Document and use 'legacy' border color mode

It's very convenient that the hardware supports this non-default
mode since it's exactly what is specified by the Render extension.
This provides a more efficient means of fixing bug #16820:

        [EXA] Composition result in black for areas outside of source-surface bo
        https://bugs.freedesktop.org/show_bug.cgi?id=16820

without the software fallback we had in the earlier fix,
(commit 76c9ece36e6400fd10f364ee330faea470e2da64 ).

3 files changed, 31 insertions, 9 deletions

diff --git a/src/brw_defines.h b/src/brw_defines.h
index 13cb4396..0df2491c 100644
--- a/src/brw_defines.h
+++ b/src/brw_defines.h
@@ -466,6 +466,9 @@
 #define BRW_SURFACE_BUFFER  4
 #define BRW_SURFACE_NULL    7
 
+#define BRW_BORDER_COLOR_MODE_DEFAULT	0
+#define BRW_BORDER_COLOR_MODE_LEGACY	1
+
 #define BRW_TEXCOORDMODE_WRAP            0
 #define BRW_TEXCOORDMODE_MIRROR          1
 #define BRW_TEXCOORDMODE_CLAMP           2
diff --git a/src/brw_structs.h b/src/brw_structs.h
index 553aeebb..022915d1 100644
--- a/src/brw_structs.h
+++ b/src/brw_structs.h
@@ -840,10 +840,26 @@ struct brw_wm_unit_state
    float global_depth_offset_scale;   
 };
 
-struct brw_sampler_border_color {
+/* The hardware supports two different modes for border color. The
+ * default (OpenGL) mode uses floating-point color channels, while the
+ * legacy mode uses 4 bytes.
+ *
+ * More significantly, the legacy mode respects the components of the
+ * border color for channels not present in the source, (whereas the
+ * default mode will ignore the border color's alpha channel and use
+ * alpha==1 for an RGB source, for example).
+ *
+ * The legacy mode matches the semantics specified by the Render
+ * extension.
+ */
+struct brw_sampler_default_border_color {
    float color[4];
 };
 
+struct brw_sampler_legacy_border_color {
+   uint8_t color[4];
+};
+
 struct brw_sampler_state
 {
    
diff --git a/src/i965_render.c b/src/i965_render.c
index 4bffe0d7..9ee273c0 100644
--- a/src/i965_render.c
+++ b/src/i965_render.c
@@ -496,8 +496,8 @@ typedef struct _gen4_state {
 					  [SAMPLER_STATE_FILTER_COUNT]
 					  [SAMPLER_STATE_EXTEND_COUNT][2];
 
-    struct brw_sampler_border_color sampler_border_color;
-    PAD64 (brw_sampler_border_color, 0);
+    struct brw_sampler_legacy_border_color sampler_border_color;
+    PAD64 (brw_sampler_legacy_border_color, 0);
 
     /* Index by [src_blend][dst_blend] */
     brw_cc_unit_state_padded cc_state[BRW_BLENDFACTOR_COUNT]
@@ -572,7 +572,10 @@ sampler_state_init (struct brw_sampler_state *sampler_state,
     memset(sampler_state, 0, sizeof(*sampler_state));
 
     sampler_state->ss0.lod_preclamp = 1; /* GL mode */
-    sampler_state->ss0.border_color_mode = 0; /* GL mode */
+
+    /* We use the legacy mode to get the semantics specified by
+     * the Render extension. */
+    sampler_state->ss0.border_color_mode = BRW_BORDER_COLOR_MODE_LEGACY;
 
     switch(filter) {
     default:
@@ -734,13 +737,13 @@ gen4_state_init (struct gen4_render_state *render_state)
     card_state->vs_state.vs6.vs_enable = 0;
     card_state->vs_state.vs6.vert_cache_disable = 1;
 
-    /* Set up the sampler default color (always transparent black) */
+    /* Set up the sampler border color (always transparent black) */
     memset(&card_state->sampler_border_color, 0,
 	   sizeof(card_state->sampler_border_color));
-    card_state->sampler_border_color.color[0] = 0.0; /* R */
-    card_state->sampler_border_color.color[1] = 0.0; /* G */
-    card_state->sampler_border_color.color[2] = 0.0; /* B */
-    card_state->sampler_border_color.color[3] = 0.0; /* A */
+    card_state->sampler_border_color.color[0] = 0; /* R */
+    card_state->sampler_border_color.color[1] = 0; /* G */
+    card_state->sampler_border_color.color[2] = 0; /* B */
+    card_state->sampler_border_color.color[3] = 0; /* A */
 
     card_state->cc_viewport.min_depth = -1.e35;
     card_state->cc_viewport.max_depth = 1.e35;