panfrost: Take into account a index_bias for glDrawElementsBaseVertex calls

Midgard does not accept a index_bias directly and relies instead on a
bias correction offset (offset_bias_correction) in order to calculate
the unbiased vertex index.

We need to make sure we adjust offset_start and vertex_count in order to
take into account the index_bias as required by a
glDrawElementsBaseVertex call and then supply a additional
offset_bias_correction to the hardware.

Signed-off-by: Rohan Garg <[email protected]>
Reviewed-by: Alyssa Rosenzweig <[email protected]>

2 files changed, 22 insertions, 7 deletions

diff --git a/src/panfrost/include/panfrost-job.h b/src/panfrost/include/panfrost-job.h
index 9179ceff2ec..98d77e718fc 100644
--- a/src/panfrost/include/panfrost-job.h
+++ b/src/panfrost/include/panfrost-job.h
@@ -935,8 +935,23 @@ struct mali_vertex_tiler_prefix {
         u32 workgroups_x_shift_3 : 6;
 
 
-        /* Negative of draw_start for TILER jobs from what I've seen */
-        int32_t negative_start;
+        /* Negative of min_index. This is used to compute
+         * the unbiased index in tiler/fragment shader runs.
+         * 
+         * The hardware adds offset_bias_correction in each run,
+         * so that absent an index bias, the first vertex processed is
+         * genuinely the first vertex (0). But with an index bias,
+         * the first vertex process is numbered the same as the bias.
+         *
+         * To represent this more conviniently:
+         * unbiased_index = lower_bound_index +
+         *                  index_bias +
+         *                  offset_bias_correction
+         *
+         * This is done since the hardware doesn't accept a index_bias
+         * and this allows it to recover the unbiased index.
+         */
+        int32_t offset_bias_correction;
         u32 zero1;
 
         /* Like many other strictly nonzero quantities, index_count is
@@ -1080,7 +1095,7 @@ struct midgard_payload_vertex_tiler {
         u8 zero4;
 
         /* Offset for first vertex in buffer */
-        u32 draw_start;
+        u32 offset_start;
 
 	u64 zero5;
 
diff --git a/src/panfrost/pandecode/decode.c b/src/panfrost/pandecode/decode.c
index 080d650fc5a..78f6da6e56c 100644
--- a/src/panfrost/pandecode/decode.c
+++ b/src/panfrost/pandecode/decode.c
@@ -1334,8 +1334,8 @@ pandecode_vertex_tiler_prefix(struct mali_vertex_tiler_prefix *p, int job_no)
         if (p->index_count)
                 pandecode_prop("index_count = MALI_POSITIVE(%" PRId32 ")", p->index_count + 1);
 
-        if (p->negative_start)
-                pandecode_prop("negative_start = %d", p->negative_start);
+        if (p->offset_bias_correction)
+                pandecode_prop("offset_bias_correction = %d", p->offset_bias_correction);
 
         DYN_MEMORY_PROP(p, job_no, indices);
 
@@ -2181,8 +2181,8 @@ pandecode_vertex_or_tiler_job_mdg(const struct mali_job_descriptor_header *h,
                 pandecode_padded_vertices(v->instance_shift, v->instance_odd);
         }
 
-        if (v->draw_start)
-                pandecode_prop("draw_start = %d", v->draw_start);
+        if (v->offset_start)
+                pandecode_prop("offset_start = %d", v->offset_start);
 
         if (v->zero5) {
                 pandecode_msg("Zero tripped\n");