vc4: fix unwanted buffer release on uploader

When converting the index buffer from 4-bytes to 2-bytes, we use the
uploader for the job. Since commit b3133e250e we do an uploader alloc
ref, which releases the uploader buffer if there is no enough space,
creating a new one.

The problem happens when we also need this buffer because it is the one
containing the index buffer to convert. This happens, for instance, if
we need to convert the primitives because they are not supported (e.g.,
converting quads to triangles), as this is done
also using the uploader.

The solution is to ensure the uploader's buffer has an extra reference
so when released, it is not destroyed. This can easily achieved by
calling first pipe_buffer_map_range(), which is required to access the
buffer, and it increases the references.

This fixes `spec@!opengl 1.1@longprim`.

Fixes: b3133e250e ("gallium: add pipe_context::resource_release to eliminate buffer refcounting")
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
(cherry picked from commit 48c086cb42)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40752>

.pick_status.json

@@ -4,7 +4,7 @@
         "description": "vc4: fix unwanted buffer release on uploader",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "b3133e250e1c40496d98c4ba52386b7ae423d194",
         "notes": null

src/broadcom/ci/broadcom-rpi3-fails.txt

@@ -765,9 +765,6 @@ spec@glsl-1.10@execution@glsl-vs-inline-explosion,Crash
 # stipple
 spec@!opengl 1.0@gl-1.0-no-op-paths,Fail
 
-# Bisected to b3133e250e1 ("gallium: add pipe_context::resource_release to eliminate buffer refcounting")
-spec@!opengl 1.1@longprim,Crash
-
 # fails on arm64, passes on armhf
 spec@arb_depth_buffer_float@depthstencil-render-miplevels 1024 s=z24_s8_d=z32f,Fail
 

src/gallium/drivers/vc4/vc4_resource.c

@@ -1148,12 +1148,6 @@ vc4_get_shadow_index_buffer(struct pipe_context *pctx,
         struct vc4_resource *orig = vc4_resource(info->index.resource);
         perf_debug("Fallback conversion for %d uint indices\n", count);
 
-        void *data;
-        struct pipe_resource *shadow_rsc = NULL;
-        u_upload_alloc_ref(vc4->uploader, 0, count * 2, 4,
-                       shadow_offset, &shadow_rsc, &data);
-        uint16_t *dst = data;
-
         struct pipe_transfer *src_transfer = NULL;
         const uint32_t *src;
         if (info->has_user_indices) {
@@ -1165,6 +1159,20 @@ vc4_get_shadow_index_buffer(struct pipe_context *pctx,
                                             PIPE_MAP_READ, &src_transfer);
         }
 
+        /* We need to do the upload alloc ref after the
+         * pipe_buffer_map_range() because there is a risk that the alloc
+         * frees and destroy its internal buffer, which might be the original
+         * base buffer we want to copy. Calling it afterwards, we guarantee
+         * that pipe_buffer_map_range() increases the reference counter so if
+         * upload alloc ref needs to unreference it, the buffer doesn't reach
+         * 0 refcounts and thus it won't be destroyed.
+         */
+        void *data;
+        struct pipe_resource *shadow_rsc = NULL;
+        u_upload_alloc_ref(vc4->uploader, 0, count * 2, 4,
+                       shadow_offset, &shadow_rsc, &data);
+        uint16_t *dst = data;
+
         for (int i = 0; i < count; i++) {
                 uint32_t src_index = src[i];
                 assert(src_index <= 0xffff);