From 106cc98380277499fbf43136acdcc31f97ae11e1 Mon Sep 17 00:00:00 2001 From: spencer-lunarg Date: Fri, 10 Oct 2025 11:26:10 -0400 Subject: [PATCH] More Descriptor Buffer --- chapters/descriptor_buffer.adoc | 82 ++++++++++++++++++++++++ chapters/images/descriptor_buffer_10.svg | 4 ++ chapters/images/descriptor_buffer_9.svg | 4 ++ 3 files changed, 90 insertions(+) create mode 100644 chapters/images/descriptor_buffer_10.svg create mode 100644 chapters/images/descriptor_buffer_9.svg diff --git a/chapters/descriptor_buffer.adoc b/chapters/descriptor_buffer.adoc index eacce32..9fb4685 100644 --- a/chapters/descriptor_buffer.adoc +++ b/chapters/descriptor_buffer.adoc @@ -131,6 +131,11 @@ image::{images}descriptor_buffer_2.svg[descriptor_buffer_2.svg] Using `vkGetDescriptorEXT` we find a spot in the "descriptor buffer" and map it you our resources. +[NOTE] +==== +You can actually use any host memory for this, but for simplicity, we will map it directly to the descriptor buffer for now. +==== + The following code will map the 3 of the descriptors using a single resource buffer. [source,c++] @@ -253,3 +258,80 @@ image::{images}descriptor_buffer_7.svg[descriptor_buffer_7.svg] That is it, from here you can just call `vkCmdDraw` (or other action commands such as `vkCmdDispatch`) and everything should be working! image::{images}descriptor_buffer_8.svg[descriptor_buffer_8.svg] + +== Descriptor is actually just memory + +When you call `vkGetDescriptorEXT` what is really happening? The driver is actually just taking the `VkDescriptorGetInfoEXT` information and turning it into a binary blob, which even the application can read now! + +[source,c++] +---- +// Can be used to print on your machine as well +void print_bytes(const void* memory, size_t size) { + const uint8_t* bytes = (uint8_t*)memory; + printf("--- (at %p) ---\n", memory); + for (size_t i = 0; i < size; ++i) { + printf("%02X ", bytes[i]); + if ((i + 1) % 16 == 0) { + printf("\n"); + } + } + printf("\n"); +} + + +void* some_host_memory = buffer.GetMappedMemory(); +vkGetDescriptorEXT(device, get_info, descriptor_size, some_host_memory); + +print_bytes(some_host_memory, descriptor_size); + +// printf output running on Lavapipe +// This represents what a "descriptor" is as a binary blob +--- (at 0x71841c1e7240) --- +00 80 1E 1C 84 71 00 00 10 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +---- + +=== Copying the descriptor yourself + +So with this knowledge, we should now realize we can actually just `memcpy` the descriptor ourselves. + +[source,c++] +---- +void* host_memory = malloc(descriptor_size); + +VkDescriptorGetInfoEXT get_info; +get_info.type = VK_DESCRIPTOR_TYPE_STORAGE_BUFFER; +get_info.data.pStorageBuffer->range = 4; +get_info.data.pStorageBuffer->address = buffer_x_address; +vkGetDescriptorEXT(get_info, descriptor_size, host_memory); + +void* descriptor_ptr = descriptor_buffer_a.GetMappedMemory(); + +memcpy(descriptor_ptr, host_memory, descriptor_size) +---- + +image::{images}descriptor_buffer_9.svg[descriptor_buffer_9.svg] + + +=== Copying the descriptor on the GPU + +So we can go another step and make our descriptor buffer not even host visible. + +We can write the descriptor into a "staging" `VkBuffer` and then copy it on the GPU. + +[source,c++] +---- +void* staging_buffer_ptr = staging_buffer.GetMappedMemory(); + +vkGetDescriptorEXT(get_info, descriptor_size, staging_buffer_ptr); + +vkCmdCopyBuffer(srcBuffer = staging_buffer, dstBuffer = descriptor_buffer_a); +---- + +image::{images}descriptor_buffer_10.svg[descriptor_buffer_10.svg] + +=== Don't forget to add synchronization + +When copying memory into your descriptor buffer, make sure to add a barrier with `VK_ACCESS_2_DESCRIPTOR_BUFFER_READ_BIT_EXT` to ensure that any writes will be visible by the GPU when it needs to access it! \ No newline at end of file diff --git a/chapters/images/descriptor_buffer_10.svg b/chapters/images/descriptor_buffer_10.svg new file mode 100644 index 0000000..2bcda3d --- /dev/null +++ b/chapters/images/descriptor_buffer_10.svg @@ -0,0 +1,4 @@ + + + +
Descriptor Buffer A
Not HOST_VISIBLE
0x1000
Resource Buffer X
0x1040
0
12
4
8
binary blob representation
VkBuffer
HOST_VISIBLE
Driver creates this via vkGetDescriptorEXT
vkCmdCopyBuffer
 \ No newline at end of file diff --git a/chapters/images/descriptor_buffer_9.svg b/chapters/images/descriptor_buffer_9.svg new file mode 100644 index 0000000..6a97455 --- /dev/null +++ b/chapters/images/descriptor_buffer_9.svg @@ -0,0 +1,4 @@ + + + +
Descriptor Buffer A
0x1000
Resource Buffer X
0x1040
0
12
4
8
binary blob representation
host_memory = malloc()
Driver creates this via vkGetDescriptorEXT
memcpy
 \ No newline at end of file