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fc1397d1d7
The optimized routine documented the tiling format together with the software algorithm. The reference implementation wants the tiling format alone documented. Let's break out the high level documentation into somewhere centrally accessible, and refocus the comments in the optimized file on the optimization. This documentation is linked bidirectionally with both implementations, so it should be easy to find. Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15803>
94 lines
4.1 KiB
ReStructuredText
94 lines
4.1 KiB
ReStructuredText
Panfrost
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========
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The Panfrost driver stack includes an OpenGL ES implementation for Arm Mali
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GPUs based on the Midgard and Bifrost microarchitectures. It is **conformant**
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on Mali G52 but **non-conformant** on other GPUs. The following hardware is
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currently supported:
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========= ============ ============ =======
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Product Architecture OpenGL ES OpenGL
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========= ============ ============ =======
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Mali T720 Midgard (v4) 2.0 2.1
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Mali T760 Midgard (v5) 3.1 3.1
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Mali T820 Midgard (v5) 3.1 3.1
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Mali T830 Midgard (v5) 3.1 3.1
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Mali T860 Midgard (v5) 3.1 3.1
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Mali T880 Midgard (v5) 3.1 3.1
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Mali G72 Bifrost (v6) 3.1 3.1
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Mali G31 Bifrost (v7) 3.1 3.1
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Mali G51 Bifrost (v7) 3.1 3.1
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Mali G52 Bifrost (v7) 3.1 3.1
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Mali G76 Bifrost (v7) 3.1 3.1
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========= ============ ============ =======
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Other Midgard and Bifrost chips (T604, T628, G71) are not yet supported.
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Older Mali chips based on the Utgard architecture (Mali 400, Mali 450) are
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supported in the Lima driver, not Panfrost. Lima is also available in Mesa.
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Other graphics APIs (Vulkan, OpenCL) are not supported at this time.
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Building
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--------
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Panfrost's OpenGL support is a Gallium driver. Since Mali GPUs are 3D-only and
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do not include a display controller, Mesa uses kmsro to support display
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controllers paired with Mali GPUs. If your board with a Panfrost supported GPU
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has a display controller with mainline Linux support not supported by kmsro,
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it's easy to add support, see the commit ``cff7de4bb597e9`` as an example.
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LLVM is *not* required by Panfrost's compilers. LLVM support in Mesa can
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safely be disabled for most OpenGL ES users with Panfrost.
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Build like ``meson . build/ -Ddri-drivers= -Dvulkan-drivers=
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-Dgallium-drivers=panfrost -Dllvm=disabled`` for a build directory
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``build``.
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For general information on building Mesa, read :doc:`the install documentation
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<../install>`.
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Chat
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----
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Panfrost developers and users hang out on IRC at ``#panfrost`` on OFTC. Note
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that registering and authenticating with `NickServ` is required to prevent
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spam. `Join the chat. <https://webchat.oftc.net/?channels=#panfrost>`_
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U-interleaved tiling
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---------------------
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Panfrost supports u-interleaved tiling. U-interleaved tiling is
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indicated by the ``DRM_FORMAT_MOD_ARM_16X16_BLOCK_U_INTERLEAVED`` modifier.
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The tiling reorders whole pixels (blocks). It does not compress or modify the
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pixels themselves, so it can be used for any image format. Internally, images
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are divided into tiles. Tiles occur in source order, but pixels (blocks) within
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each tile are reordered according to a space-filling curve.
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For regular formats, 16x16 tiles are used. This harmonizes with the default tile
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size for binning and CRCs (transaction elimination). It also means a single line
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(16 pixels) at 4 bytes per pixel equals a single 64-byte cache line.
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For formats that are already block compressed (S3TC, RGTC, etc), 4x4 tiles are
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used, where entire blocks are reorder. Most of these formats compress 4x4
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blocks, so this gives an effective 16x16 tiling. This justifies the tile size
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intuitively, though it's not a rule: ASTC may uses larger blocks.
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Within a tile, the X and Y bits are interleaved (like Morton order), but with a
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twist: adjacent bit pairs are XORed. The reason to add XORs is not obvious.
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Visually, addresses take the form::
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| y3 | (x3 ^ y3) | y2 | (y2 ^ x2) | y1 | (y1 ^ x1) | y0 | (y0 ^ x0) |
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Reference routines to encode/decode u-interleaved images are available in
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``src/panfrost/shared/test/test-tiling.cpp``, which documents the space-filling
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curve. This reference implementation is used to unit test the optimized
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implementation used in production. The optimized implementation is available in
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``src/panfrost/shared/pan_tiling.c``.
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Although these routines are part of Panfrost, they are also used by Lima, as Arm
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introduced the format with Utgard. It is the only tiling supported on Utgard. On
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Mali-T760 and newer, Arm Framebuffer Compression (AFBC) is more efficient and
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should be used instead where possible. However, not all formats are
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compressible, so u-interleaved tiling remains an important fallback on Panfrost.
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