[FFmpeg-cvslog] avutil/hwcontext_vulkan: disable host transfers if ReBAR is disabled

[Niklas Haas]

ffmpeg | branch: master | Niklas Haas <git at haasn.dev> | Fri Jun 20 14:21:51 2025 +0200| [27590d86534b0c94f12b9a206410f475fb008e72] | committer: Niklas Haas

avutil/hwcontext_vulkan: disable host transfers if ReBAR is disabled

This feature fundamentally relies on host-visible VRAM, which restricts the
set of available memory types to (typically) host-visible device-local ones.

When resizable BAR is disabled, this memory type is usually limited to
e.g. 256 MiB in size, which is just plain insufficient for allocation of
general purpose GPU images, causing OOM errors on even the simplest of
commands.

The easiest solution is to disable host transfers entirely on machines
without host-addressable VRAM. In theory, we could try and recover the use
of host transfers for images which are *not* restricted to device-local
memory types, but this is rarely the case in practice, and the effort
required would exceed the benefit, especially since ReBAR is a standard
feature on all platforms recent enough to have Vulkan drivers, and only
occasionally disabled in the UEFI for by default for some hare-brained
notion of "backwards compatibiility" with ancient software.

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=27590d86534b0c94f12b9a206410f475fb008e72
---

 libavutil/hwcontext_vulkan.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/libavutil/hwcontext_vulkan.c b/libavutil/hwcontext_vulkan.c
index edff89af26..b0a765e370 100644
--- a/libavutil/hwcontext_vulkan.c
+++ b/libavutil/hwcontext_vulkan.c
@@ -144,6 +144,9 @@ typedef struct VulkanDevicePriv {
     /* Disable multiplane images */
     int disable_multiplane;
 
+    /* Disable host image transfer */
+    int disable_host_transfer;
+
     /* Maximum queues */
     int limit_queues;
 
@@ -1694,6 +1697,23 @@ static int vulkan_device_create_internal(AVHWDeviceContext *ctx,
         goto end;
     }
 
+    /* Get device memory properties */
+    vk->GetPhysicalDeviceMemoryProperties(hwctx->phys_dev, &p->mprops);
+    VkDeviceSize max_vram = 0, max_visible_vram = 0;
+    for (int i = 0; i < p->mprops.memoryTypeCount; i++) {
+        const VkMemoryType type = p->mprops.memoryTypes[i];
+        const VkMemoryHeap heap = p->mprops.memoryHeaps[type.heapIndex];
+        if (!(type.propertyFlags & VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT))
+            continue;
+        max_vram = FFMAX(max_vram, heap.size);
+        if (type.propertyFlags & VK_MEMORY_PROPERTY_HOST_VISIBLE_BIT)
+            max_visible_vram = FFMAX(max_visible_vram, heap.size);
+    }
+
+    /* Only use host image transfers if ReBAR is enabled */
+    const int has_rebar = max_vram - max_visible_vram < 1024; /* 1 kB tolerance */
+    p->disable_host_transfer = !has_rebar;
+
     /* Get all supported features for the physical device */
     device_features_init(ctx, &supported_feats);
     vk->GetPhysicalDeviceFeatures2(hwctx->phys_dev, &supported_feats.device);
@@ -1990,9 +2010,6 @@ FF_ENABLE_DEPRECATION_WARNINGS
     if (!hwctx->unlock_queue)
         hwctx->unlock_queue = unlock_queue;
 
-    /* Get device capabilities */
-    vk->GetPhysicalDeviceMemoryProperties(hwctx->phys_dev, &p->mprops);
-
     p->vkctx.device = ctx;
     p->vkctx.hwctx = hwctx;
 
@@ -2835,7 +2852,7 @@ static int vulkan_frames_init(AVHWFramesContext *hwfc)
                                           VK_IMAGE_USAGE_STORAGE_BIT      |
                                           VK_IMAGE_USAGE_SAMPLED_BIT);
 
-        if (p->vkctx.extensions & FF_VK_EXT_HOST_IMAGE_COPY)
+        if ((p->vkctx.extensions & FF_VK_EXT_HOST_IMAGE_COPY) && !p->disable_host_transfer)
             hwctx->usage |= supported_usage & VK_IMAGE_USAGE_HOST_TRANSFER_BIT_EXT;
 
         /* Enables encoding of images, if supported by format and extensions */