMTG clock gating is
disabled. This in turn can lead to at least the DPLL0/1 configuration
getting stuck at their last state, which means we can't reprogram them
to a new config.

Add the corresponding Bspec workaround to prevent the above.

v2: Fix checkpatch errors. (CI, Jose)

Cc: Uma Shankar <uma.shankar@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Anshuman Gupta <anshuman.gupta@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210727134400.101290-1-imre.deak@intel.com

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

@@ -13237,6 +13237,24 @@ static void intel_early_display_was(struct drm_i915_private *dev_priv)
 			     KBL_ARB_FILL_SPARE_13 | KBL_ARB_FILL_SPARE_14,
 			     KBL_ARB_FILL_SPARE_14);
 	}
+
+	if (IS_ADLP_DISPLAY_STEP(dev_priv, STEP_A0, STEP_B0)) {
+		u32 val;
+
+		/*
+		 * Wa_16011069516:adl-p[a0]
+		 *
+		 * All CMTG regs are unreliable until CMTG clock gating is
+		 * disabled, so we can only assume the default CMTG_CHICKEN
+		 * reg value and sanity check this assumption with a double
+		 * read, which presumably returns the correct value even with
+		 * clock gating on.
+		 */
+		val = intel_de_read(dev_priv, TRANS_CMTG_CHICKEN);
+		val = intel_de_read(dev_priv, TRANS_CMTG_CHICKEN);
+		intel_de_write(dev_priv, TRANS_CMTG_CHICKEN, DISABLE_DPT_CLK_GATING);
+		drm_WARN_ON(&dev_priv->drm, val & ~DISABLE_DPT_CLK_GATING);
+	}
 }
 
 static void ibx_sanitize_pch_hdmi_port(struct drm_i915_private *dev_priv,

drivers/gpu/drm/i915/i915_reg.h

@@ -10177,6 +10177,9 @@ enum skl_power_gate {
 #define  PORT_SYNC_MODE_MASTER_SELECT_MASK	REG_GENMASK(2, 0)
 #define  PORT_SYNC_MODE_MASTER_SELECT(x)	REG_FIELD_PREP(PORT_SYNC_MODE_MASTER_SELECT_MASK, (x))
 
+#define TRANS_CMTG_CHICKEN		_MMIO(0x6fa90)
+#define  DISABLE_DPT_CLK_GATING		REG_BIT(1)
+
 /* DisplayPort Transport Control */
 #define _DP_TP_CTL_A			0x64040
 #define _DP_TP_CTL_B			0x64140