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docs/drivers/nvk/isbe_layout.rst

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