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The current workaround for this hardware bug involved marking the ADD instruction used to initialize the address register as NoMask on Gen12, which was based on the assumption that the problem was caused by a hardware bug affecting the application of the execution mask to the address register write. However that doesn't seem to be the case: The address register write was working correctly, the real problem leading to hangs on TGL is that the indirect addressing logic is unable to deal with garbage values in the address register (e.g. misaligned offsets), even for channels which are currently inactive due to non-uniform control flow. The current workaround isn't able to avoid that situation in general, since the result of the NoMask ADD instruction for a dead channel is calculated based on the corresponding (dead) component of the indirect_byte_offset source, which would still be undefined in the likely case that the source was initialized under control flow itself. This would lead to hangs whenever MOV_INDIRECT was used under non-uniform control flow in some scenarios like a tessellation shader from GFXBench5/gl_4 (AKA Car Chase) on TGL. In addition I've managed to reproduce the same issue on earlier platforms by initializing the whole address register with garbage before the ADD instruction, so this seems to be a long-standing issue we have avoided mostly by luck. This patch fixes the problem and applies the workaround to all platforms, since even when the hardware is able to deal with garbage address values without hanging there might be a significant performance cost from reading random GRF registers due to the useless extra EU cycles spent fetching registers for dead channels and due to the potential for unintended serialization with respect to other random instructions that could be executed in parallel, which may have had a cost of the order of hundreds of cycles in the worst case scenario. Fixes: |
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`Mesa <https://mesa3d.org>`_ - The 3D Graphics Library ====================================================== Source ------ This repository lives at https://gitlab.freedesktop.org/mesa/mesa. Other repositories are likely forks, and code found there is not supported. Build & install --------------- You can find more information in our documentation (`docs/install.html <https://mesa3d.org/install.html>`_), but the recommended way is to use Meson (`docs/meson.html <https://mesa3d.org/meson.html>`_): .. code-block:: sh $ mkdir build $ cd build $ meson .. $ sudo ninja install Support ------- Many Mesa devs hang on IRC; if you're not sure which channel is appropriate, you should ask your question on `Freenode's #dri-devel <irc://chat.freenode.net#dri-devel>`_, someone will redirect you if necessary. Remember that not everyone is in the same timezone as you, so it might take a while before someone qualified sees your question. To figure out who you're talking to, or which nick to ping for your question, check out `Who's Who on IRC <https://dri.freedesktop.org/wiki/WhosWho/>`_. The next best option is to ask your question in an email to the mailing lists: `mesa-dev\@lists.freedesktop.org <https://lists.freedesktop.org/mailman/listinfo/mesa-dev>`_ Bug reports ----------- If you think something isn't working properly, please file a bug report (`docs/bugs.html <https://mesa3d.org/bugs.html>`_). Contributing ------------ Contributions are welcome, and step-by-step instructions can be found in our documentation (`docs/submittingpatches.html <https://mesa3d.org/submittingpatches.html>`_). Note that Mesa uses gitlab for patches submission, review and discussions.