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  • Hash : fc4fc174
    Author : Shahbaz Youssefi
    Date : 2024-12-10T22:01:28

    Vulkan: Prevent crash with D/S FF without D/S attachment
    
    The spec says that the values for gl_LastFragDepth/StencilARM are
    undefined if there is no depth/stencil attachment.  This "just" works on
    tiling GPUs, because reading input attachments simply translates to
    reading _something_ from the tile memory.
    
    For ANGLE, the situation is a little more complicated.  ANGLE has to
    bind descriptors for input attachments (because non-tilers read from the
    input attachment descriptor instead of using the knowledge that input
    and color/depth/stencil attachments are one and the same thing in tile
    memory).  When a depth/stencil attachment is missing, there is no image
    to bind to the descriptor set.
    
    ANGLE cannot skip binding an image to the descriptor set, because
    OpImageRead (translated from subpassLoad()) attempts to access the
    input descriptor; skipping this causes an internal crash in SwiftShader
    for example.
    
    ANGLE cannot bind a bogus image as input attachment, as Vulkan requires
    that input attachments are also color/depth/stencil attachments.
    
    ANGLE _could_ bind a bogus image as input attachment and also as
    depth/stencil attachment.  This is rather risky, as it then also has to
    be careful to make sure that depth/stencil attachment is never actually
    used (i.e. it affects the depth/stencil state, load/store ops etc).
    
    In this change, the shader itself is modified to remove references to
    the depth/stencil input attachments if the attachment is missing.  This
    is rather inefficient, as it means the pipeline warmup will not produce
    a usable pipeline, but it's accepted as a workaround for something apps
    shouldn't really be doing.
    
    Bug: angleproject:376572258
    Change-Id: I0de68252b61615cb82cba7d1730699aadf41e92f
    Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/6085368
    Reviewed-by: Charlie Lao <cclao@google.com>
    Reviewed-by: Yuxin Hu <yuxinhu@google.com>
    Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
    

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    Description

    A conformant OpenGL ES implementation for Windows, Mac, Linux, iOS and Android.

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  • README.md

  • ANGLE: Vulkan Back-end

    ANGLE’s Vulkan back-end implementation lives in this folder.

    Vulkan is an explicit graphics API. Compared to APIs like OpenGL or D3D11 explicit APIs can offer a number of significant benefits:

    • Lower API call CPU overhead.
    • A smaller API surface with more direct hardware control.
    • Better support for multi-core programming.
    • Vulkan in particular has open-source tooling and tests.

    Back-end Design

    The vk::Renderer class represents an EGLDisplay. vk::Renderer owns shared global resources like the VkDevice, VkQueue, the Vulkan format tables and internal Vulkan shaders. The ContextVk class implements the back-end of a front-end OpenGL Context. ContextVk processes state changes and handles action commands like glDrawArrays and glDrawElements.

    Command recording

    A render pass has three states: unstarted, started and active (we call it active in short), started but inactive (we call it inactive in short). The back-end records commands into command buffers via the following ContextVk APIs:

    • beginNewRenderPass: Writes out (aka flushes) prior pending commands into a primary command buffer, then starts a new render pass. Returns a secondary command buffer inside a render pass instance.
    • getOutsideRenderPassCommandBuffer: May flush prior command buffers and close the render pass if necessary, in addition to issuing the appropriate barriers. Returns a secondary command buffer outside a render pass instance.
    • getStartedRenderPassCommands: Returns a reference to the currently open render pass’ commands buffer.
    • onRenderPassFinished: Puts render pass into inactive state where you can not record more commands into secondary command buffer, except in some special cases where ANGLE does some optimization internally.
    • flushCommandsAndEndRenderPassWithoutSubmit: Marks the end of render pass. It flushes secondary command buffer into vulkan’s primary command buffer, puts secondary command buffer back to unstarted state and then goes into recycler for reuse.

    The back-end (mostly) records Image and Buffer barriers through additional CommandBufferAccess APIs, the result of which is passed to getOutsideRenderPassCommandBuffer. Note that the barriers are not actually recorded until getOutsideRenderPassCommandBuffer is called:

    • onBufferTransferRead and onBufferComputeShaderRead accumulate VkBuffer read barriers.
    • onBufferTransferWrite and onBufferComputeShaderWrite accumulate VkBuffer write barriers.
    • onBuffferSelfCopy is a special case for VkBuffer self copies. It behaves the same as write.
    • onImageTransferRead and onImageComputerShadeRead accumulate VkImage read barriers.
    • onImageTransferWrite and onImageComputerShadeWrite accumulate VkImage write barriers.
    • onImageRenderPassRead and onImageRenderPassWrite accumulate VkImage barriers inside a started RenderPass.

    After the back-end records commands to the primary buffer and we flush (e.g. on swap) or when we call vk::Renderer::finishQueueSerial, ANGLE submits the primary command buffer to a VkQueue.

    See the code for more details.

    Simple command recording example

    In this example we’ll be recording a buffer copy command:

        // Ensure that ANGLE sets proper read and write barriers for the Buffers.
        vk::CommandBufferAccess access;
        access.onBufferTransferWrite(dstBuffer);
        access.onBufferTransferRead(srcBuffer);
    
        // Get a pointer to a secondary command buffer for command recording.
        vk::OutsideRenderPassCommandBuffer *commandBuffer;
        ANGLE_TRY(contextVk->getOutsideRenderPassCommandBuffer(access, &commandBuffer));
    
        // Record the copy command into the secondary buffer. We're done!
        commandBuffer->copyBuffer(srcBuffer->getBuffer(), dstBuffer->getBuffer(), copyCount, copies);

    Additional Reading

    More implementation details can be found in the doc directory: