Intel Surface Layout

Introduction
============
isl is a small library that calculates the layout of Intel GPU surfaces, queries
those layouts, and queries the properties of surface formats.


Independence from User APIs
===========================
isl's API is independent of any user-facing graphics API, such as OpenGL and
Vulkan. This independence allows isl to be used a shared component by multiple
Intel drivers.

Rather than mimic the user-facing APIs, the isl API attempts to reflect Intel
hardware: the actual memory layout of Intel GPU surfaces and how one programs
the GPU to use those surfaces. For example:

  - The tokens of `enum isl_format` (such as `ISL_FORMAT_R8G8B8A8_UNORM`)
    match those of the hardware enum `SURFACE_FORMAT` rather than the OpenGL
    or Vulkan format tokens.  And the values of `isl_format` and
    `SURFACE_FORMAT` are identical.

  - The OpenGL and Vulkan APIs contain depth and stencil formats. However the
    hardware enum `SURFACE_FORMAT` does not, and therefore neither does `enum
    isl_format`. Rather than define new pixel formats that have no hardware
    counterpart, isl records the intent to use a surface as a depth or stencil
    buffer with the usage flags `ISL_SURF_USAGE_DEPTH_BIT` and
    `ISL_SURF_USAGE_STENCIL_BIT`.

  - `struct isl_surf` distinguishes between the surface's logical dimension
    from the user API's perspective (`enum isl_surf_dim`, which may be 1D, 2D,
    or 3D) and the layout of those dimensions in memory (`enum isl_dim_layout`).


Surface Units
=============

Intro
-----
ISL takes care in its equations to correctly handle conversion among surface
units (such as pixels and compression blocks) and to carefully distinguish
between a surface's logical layout in the client API and its physical layout
in memory.

Symbol names often explicitly declare their unit with a suffix:

   - px: logical pixels
   - sa: physical surface samples
   - el: physical surface elements
   - sa_rows: rows of physical surface samples
   - el_rows: rows of physical surface elements

Logical units are independent of hardware generation and are closely related
to the user-facing API (OpenGL and Vulkan). Physical units are dependent on
hardware generation and reflect the surface's layout in memory.

Definitions
-----------
- Logical Pixels (px):

  The surface's layout from the perspective of the client API (OpenGL and
  Vulkan) is in units of logical pixels. Logical pixels are independent of the
  surface's layout in memory.

  A surface's width and height, in units of logical pixels, is not affected by
  the surface's sample count. For example, consider a VkImage created with
  VkImageCreateInfo{width=w0, height=h0, samples=s0}. The surface's width and
  height at level 0 is, in units of logical pixels, w0 and h0 regardless of
  the value of s0.

  For example, the logical array length of a 3D surface is always 1, even on
  Gen9 where the surface's memory layout is that of an array surface
  (ISL_DIM_LAYOUT_GEN4_2D).

- Physical Surface Samples (sa):

  For a multisampled surface, this unit has the obvious meaning.
  A singlesampled surface, from ISL's perspective, is simply a multisampled
  surface whose sample count is 1.

  For example, consider a 2D single-level non-array surface with samples=4,
  width_px=64, and height_px=64 (note that the suffix 'px' indicates logical
  pixels). If the surface's multisample layout is ISL_MSAA_LAYOUT_INTERLEAVED,
  then the extent of level 0 is, in units of physical surface samples,
  width_sa=128, height_sa=128, depth_sa=1, array_length_sa=1. If
  ISL_MSAA_LAYOUT_ARRAY, then width_sa=64, height_sa=64, depth_sa=1,
  array_length_sa=4.

- Physical Surface Elements (el):

  This unit allows ISL to treat compressed and uncompressed formats
  identically in many calculations.

  If the surface's pixel format is compressed, such as ETC2, then a surface
  element is equivalent to a compression block. If uncompressed, then
  a surface element is equivalent to a surface sample. As a corollary, for
  a given surface a surface element is at least as large as a surface sample.

Errata
------
ISL acquired the term 'surface element' from the Broadwell PRM [1], which
defines it as follows:

   An element is defined as a pixel in uncompresed surface formats, and as
   a compression block in compressed surface formats. For MSFMT_DEPTH_STENCIL
   type multisampled surfaces, an element is a sample.


References
==========
[1]: Broadwell PRM >> Volume 2d: Command Reference: Structures >>
     RENDER_SURFACE_STATE Surface Vertical Alignment (p325)
