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VK_AMD_device_coherent_memory

VK_AMD_device_coherent_memory is a device extension that enables access to additional memory types with VK_MEMORY_PROPERTY_DEVICE_COHERENT_BIT_AMD and VK_MEMORY_PROPERTY_DEVICE_UNCACHED_BIT_AMD flag. It is useful mostly for allocation of buffers intended for writing "breadcrumb markers" in between passes or draw calls, which in turn are useful for debugging GPU crash/hang/TDR cases.

When the extension is available but has not been enabled, Vulkan physical device still exposes those memory types, but their usage is forbidden. VMA automatically takes care of that - it returns VK_ERROR_FEATURE_NOT_PRESENT when an attempt to allocate memory of such type is made.

If you want to use this extension in connection with VMA, follow these steps:

# Initialization

1) Call vkEnumerateDeviceExtensionProperties for the physical device. Check if the extension is supported - if returned array of VkExtensionProperties contains "VK_AMD_device_coherent_memory".

2) Call vkGetPhysicalDeviceFeatures2 for the physical device instead of old vkGetPhysicalDeviceFeatures. Attach additional structure VkPhysicalDeviceCoherentMemoryFeaturesAMD to VkPhysicalDeviceFeatures2::pNext to be returned. Check if the device feature is really supported - check if VkPhysicalDeviceCoherentMemoryFeaturesAMD::deviceCoherentMemory is true.

3) While creating device with vkCreateDevice, enable this extension - add "VK_AMD_device_coherent_memory" to the list passed as VkDeviceCreateInfo::ppEnabledExtensionNames.

4) While creating the device, also don't set VkDeviceCreateInfo::pEnabledFeatures. Fill in VkPhysicalDeviceFeatures2 structure instead and pass it as VkDeviceCreateInfo::pNext. Enable this device feature - attach additional structure VkPhysicalDeviceCoherentMemoryFeaturesAMD to VkPhysicalDeviceFeatures2::pNext and set its member deviceCoherentMemory to VK_TRUE.

5) While creating VmaAllocator with vmaCreateAllocator() inform VMA that you have enabled this extension and feature - add VMA_ALLOCATOR_CREATE_AMD_DEVICE_COHERENT_MEMORY_BIT to VmaAllocatorCreateInfo::flags.

# Usage

After following steps described above, you can create VMA allocations and custom pools out of the special DEVICE_COHERENT and DEVICE_UNCACHED memory types on eligible devices. There are multiple ways to do it, for example:

- You can request or prefer to allocate out of such memory types by adding VK_MEMORY_PROPERTY_DEVICE_UNCACHED_BIT_AMD to VmaAllocationCreateInfo::requiredFlags or VmaAllocationCreateInfo::preferredFlags. Those flags can be freely mixed with other ways of Choosing memory type, like setting VmaAllocationCreateInfo::usage.
- If you manually found memory type index to use for this purpose, force allocation from this specific index by setting VmaAllocationCreateInfo::memoryTypeBits = 1U << index.

# More information

To learn more about this extension, see VK_AMD_device_coherent_memory in Vulkan specification

Example use of this extension can be found in the code of the sample and test suite accompanying this library.