drm/i915/skl: While sanitizing cdclock check the SWF18 as well

SWF18 is set if the display has been initialized by the pre-os. It also
gives what configuration is enabled on which pipe. In skl_sanitize_cdclk,
the DPLL sanity check can pass even if GOP/VBIOS is not loaded as BIOS
enables DPLL for integrated audio codec related programming.
So fisrt check if SWF18 is set and then follow through with other DPLL
and CDCLK verification. If not set then for sure we need to sanitize the
cdclock.

v2: Update the commit message for clarity (Siva)
v3: Correct the mask to check for bits[23:0] instead of only bits[16:0].
    Had missed checking for PIPE C altogether. Remaining are reserved (Siva)
v4: Use ILK_SWF macro for SWF register definitions. Taken from Ville's patch
    http://lists.freedesktop.org/archives/intel-gfx/2015-November/079480.html

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Shobhit Kumar <shobhit.kumar@intel.com>
Reviewed-by: Sivakumar Thulasimani <sivakumar.thulasimani@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446726932-14078-1-git-send-email-shobhit.kumar@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -5005,6 +5005,7 @@ enum skl_disp_power_wells {
 #define SWF0(i)	(dev_priv->info.display_mmio_offset + 0x70410 + (i) * 4)
 #define SWF1(i)	(dev_priv->info.display_mmio_offset + 0x71410 + (i) * 4)
 #define SWF3(i)	(dev_priv->info.display_mmio_offset + 0x72414 + (i) * 4)
+#define SWF_ILK(i)	(0x4F000 + (i) * 4)
 
 /* Pipe B */
 #define _PIPEBDSL		(dev_priv->info.display_mmio_offset + 0x71000)

drivers/gpu/drm/i915/intel_display.c

@@ -5734,6 +5734,14 @@ int skl_sanitize_cdclk(struct drm_i915_private *dev_priv)
 	uint32_t cdctl = I915_READ(CDCLK_CTL);
 	int freq = dev_priv->skl_boot_cdclk;
 
+	/*
+	 * check if the pre-os intialized the display
+	 * There is SWF18 scratchpad register defined which is set by the
+	 * pre-os which can be used by the OS drivers to check the status
+	 */
+	if ((I915_READ(SWF_ILK(0x18)) & 0x00FFFFFF) == 0)
+		goto sanitize;
+
 	/* Is PLL enabled and locked ? */
 	if (!((lcpll1 & LCPLL_PLL_ENABLE) && (lcpll1 & LCPLL_PLL_LOCK)))
 		goto sanitize;