[Mesa-dev] [PATCH 2/6] panfrost: Add tilebuffer load? branch

[Alyssa Rosenzweig]

Also document branches better.

Signed-off-by: Alyssa Rosenzweig <alyssa at rosenzweig.io>
---
 .../drivers/panfrost/midgard/disassemble.c    |  4 +++
 .../drivers/panfrost/midgard/midgard.h        | 34 ++++++++++++++++++-
 2 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/src/gallium/drivers/panfrost/midgard/disassemble.c b/src/gallium/drivers/panfrost/midgard/disassemble.c
index 376c8f46956..e9ff3808318 100644
--- a/src/gallium/drivers/panfrost/midgard/disassemble.c
+++ b/src/gallium/drivers/panfrost/midgard/disassemble.c
@@ -385,6 +385,10 @@ print_branch_op(int op)
                 printf("write.");
                 break;
 
+        case midgard_jmp_writeout_op_tilebuffer_pending:
+                printf("tilebuffer.");
+                break;
+
         case midgard_jmp_writeout_op_discard:
                 printf("discard.");
                 break;
diff --git a/src/gallium/drivers/panfrost/midgard/midgard.h b/src/gallium/drivers/panfrost/midgard/midgard.h
index b2849aa0fee..335ea5c56cc 100644
--- a/src/gallium/drivers/panfrost/midgard/midgard.h
+++ b/src/gallium/drivers/panfrost/midgard/midgard.h
@@ -219,18 +219,50 @@ __attribute__((__packed__))
 }
 midgard_reg_info;
 
+/* In addition to conditional branches and jumps (unconditional branches),
+ * Midgard implements a bit of fixed function functionality used in fragment
+ * shaders via specially crafted branches. These have special branch opcodes,
+ * which perform a fixed-function operation and/or use the results of a
+ * fixed-function operation as the branch condition.  */
+
 typedef enum {
+        /* Regular branches */
         midgard_jmp_writeout_op_branch_uncond = 1,
         midgard_jmp_writeout_op_branch_cond = 2,
+
+        /* In a fragment shader, execute a discard_if instruction, with the
+         * corresponding condition code. Terminates the shader, so generally
+         * set the branch target to out of the shader */
         midgard_jmp_writeout_op_discard = 4,
+
+        /* Branch if the tilebuffer is not yet ready. At the beginning of a
+         * fragment shader that reads from the tile buffer, for instance via
+         * ARM_shader_framebuffer_fetch or EXT_pixel_local_storage, this branch
+         * operation should be used as a loop. An instruction like
+         * "br.tilebuffer.always -1" does the trick, corresponding to
+         * "while(!is_tilebuffer_ready) */
+        midgard_jmp_writeout_op_tilebuffer_pending = 6,
+
+        /* In a fragment shader, try to write out the value pushed to r0 to the
+         * tilebuffer, subject to unknown state in r1.z and r1.w. If this
+         * succeeds, the shader terminates. If it fails, it branches to the
+         * specified branch target. Generally, this should be used in a loop to
+         * itself, acting as "do { write(r0); } while(!write_successful);" */
         midgard_jmp_writeout_op_writeout = 7,
 } midgard_jmp_writeout_op;
 
 typedef enum {
         midgard_condition_write0 = 0,
+
+        /* These condition codes denote a conditional branch on FALSE and on
+         * TRUE respectively */
         midgard_condition_false = 1,
         midgard_condition_true = 2,
-        midgard_condition_always = 3, /* Special for writeout/uncond discard */
+
+        /* This condition code always branches. For a pure branch, the
+         * unconditional branch coding should be used instead, but for
+         * fixed-function branch opcodes, this is still useful */
+        midgard_condition_always = 3,
 } midgard_condition;
 
 typedef struct
-- 
2.20.1