drm/i915: print FBC compression status on debugfs

We already had a few bugs in the past where FBC was compressing
nothing when it was enabled, which makes the feature quite useless.
Add this information to debugfs so the test suites can check for
regressions in this piece of the code.

Our igt/tests/kms_frontbuffer_tracking already has support for this
message.

v2: - Remove pointless VLV check (Ville).
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -1638,6 +1638,11 @@ static int i915_fbc_status(struct seq_file *m, void *unused)
 		seq_putc(m, '\n');
 	}
 
+	if (INTEL_INFO(dev_priv)->gen >= 7)
+		seq_printf(m, "Compressing: %s\n",
+			   yesno(I915_READ(FBC_STATUS2) &
+				 FBC_COMPRESSION_MASK));
+
 	intel_runtime_pm_put(dev_priv);
 
 	return 0;

drivers/gpu/drm/i915/i915_reg.h

@@ -1951,6 +1951,9 @@ enum skl_disp_power_wells {
 #define FBC_FENCE_OFF		0x03218 /* BSpec typo has 321Bh */
 #define FBC_TAG			0x03300
 
+#define FBC_STATUS2		0x43214
+#define  FBC_COMPRESSION_MASK	0x7ff
+
 #define FBC_LL_SIZE		(1536)
 
 /* Framebuffer compression for GM45+ */