Signed-off-by: Richard Sandiford <rsand...@linux.vnet.ibm.com>
---
src/gallium/auxiliary/util/u_format.h | 42 ++++++++++++++++++++++++++++++++++-
1 file changed, 41 insertions(+), 1 deletion(-)
diff --git a/src/gallium/auxiliary/util/u_format.h
b/src/gallium/auxiliary/util/u_format.h
index e4b9c36..db6c290 100644
--- a/src/gallium/auxiliary/util/u_format.h
+++ b/src/gallium/auxiliary/util/u_format.h
@@ -178,9 +178,49 @@ struct util_format_description
unsigned is_mixed:1;
/**
- * Input channel description.
+ * Input channel description, in the order XYZW.
*
* Only valid for UTIL_FORMAT_LAYOUT_PLAIN formats.
+ *
+ * The general rule is that the order and layout of the channels is the
+ * same as they would be in a C struct:
+ *
+ * struct {
+ * ...X...;
+ * ...Y...;
+ * ...Z...;
+ * ...W...;
+ * };
+ *
+ * with bitfields being used for all integer channels.
+ *
+ * This means that if some channels can be accessed as individual N-byte
+ * values, the order of those channels in this array matches their order
+ * in memory. Each channel has native endianness.
+ *
+ * If instead a group of channels is accessed as a single N-byte value,
+ * the order of the channels within that value depends on endianness.
+ * For big-endian targets, the first channel in the group will be
+ * the most significant, otherwise it will be the least significant.
+ *
+ * For example, if X, Y, Z and W are all 16 bits, the memory order is:
+ *
+ * 0 1 2 3 4 5 6 7
+ * little-endian: Xl Xu Yl Yu Zl Zu Wl Wu (l = lower, u = upper)
+ * big-endian: Xu Xl Yu Yl Zu Zl Wu Wl
+ *
+ * If X is 8 bits and Y is 24 bits, the memory order is:
+ *
+ * 0 1 2 3
+ * little-endian: X Yl Ym Yu (l = lower, m = middle, u = upper)
+ * big-endian: X Yu Ym Yl
+ *
+ * If X is 5 bits, Y is 5 bits, Z is 5 bits and W is 1 bit, the layout is:
+ *
+ * 0 1
+ * msb lsb msb lsb
+ * little-endian: YYYXXXXX WZZZZZYY
+ * big-endian: XXXXXYYY YYZZZZZW
*/
struct util_format_channel_description channel[4];
--
1.7.11.7
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