39 lines
2.0 KiB
ReStructuredText
39 lines
2.0 KiB
ReStructuredText
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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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