[PATCH 04/12] drm/i915: Add a comment exlaining CCS hsub/vsub

[ville.syrjala@linux.intel.com]

From: Ville Syrjälä <ville.syrjala@linux.intel.com>

Let's document why we claim hsub==8,vsub==16 for CCS even though the
memory layout would suggest that we use 16x8 instead.

Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Ben Widawsky <ben@bwidawsk.net>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: Daniel Stone <daniels@collabora.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
---
 drivers/gpu/drm/i915/intel_display.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index a8809bc64475..27c1a4cbce7d 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -2484,6 +2484,13 @@ static unsigned int intel_fb_modifier_to_tiling(uint64_t fb_modifier)
 	}
 }
 
+/*
+ * 1 byte of CCS actually corresponds to 16x8 pixels on the main
+ * surface, and the memory layout for the CCS tile is 64x64 bytes.
+ * But since we're pretending the CCS tile is 128 bytes wide we
+ * adjust hsub/vsub here accordingly to 8x16 so that the
+ * bytes<->x/y conversions come out correct.
+ */
 static const struct drm_format_info ccs_formats[] = {
 	{ .format = DRM_FORMAT_XRGB8888, .depth = 24, .num_planes = 2, .cpp = { 4, 1, }, .hsub = 8, .vsub = 16, },
 	{ .format = DRM_FORMAT_XBGR8888, .depth = 24, .num_planes = 2, .cpp = { 4, 1, }, .hsub = 8, .vsub = 16, },
-- 
2.13.0

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