texture_cache: keep the allocator's block slack out of the usable VRAM

The collector's usage leaves out the unused part of VMA's blocks, but
the driver counts it and a heap at its budget cannot give it to a new
image. On an RX 7800 XT that slack was 1.06 GiB, the whole 1 GiB margin:
the heap sat at its budget for a minute while the collector saw room,
then images stopped fitting and the process died. Reserve the slack
(capped at 1.5 GiB) on top of the margin: at once when it grows, slowly
when it shrinks, so eviction leaving partly used blocks doesn't chase
the threshold down.

Assisted-by: Claude Sonnet 5.5 (diagnosis and code)
(cherry picked from commit 73fd5a489ea28226fa16a55f5008e622d631b54c)

2 files changed, 6 insertions, 0 deletions

diff --git a/src/video_core/renderer_vulkan/vk_texture_cache.cpp b/src/video_core/renderer_vulkan/vk_texture_cache.cpp
index 09e7961ff9..cb0453c178 100644
--- a/src/video_core/renderer_vulkan/vk_texture_cache.cpp
+++ b/src/video_core/renderer_vulkan/vk_texture_cache.cpp
@@ -2082,6 +2082,10 @@ u64 TextureCacheRuntime::GetDeviceMemoryBudget() const {
     return device.GetDeviceMemoryBudget();
 }
 
+u64 TextureCacheRuntime::GetDeviceMemorySlack() const {
+    return device.GetDeviceMemorySlack();
+}
+
 bool TextureCacheRuntime::CanDownloadMsaa(const VideoCommon::ImageInfo& info) const {
     const VkImageAspectFlags aspect_mask = ImageAspectMask(info.format);
     if ((aspect_mask & VK_IMAGE_ASPECT_COLOR_BIT) != 0) {
diff --git a/src/video_core/renderer_vulkan/vk_texture_cache.h b/src/video_core/renderer_vulkan/vk_texture_cache.h
index 5ac833e787..d5f8756486 100644
--- a/src/video_core/renderer_vulkan/vk_texture_cache.h
+++ b/src/video_core/renderer_vulkan/vk_texture_cache.h
@@ -74,6 +74,8 @@ public:
 
     u64 GetDeviceMemoryBudget() const;
 
+    u64 GetDeviceMemorySlack() const;
+
     bool CanReportMemoryUsage() const;
 
     /// Images that did not fit the device memory budget so far (see MemoryAllocator).