VK_KHR_depth_stencil_resolve support on Turnip

Posted by Samuel Iglesias on March 17, 2021

Since last year, I have been working on Turnip driver development as my daily job at Igalia. One of those tasks was implementing the support for VK_KHR_depth_stencil_resolve extension.

VK_KHR_depth_stencil_resolve

This extension adds support for automatically resolving multisampled depth/stencil attachments in a subpass in a similar manner as for color attachments. This extension, however, does not add support for resolving msaa depth/stencil images with vkCmdResolveImage() command.

As you can imagine, this extension is easy to use by any application. Unless you are using a driver that supports Vulkan 1.2 or higher (VK_KHR_depth_stencil_resolve was promoted to core in Vulkan 1.2), you should check first if it is supported. Once done that, ask the driver which features are supported by extending VkPhysicalDeviceProperties2 with VkPhysicalDeviceDepthStencilResolveProperties structure when calling vkGetPhysicalDeviceProperties2().

struct VkPhysicalDeviceDepthStencilResolveProperties {
    VkStructureType       sType;
    void*                 pNext;
    VkResolveModeFlags    supportedDepthResolveModes;
    VkResolveModeFlags    supportedStencilResolveModes;
    VkBool32              independentResolveNone;
    VkBool32              independentResolve;
}

This structure will be filled by the driver to indicate the different depth/stencil resolve modes supported by the driver (more info about their meaning and possible values in the spec).

Next step: just fill a VkSubpassDescriptionDepthStencilResolve struct with the resolve mode wanted and the depth/stencil attachment used for resolve. Then, extend the VkSubpassDescription2 struct with it. And that’s all.

struct VkSubpassDescriptionDepthStencilResolve {
    VkStructureType                  sType;
    const void*                      pNext;
    VkResolveModeFlagBits            depthResolveMode;
    VkResolveModeFlagBits            stencilResolveMode;
    const VkAttachmentReference2*    pDepthStencilResolveAttachment;
}

Turnip implementation

Implementing this extension on Turnip was more or less direct, although there were some issues to fix.

For all depth/stencil formats, it was added its support in the both resolve paths used by the driver, one for sysmem (system memory) and other for gmem (tile buffer), including flushing the depth cache when needed. However, for VK_FORMAT_D32_SFLOAT_S8_UINT format, which has the stencil part in a separate plane, it was needed to add specific code to extend the 2D path used in the driver for sysmem resolves.

However the main issue when was testing the extension implementation on Adreno 630 GPU. It turns out that VK-GL-CTS tests exercising this extension for MSAA VK_FORMAT_D24_UNORM_S8_UINT formats were failing always, except when disabling UBWC via TU_DEBUG=noubwc environment variable. UBWC (Universal Bandwidth Compression) is a HW feature designed to improve throughput to system memory by minimizing the bandwidth of data (which also gives some power savings). The problem was that the UBWC support for MSAA VK_FORMAT_D24_UNORM_S8_UINT format is known to be failing on Adreno 630 and Adreno 618 (see merge request for freedreno driver). I just needed to disable it in Turnip to fix these failures.

I also found other VK_KHR_depth_stencil_resolve CTS tests failing: the ones testing the format compatibility for VK_FORMAT_D32_SFLOAT_S8_UINT and VK_FORMAT_D24_UNORM_S8_UINT formats. For VK_FORMAT_D32_SFLOAT_S8_UINT failures, it was needed to take into account the particularity that it has a separate plane for the stencil part when resolving it to VK_FORMAT_S8_UINT. In the VK_FORMAT_D24_UNORM_S8_UINT failures, the problem was that we were setting wrongly the resolve mode used by the HW: it was wrongly doing a sample average, when we wanted to use the value of sample 0. This merge request fixed both issues.

And that’s all, this was a extension that allowed me to dive into the different resolve paths used by Turnip and learn one or two things about the HW ;-) Thanks a lot to Jonathan Marek for his reviews and suggestions to improve the implementation of this extension.

Happy hacking!