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Signed-off-by: Mary Guillemard <mary@mary.zone> Reviewed-by: Mel Henning <mhenning@darkrefraction.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27196>
66 lines
3.4 KiB
ReStructuredText
66 lines
3.4 KiB
ReStructuredText
ISBE Layout
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=============================================
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The ISBE is a piece of staging memory shared between the :abbr:`PE (Primitive Engine)`
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and :abbr:`SM (Streaming Multiprocessor)` containing the attributes for
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:abbr:`VTG (Vertex, Tesselation, Geometry)` stages.
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In normal cases, no shader needs to access it and instead use ``ALD`` or ``AST``
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but in some some cases (like ``ALD`` vtx handling or mesh shaders) direct access to
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it is necessary.
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This document the observations made about the layout of it.
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The ISBE "space" is separated in multiple regions (map, patch, primitive and attribute)
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and can be accessed with ``ISBERD`` and ``ISBEWR`` instructions.
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There is always two ISBE "space" present: one for inputs and one for outputs.
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Input and output ISBE memory can be shared by setting bit 25 of the
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:abbr:`SPH (Shader Program Header)`.
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**NOTE**: Before Turing, only ``ISBERD`` was supported with the ``.MAP`` flag.
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Map region
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""""""""""
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The Map region contains all the vertex indices for all primitives.
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When used as output, it starts with the primitives count.
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==================================================== ==================== ===================================================
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Byte Range Name Note
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==================================================== ==================== ===================================================
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0x0 primitive_count 32-bit, Only present as output ISBE
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0x0?..(0x0? + primitive_count * $vtx_per_prim_count) primitive_indices[i] 8-bit, Offset is 0x04 as output ISBE otherwise 0x00
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==================================================== ==================== ===================================================
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Attribute region
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""""""""""""""""
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The Attribute region contains all attributes and is allocated based on the
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:abbr:`SPH (Shader Program Header)` input/output mask and :doc:`mesh shader methods <mesh_shading_notes>`.
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With the ``SKEW`` flag applied on ``ISBERD`` and ``ISBEWR``, each type of attribute is packed with
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32 values of the same type forming "memory lines" of 128 bytes each.
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Additionally, the order of the attribute packs is determined by the unique attribute id.
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(see `nak_attr` or :abbr:`SPH (Shader Program Header)` headers definitions for the values)
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Finally, if more than 32 values are needed, the layout repeat itself.
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Here is an example for 256 vertices being defined with ``ATTR_POINT_SIZE`` (0x6C) and
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``ATTR_POSITION_X`` (0x70) active:
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==================================================== ========================
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Byte Range Name
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==================================================== ========================
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0x000..0x080 ATTR_POINT_SIZE[0..31]
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0x000..0x100 ATTR_POSITION_X[0..31]
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0x180..0x200 ATTR_POINT_SIZE[32..63]
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0x200..0x280 ATTR_POSITION_X[32..63]
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0x280..0x300 ATTR_POINT_SIZE[63..95]
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0x300..0x380 ATTR_POSITION_X[63..95]
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... ...
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0x700..0x780 ATTR_POINT_SIZE[224..255]
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0x780..0x800 ATTR_POSITION_X[224..255]
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==================================================== ========================
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