pan/cs: Allow lazy root chunk allocation

We don't necessarily want to allocate the root CS chunk upfront if we
don't know if there will be instructions emitted on the CS.

Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Mary Guillemard <mary.guillemard@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30695>

src/panfrost/lib/genxml/cs_builder.h

@@ -87,6 +87,9 @@ struct cs_builder {
    /* CS builder configuration */
    struct cs_builder_conf conf;
 
+   /* True if an allocation failed, making the whole CS invalid. */
+   bool invalid;
+
    /* Initial (root) CS chunk. */
    struct cs_chunk root_chunk;
 
@@ -124,7 +127,7 @@ cs_builder_init(struct cs_builder *b, const struct cs_builder_conf *conf,
 static bool
 cs_is_valid(struct cs_builder *b)
 {
-   return b->cur_chunk.buffer.cpu != NULL;
+   return !b->invalid;
 }
 
 /*
@@ -275,9 +278,19 @@ cs_alloc_ins(struct cs_builder *b)
    /* If an allocation failure happened before, we just discard all following
     * instructions.
     */
-   if (unlikely(!b->cur_chunk.buffer.cpu))
+   if (unlikely(!cs_is_valid(b)))
       return &b->discard_instr_slot;
 
+   /* Lazy root chunk allocation. */
+   if (unlikely(!b->root_chunk.buffer.cpu)) {
+      b->root_chunk.buffer = b->conf.alloc_buffer(b->conf.cookie);
+      b->cur_chunk.buffer = b->root_chunk.buffer;
+      if (!b->cur_chunk.buffer.cpu) {
+         b->invalid = true;
+         return &b->discard_instr_slot;
+      }
+   }
+
    /* If the current chunk runs out of space, allocate a new one and jump to it.
     * We actually do this a few instructions before running out, because the
     * sequence to jump to a new queue takes multiple instructions.