nvk/cmd_pool: NVK_DEBUG=trash_mem for alloc_mem

These buffers get recycled, so we can't rely on their contents.
Unfortunately, conditional rendering was relying on these buffers being
all zero, implicitly, because we didn't fully understand the hardware.
By supporting this debug flag here, we add a way to check for code that
accidentally assumes zero-initialization.

Reviewed-by: Faith Ekstrand <faith.ekstrand@collabora.com>
Reviewed-by: Mary Guillemard <mary@mary.zone>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/37153>

src/nouveau/vulkan/nvk_cmd_pool.c

@@ -129,7 +129,10 @@ VkResult
 nvk_cmd_pool_alloc_mem(struct nvk_cmd_pool *pool, bool force_gart,
                        struct nvk_cmd_mem **mem_out)
 {
+   const struct nvk_device *dev = nvk_cmd_pool_device(pool);
+   const struct nvk_physical_device *pdev = nvk_device_physical(dev);
    struct nvk_cmd_mem *mem = NULL;
+
    if (force_gart) {
       if (!list_is_empty(&pool->free_gart_mem))
          mem = list_first_entry(&pool->free_gart_mem, struct nvk_cmd_mem, link);
@@ -137,13 +140,21 @@ nvk_cmd_pool_alloc_mem(struct nvk_cmd_pool *pool, bool force_gart,
       if (!list_is_empty(&pool->free_mem))
          mem = list_first_entry(&pool->free_mem, struct nvk_cmd_mem, link);
    }
+
    if (mem) {
       list_del(&mem->link);
-      *mem_out = mem;
-      return VK_SUCCESS;
+   } else {
+      VkResult result = nvk_cmd_mem_create(pool, force_gart, &mem);
+      if (result != VK_SUCCESS)
+         return result;
    }
 
-   return nvk_cmd_mem_create(pool, force_gart, mem_out);
+   if (unlikely(pdev->debug_flags & NVK_DEBUG_TRASH_MEMORY)) {
+      memset(mem->mem->map, 0xF1, mem->mem->size_B);
+   }
+
+   *mem_out = mem;
+   return VK_SUCCESS;
 }
 
 VkResult