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Render Passes
=============
The Vulkan runtime code in Mesa provides several helpful utilities to make
managing render passes easier.
:ext:`VK_KHR_create_renderpass2`
--------------------------------
It is strongly recommended that drivers implement
:ext:`VK_KHR_create_renderpass2` directly and not bother implementing the
old Vulkan 1.0 entrypoints. If a driver does not implement them, the
following will be implemented in common code in terms of their
:ext:`VK_KHR_create_renderpass2` counterparts:
- :cpp:func:`vkCreateRenderPass`
- :cpp:func:`vkCmdBeginRenderPass`
- :cpp:func:`vkCmdNextSubpass`
- :cpp:func:`vkCmdEndRenderPass`
Common VkRenderPass implementation
----------------------------------
The Vulkan runtime code in Mesa provides a common implementation of
:cpp:type:`VkRenderPass` called :cpp:struct:`vk_render_pass` which drivers
can optionally use. Unlike most Vulkan runtime structs, it's not really
designed to be used as a base for a driver-specific struct. It does,
however, contain all the information passed to
:cpp:func:`vkCreateRenderPass2` so it can be used in a driver so long as
that driver doesn't need to do any additional compilation at
:cpp:func:`vkCreateRenderPass2` time. If a driver chooses to use
:cpp:struct:`vk_render_pass`, the Vulkan runtime provides implementations
of :cpp:func:`vkCreateRenderPass2` and :cpp:func:`vkDestroyRenderPass`.
:ext:`VK_KHR_dynamic_rendering`
-------------------------------
For drivers which don't need to do subpass combining, it is recommended
that they skip implementing render passes entirely and implement
:ext:`VK_KHR_dynamic_rendering` instead. If they choose to do so, the runtime
will provide the following, implemented in terms of
:cpp:func:`vkCmdBeginRendering` and :cpp:func:`vkCmdEndRendering`:
- :cpp:func:`vkCmdBeginRenderPass2`
- :cpp:func:`vkCmdNextSubpass2`
- :cpp:func:`vkCmdEndRenderPass2`
We also provide a no-op implementation of
:cpp:func:`vkGetRenderAreaGranularity` which returns a render area
granularity of 1x1.
Drivers which wish to use the common render pass implementation in this way
**must** also support a Mesa-specific pseudo-extension which optionally
provides an initial image layout for each attachment at
:cpp:func:`vkCmdBeginRendering` time. This is required for us to combine
render pass clears with layout transitions, often from
:cpp:enum:`VK_IMAGE_LAYOUT_UNDEFINED`. On at least Intel and AMD,
combining these transitions with clears is important for performance.
.. doxygenstruct:: VkRenderingAttachmentInitialLayoutInfoMESA
:members:
Because render passes and subpass indices are also passed into
:cpp:func:`vkCmdCreateGraphicsPipelines` and
:cpp:func:`vkCmdExecuteCommands` which we can't implement on the driver's
behalf, we provide a couple of helpers for getting the render pass
information in terms of the relevant :ext:`VK_KHR_dynamic_rendering`:
.. doxygenfunction:: vk_get_pipeline_rendering_create_info
.. doxygenfunction:: vk_get_command_buffer_inheritance_rendering_info
Apart from handling layout transitions, the common render pass
implementation mostly ignores input attachments. It is expected that the
driver call :cpp:func:`nir_lower_input_attachments` to turn them into
texturing operations. The driver **must** support texturing from an input
attachment at the same time as rendering to it in order to support Vulkan
subpass self-dependencies. ``VK_EXT_attachment_feedback_loop_layout`` provides
information on when these self dependencies are present.
vk_render_pass reference
------------------------
The following is a reference for the :cpp:struct:`vk_render_pass` structure
and its substructures.
.. doxygenstruct:: vk_render_pass
:members:
.. doxygenstruct:: vk_render_pass_attachment
:members:
.. doxygenstruct:: vk_subpass
:members:
.. doxygenstruct:: vk_subpass_attachment
:members:
.. doxygenstruct:: vk_subpass_dependency
:members:
|
