drm/i915: Program SHPD_FILTER_CNT on CNP+

The bspec tells us 'Program SHPD_FILTER_CNT with the "500 microseconds
adjusted" value before enabling hotplug detection' on CNP+.  We haven't
been touching this register at all thus far, but we should probably
follow the bspec's guidance.

The register also exists on LPT and SPT, but there isn't any specific
guidance I can find on how we should be programming it there so let's
leave it be for now.

Bspec: 4342
Bspec: 31297
Bspec: 8407
Bspec: 49305
Bspec: 50473
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127221314.575575-3-matthew.d.roper@intel.com

drivers/gpu/drm/i915/i915_irq.c

@@ -2980,6 +2980,8 @@ static void icp_hpd_irq_setup(struct drm_i915_private *dev_priv,
 	hotplug_irqs = sde_ddi_mask | sde_tc_mask;
 	enabled_irqs = intel_hpd_enabled_irqs(dev_priv, pins);
 
+	I915_WRITE(SHPD_FILTER_CNT, SHPD_FILTER_CNT_500_ADJ);
+
 	ibx_display_interrupt_update(dev_priv, hotplug_irqs, enabled_irqs);
 
 	icp_hpd_detection_setup(dev_priv, ddi_enable_mask, tc_enable_mask);
@@ -3085,6 +3087,9 @@ static void spt_hpd_irq_setup(struct drm_i915_private *dev_priv)
 {
 	u32 hotplug_irqs, enabled_irqs;
 
+	if (INTEL_PCH_TYPE(dev_priv) >= PCH_CNP)
+		I915_WRITE(SHPD_FILTER_CNT, SHPD_FILTER_CNT_500_ADJ);
+
 	hotplug_irqs = SDE_HOTPLUG_MASK_SPT;
 	enabled_irqs = intel_hpd_enabled_irqs(dev_priv, hpd_spt);
 

drivers/gpu/drm/i915/i915_reg.h

@@ -8114,6 +8114,10 @@ enum {
 
 #define SHOTPLUG_CTL_TC				_MMIO(0xc4034)
 #define   ICP_TC_HPD_ENABLE(tc_port)		(8 << (tc_port) * 4)
+
+#define SHPD_FILTER_CNT				_MMIO(0xc4038)
+#define   SHPD_FILTER_CNT_500_ADJ		0x001D9
+
 /* Icelake DSC Rate Control Range Parameter Registers */
 #define DSCA_RC_RANGE_PARAMETERS_0		_MMIO(0x6B240)
 #define DSCA_RC_RANGE_PARAMETERS_0_UDW		_MMIO(0x6B240 + 4)