[Mesa-dev] [PATCH v4 21/23] anv/batch_chain: Simplify secondary batch return chaining

[Jason Ekstrand]

Previously, we did this weird thing where we left space and an empty
relocation for use in a hypothetical MI_BATCH_BUFFER_START that would be
added to the secondary later.  Then, when it came time to chain it into
the primary, we would back that out and emit an MI_BATCH_BUFFER_START.
This worked well but it was always a bit hacky, fragile and ugly.  This
commit instead adds a helper for rewriting the MI_BATCH_BUFFER_START at
the end of an anv_batch_bo and we use that helper for both batch bo list
cloning and handling returns from secondaries.  The new helper doesn't
actually modify the batch in any way but instead just adjusts the
relocation as needed.
---
 src/intel/vulkan/anv_batch_chain.c | 76 ++++++++++++++++++--------------------
 1 file changed, 36 insertions(+), 40 deletions(-)

diff --git a/src/intel/vulkan/anv_batch_chain.c b/src/intel/vulkan/anv_batch_chain.c
index 9f85f81..9d3884c 100644
--- a/src/intel/vulkan/anv_batch_chain.c
+++ b/src/intel/vulkan/anv_batch_chain.c
@@ -416,6 +416,30 @@ anv_batch_bo_grow(struct anv_cmd_buffer *cmd_buffer, struct anv_batch_bo *bbo,
 }
 
 static void
+anv_batch_bo_link(struct anv_cmd_buffer *cmd_buffer,
+                  struct anv_batch_bo *prev_bbo,
+                  struct anv_batch_bo *next_bbo,
+                  uint32_t next_bbo_offset)
+{
+   MAYBE_UNUSED const uint32_t bb_start_offset =
+      prev_bbo->length - GEN8_MI_BATCH_BUFFER_START_length * 4;
+   MAYBE_UNUSED const uint32_t *bb_start = prev_bbo->bo.map + bb_start_offset;
+
+   /* Make sure we're looking at a MI_BATCH_BUFFER_START */
+   assert(((*bb_start >> 29) & 0x07) == 0);
+   assert(((*bb_start >> 23) & 0x3f) == 49);
+
+   uint32_t reloc_idx = prev_bbo->relocs.num_relocs - 1;
+   assert(prev_bbo->relocs.relocs[reloc_idx].offset == bb_start_offset + 4);
+
+   prev_bbo->relocs.reloc_bos[reloc_idx] = &next_bbo->bo;
+   prev_bbo->relocs.relocs[reloc_idx].delta = next_bbo_offset;
+
+   /* Use a bogus presumed offset to force a relocation */
+   prev_bbo->relocs.relocs[reloc_idx].presumed_offset = -1;
+}
+
+static void
 anv_batch_bo_destroy(struct anv_batch_bo *bbo,
                      struct anv_cmd_buffer *cmd_buffer)
 {
@@ -441,16 +465,8 @@ anv_batch_bo_list_clone(const struct list_head *list,
          break;
       list_addtail(&new_bbo->link, new_list);
 
-      if (prev_bbo) {
-         /* As we clone this list of batch_bo's, they chain one to the
-          * other using MI_BATCH_BUFFER_START commands.  We need to fix up
-          * those relocations as we go.  Fortunately, this is pretty easy
-          * as it will always be the last relocation in the list.
-          */
-         uint32_t last_idx = prev_bbo->relocs.num_relocs - 1;
-         assert(prev_bbo->relocs.reloc_bos[last_idx] == &bbo->bo);
-         prev_bbo->relocs.reloc_bos[last_idx] = &new_bbo->bo;
-      }
+      if (prev_bbo)
+         anv_batch_bo_link(cmd_buffer, prev_bbo, new_bbo, 0);
 
       prev_bbo = new_bbo;
    }
@@ -864,13 +880,13 @@ anv_cmd_buffer_end_batch_buffer(struct anv_cmd_buffer *cmd_buffer)
                    VK_COMMAND_BUFFER_USAGE_SIMULTANEOUS_USE_BIT)) {
          cmd_buffer->exec_mode = ANV_CMD_BUFFER_EXEC_MODE_CHAIN;
 
-         /* When we chain, we need to add an MI_BATCH_BUFFER_START command
-          * with its relocation.  In order to handle this we'll increment here
-          * so we can unconditionally decrement right before adding the
-          * MI_BATCH_BUFFER_START command.
+         /* In order to chain, we need this command buffer to contain an
+          * MI_BATCH_BUFFER_START which will jump back to the calling batch.
+          * It doesn't matter where it points now so long as has a valid
+          * relocation.  We'll adjust it later as part of the chaining
+          * process.
           */
-         batch_bo->relocs.num_relocs++;
-         cmd_buffer->batch.next += GEN8_MI_BATCH_BUFFER_START_length * 4;
+         emit_batch_buffer_start(cmd_buffer, &batch_bo->bo, 0);
       } else {
          cmd_buffer->exec_mode = ANV_CMD_BUFFER_EXEC_MODE_COPY_AND_CHAIN;
       }
@@ -921,33 +937,13 @@ anv_cmd_buffer_add_secondary(struct anv_cmd_buffer *primary,
       struct anv_batch_bo *this_bbo = anv_cmd_buffer_current_batch_bo(primary);
       assert(primary->batch.start == this_bbo->bo.map);
       uint32_t offset = primary->batch.next - primary->batch.start;
-      const uint32_t inst_size = GEN8_MI_BATCH_BUFFER_START_length * 4;
 
-      /* Roll back the previous MI_BATCH_BUFFER_START and its relocation so we
-       * can emit a new command and relocation for the current splice.  In
-       * order to handle the initial-use case, we incremented next and
-       * num_relocs in end_batch_buffer() so we can alyways just subtract
-       * here.
+      /* Make the tail of the secondary point back to right after the
+       * MI_BATCH_BUFFER_START in the primary batch.
        */
-      last_bbo->relocs.num_relocs--;
-      secondary->batch.next -= inst_size;
-      emit_batch_buffer_start(secondary, &this_bbo->bo, offset);
-      anv_cmd_buffer_add_seen_bbos(primary, &secondary->batch_bos);
+      anv_batch_bo_link(primary, last_bbo, this_bbo, offset);
 
-      /* After patching up the secondary buffer, we need to clflush the
-       * modified instruction in case we're on a !llc platform. We use a
-       * little loop to handle the case where the instruction crosses a cache
-       * line boundary.
-       */
-      if (!primary->device->info.has_llc) {
-         void *inst = secondary->batch.next - inst_size;
-         void *p = (void *) (((uintptr_t) inst) & ~CACHELINE_MASK);
-         __builtin_ia32_mfence();
-         while (p < secondary->batch.next) {
-            __builtin_ia32_clflush(p);
-            p += CACHELINE_SIZE;
-         }
-      }
+      anv_cmd_buffer_add_seen_bbos(primary, &secondary->batch_bos);
       break;
    }
    case ANV_CMD_BUFFER_EXEC_MODE_COPY_AND_CHAIN: {
-- 
2.5.0.400.gff86faf