Units — The Mesa 3D Graphics Library latest documentation

# Units

Almost every variable, function parameter, or struct field in ISL that carries a numeric value has explicit units associated with it. The units used in ISL are as follows:

> - Pixels (px)

> - Samples (sa)

> - Elements (el)

> - Tiles (tl)

> - Bytes (B)

> - Rows of some other unit size (<unit>_rows)

These units are fundamental to ISL because they allow us to specify information about a surface in a canonical way that isn’t dependent on hardware generation. Each field in an ISL data structure that stores any sort of dimension has a suffix that declares the units for that particular value: `_el` for elements, `_sa` for samples, etc. If the units of the particular field aren’t quite what is wanted by the hardware, we do the conversion when we emit `RENDER_SURFACE_STATE`.

This is one of the primary differences between ISL and the old miptree code and one of the core design principles of ISL. In the old miptree code, we tried to keep everything in the same units as the hardware expects but this lead to unnecessary complications as the hardware evolved. One example of this difference is QPitch which specifies the distance between array slices. On Broadwell and earlier, QPitch field in `RENDER_SURFACE_STATE` was in rows of samples. For block-compressed images, this meant it had to be a multiple of the block height. On Skylake, it changed to always being in rows of elements so you have to divide the pitch in samples by the compression block height. Since the old surface state code tries to store things in hardware units, everyone who ever reads `brw_mipmap_tree.qpitch` has to change their interpretation based on hardware generation and whether or not the surface was block-compressed. In ISL, we have `isl_surf.array_pitch_el_rows` which, as the name says, is in rows of elements. On Broadwell and earlier, we have to multiply by the block size of the texture when we finally fill out the hardware packet. However, the consistency of always being in rows of elements makes any other users of the field much simpler because they never have to look at hardware generation or whether or not the image is block-compressed.

Pixels are the most straightforward unit and are where everything starts. A pixel simply corresponds to a single pixel (or texel if you prefer) in the surface. For multisampled surfaces, a pixel may contain one or more samples. For compressed textures, a compression block may contain one or more pixels. When initially creating a surface, everything passed to isl_surf_init is implicitly in terms of pixels because this is what all of the APIs use.

The next unit in ISL’s repertoire is samples. In a multisampled surface, each pixel corresponds to some number of samples given by `isl_surf.samples`. The exact layout of the samples depends on the value of `isl_surf.msaa_layout`. If the layout is `isl_msaa_layout.ISL_MSAA_LAYOUT_ARRAY` then each logical array in the surface corresponds to `isl_surf.samples` actual slices in the resulting surface, one per array slice. If the layout is `isl_msaa_layout.ISL_MSAA_LAYOUT_INTERLEAVED` then each pixel corresponds to a 2x1, 2x2, 4x2, or 4x4 grid of samples. In order to aid in calculations, one of the first things ISL does is to compute `isl_surf.phys_level0_sa` which gives the dimensions of the base miplevel of the surface in samples. The type of `isl_surf.phys_level0_sa` is `isl_extent4d` which allows us to express both the array and interleaved cases. Most of the calculations of how the different miplevels and array slices are laid out is done in terms of samples.

Next, we have surface elements. An element is the basic unit of actual surface memory. For multisampled textures, an element is equal to a single sample. For block compressed textures, an element corresponds to an entire compression block. The conversion from samples to elements is given by dividing by the block width and block height of the surface format. This is true regardless of whether or not the surface is multisampled; for multisampled compressed textures (these exist for certain auxiliary formats), the block width and block height are expressed in samples. Pixels cannot be converted directly to elements or vice versa; any conversion between pixels and elements must go through samples.

The final surface unit is tiles. A tile is a large rectangular block of surface data that all fits in a single contiguous block of memory (usually a 4K or 64K page, depending on tile format). Tiles are used to provide an arrangement of the data in memory that yields better cache performance. The size of a tile is always specified in surface elements.