drm/i915/fbc: Allow FBC to recompress after a 3D workload on i85x/i865

Normally i85x/i865 3D activity will block FBC until a 2D blit
occurs. I suppose this was meant to avoid recompression while
3D activity is still going on but the frame hasn't yet been
presented. Unfortunately that also means that a page flipped
3D workload will permanently block FBC even if it only renders
a single frame and then does nothing.

Since we are using software render tracking anyway we might as
well flip the chicken bit so that 3D does not block FBC. This
will avoid the permament FBC blockage in the aforemention use
case, but thanks to the software tracking the compressor will
not disturb 3D rendering activity.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200702153723.24327-5-ville.syrjala@linux.intel.comReviewed-by: José Roberto de Souza <jose.souza@intel.com>

drivers/gpu/drm/i915/i915_reg.h

@@ -2827,6 +2827,7 @@ static inline bool i915_mmio_reg_valid(i915_reg_t reg)
 #define VLV_GU_CTL0	_MMIO(VLV_DISPLAY_BASE + 0x2030)
 #define VLV_GU_CTL1	_MMIO(VLV_DISPLAY_BASE + 0x2034)
 #define SCPD0		_MMIO(0x209c) /* 915+ only */
+#define  SCPD_FBC_IGNORE_3D			(1 << 6)
 #define  CSTATE_RENDER_CLOCK_GATE_DISABLE	(1 << 5)
 #define GEN2_IER	_MMIO(0x20a0)
 #define GEN2_IIR	_MMIO(0x20a4)

drivers/gpu/drm/i915/intel_pm.c

@@ -7471,6 +7471,16 @@ static void i85x_init_clock_gating(struct drm_i915_private *dev_priv)
 
 	I915_WRITE(MEM_MODE,
 		   _MASKED_BIT_ENABLE(MEM_DISPLAY_TRICKLE_FEED_DISABLE));
+
+	/*
+	 * Have FBC ignore 3D activity since we use software
+	 * render tracking, and otherwise a pure 3D workload
+	 * (even if it just renders a single frame and then does
+	 * abosultely nothing) would not allow FBC to recompress
+	 * until a 2D blit occurs.
+	 */
+	I915_WRITE(SCPD0,
+		   _MASKED_BIT_ENABLE(SCPD_FBC_IGNORE_3D));
 }
 
 static void i830_init_clock_gating(struct drm_i915_private *dev_priv)