**What are push constants?**
Push constants is a way to quickly provide a small amount of uniform data to shaders.
It should be much quicker than UBOs but a huge limitation is the size of data - spec
requires 128 bytes to be available for a push constant range.
**What are the challenges with push constants?**
The challenge with push constants is that the limited available size. According to
the Vulkan spec each platform should at least have 128 bytes reserved for push
constants. Current Mesa/AMD drivers supports 256 bytes, but Mesa/Intel is only 128
bytes.
**What is our solution?**
Some shaders of Blender uses more than these boundaries. When more data is needed
push constants will not be used, but the shader will be patched to use an uniform
buffer instead. This mechanism will be part of the Vulkan backend and shader
developers should not see any difference on API level.
**Known limitations**
Current state of the vulkan backend does not track resources that are in the
command queue. This patch includes some test cases that identified this issue as
well. See #104771.
Pull Request #104880
Descriptor set locations are now determined in the
VKShaderInterface. Issues with the previous solution:
- Due to legacy code in GPU module the locations/bindings
must be the same. Using one for something else might
result in undesired lookups, incorrect resource
bindings.
- Images/Textures reuses the same namespace, that didn't
work as expected when looking up the resources via
its binding.
This refactoring is required for adding support for
push constants.
Pull Request #105073
This patch adds initial support for compute shaders to
the vulkan backend. As the development is oriented to the test-
cases we have the implementation is limited to what is used there.
It has been validated that with this patch that the following test
cases are running as expected
- `GPUVulkanTest.gpu_shader_compute_vbo`
- `GPUVulkanTest.gpu_shader_compute_ibo`
- `GPUVulkanTest.gpu_shader_compute_ssbo`
- `GPUVulkanTest.gpu_storage_buffer_create_update_read`
- `GPUVulkanTest.gpu_shader_compute_2d`
This patch includes:
- Allocating VkBuffer on device.
- Uploading data from CPU to VkBuffer.
- Binding VkBuffer as SSBO to a compute shader.
- Execute compute shader and altering VkBuffer.
- Download the VkBuffer to CPU ram.
- Validate that it worked.
- Use device only vertex buffer as SSBO
- Use device only index buffer as SSBO
- Use device only image buffers
GHOST API has been changed as the original design was created before
we even had support for compute shaders in blender. The function
`GHOST_getVulkanBackbuffer` has been separated to retrieve the command
buffer without a backbuffer (`GHOST_getVulkanCommandBuffer`). In order
to do correct command buffer processing we needed access to the queue
owned by GHOST. This is returned as part of the `GHOST_getVulkanHandles`
function.
Open topics (not considered part of this patch)
- Memory barriers & command buffer encoding
- Indirect compute dispatching
- Rest of the test cases
- Data conversions when requested data format is different than on device.
- GPUVulkanTest.gpu_shader_compute_1d is supported on AMD devices.
NVIDIA doesn't seem to support 1d textures.
Pull-request: #104518
