Description
Modern GPUs typically have a (relatively small) memory region which is both: vk::MemoryPropertyFlagBits::eDeviceLocal | vk::MemoryPropertyFlagBits::eHostCoherent. See the following output of context().print_available_memory_types();:
INFO: ========== MEMORY PROPERTIES OF DEVICE 'NVIDIA GeForce RTX 3070' | file[avk.cpp] line[118]
INFO: ------------------------------------------------------------- | file[avk.cpp] line[119]
INFO: HEAP TYPES: | file[avk.cpp] line[120]
INFO: heap-idx | bytes | heap flags | file[avk.cpp] line[121]
INFO: ------------------------------------------------------------- | file[avk.cpp] line[122]
INFO: 0 | 8,433,696,768 | { DeviceLocal } | file[avk.cpp] line[139]
INFO: 1 | 17,141,145,600 | {} | file[avk.cpp] line[139]
INFO: 2 | 224,395,264 | { DeviceLocal } | file[avk.cpp] line[139]
INFO: ============================================================= | file[avk.cpp] line[141]
INFO: MEMORY TYPES: | file[avk.cpp] line[142]
INFO: mem-idx | heap-idx | memory propety flags | file[avk.cpp] line[143]
INFO: ------------------------------------------------------------- | file[avk.cpp] line[144]
INFO: 0 | 1 | {} | file[avk.cpp] line[154]
INFO: 1 | 0 | { DeviceLocal } | file[avk.cpp] line[154]
INFO: 2 | 1 | { HostVisible | HostCoherent } | file[avk.cpp] line[154]
INFO: 3 | 1 | { HostVisible | HostCoherent | HostCached } | file[avk.cpp] line[154]
INFO: 4 | 2 | { DeviceLocal | HostVisible | HostCoherent } | file[avk.cpp] line[154]
INFO: ============================================================= | file[avk.cpp] line[156]
The problem is just, that enum struct memory_usage only supports device OR host_coherent, but not the combination of both.
Furthermore, the framework does probably disregard which memory region exactly a memory_usage::device is allocated from (in the example above, the options would be memory regions at heap indices 0 or 2, but the framework does probably not really care which one it chooses -- but it should!)
memory_usage::device should allocate from the memory region that has the flag vk::MemoryPropertyFlagBits::eDeviceLocal, but does not have vk::MemoryPropertyFlagBits::eHostCoherent! Furthermore, this flag should probably be renamed into memory_usage::device_local!
memory_usage::host_coherent should allocate from the memory region that has the flag vk::MemoryPropertyFlagBits::eHostCoherent, but does not have vk::MemoryPropertyFlagBits::eDeviceLocal!
And a new memory_usage::device_local_and_host_coherent should allocate from the memory region that has both flags: vk::MemoryPropertyFlagBits::eDeviceLocal | vk::MemoryPropertyFlagBits::eHostCoherent. The question is just if there is a better name for this. (I think, there was a dedicated name for this memory region, but I can't remember.)