Asahi

The Asahi driver aims to provide an OpenGL implementation for the Apple M1.

Testing on macOS

On macOS, the experimental Asahi driver may built with options:

-Dosmesa=true -Dglx=xlib -Dgallium-drivers=asahi,swrast

To use, set the DYLD_LIBRARY_PATH environment variable:

DYLD_LIBRARY_PATH=/Users/nobody/mesa/build/src/gallium/targets/libgl-xlib/ glmark2 --reuse-context

Only X11 apps are supported. XQuartz must be setup separately.

Wrap (macOS only)

Mesa includes a library that wraps the key IOKit entrypoints used in the macOS UABI for AGX. The wrapped routines print information about the kernel calls made and dump work submitted to the GPU using agxdecode.

This library allows debugging Mesa, particularly around the undocumented macOS user-kernel interface. Logs from Mesa may compared to Metal to check that the UABI is being used correctly.

Furthermore, it allows reverse-engineering the hardware, as glue to get at the “interesting” GPU memory.

The library is only built if -Dtools=asahi is passed. It builds a single wrap.dylib file, which should be inserted into a process with the DYLD_INSERT_LIBRARIES environment variable.

For example, to trace an app ./app, run:

DYLD_INSERT_LIBRARIES=~/mesa/build/src/asahi/lib/libwrap.dylib ./app

Hardware varyings

At an API level, vertex shader outputs need to be interpolated to become fragment shader inputs. This process is logically pipelined in AGX, with a value traveling from a vertex shader to remapping hardware to coefficient register setup to the fragment shader to the iterator hardware. Each stage is described below.

Vertex shader

A vertex shader (running on the Unified Shader Cores) outputs varyings with the st_var instruction. st_var takes a vertex output index and a 32-bit value. The maximum number of vertex outputs is specified as the “output count” of the shader in the “Bind Vertex Pipeline” packet. The value may be interpreted consist of a single 32-bit value or an aligned 16-bit register pair, depending on whether interpolation should happen at 32-bit or 16-bit. Vertex outputs are indexed starting from 0, with the vertex position always coming first, the 32-bit user varyings coming next, then 16-bit user varyings, and finally point size at the end if present.

Ordering of vertex outputs with all outputs used

Index

Value

0

Vertex position

4

32-bit varying 0

4 + m

32-bit varying m

4 + m + 1

Packed pair of 16-bit varyings 0

4 + m + 1 + n

Packed pair of 16-bit varyings n

4 + m + 1 + n + 1

Point size

Remapping

Vertex outputs are remapped to varying slots to be interpolated. The output of remapping consists of the following items: the W fragment coordinate, the Z fragment coordinate, user varyings in the vertex output order. Z may be omitted, but W may not be. This remapping is configured by the “Linkage” packet.

Ordering of remapped slots

Index

Value

0

Fragment coord W

1

Fragment coord Z

2

32-bit varying 0

2 + m

32-bit varying m

2 + m + 1

Packed pair of 16-bit varyings 0

2 + m + n + 1

Packed pair of 16-bit varyings n

Coefficient registers

The fragment shader does not see the physical slots. Instead, it references varyings through coefficient registers. A coefficient register is a register allocated constant for all fragment shader invocations in a given polygon. Physically, it contains the values output by the vertex shader for each vertex of the polygon. Coefficient registers are preloaded with values from varying slots. This preloading appears to occur in fixed function hardware, a simplification from PowerVR which requires a specialized program for the programmable data sequencer to do the preload.

The “Bind fragment pipeline” packet points to coefficient register bindings, preceded by a header. The header contains the number of 32-bit varying slots. As the W slot is always present, this field is always nonzero. Slots whose index is below this count are treated as 32-bit. The remaining slots are treated as 16-bits.

The header also contains the total number of coefficient registers bound.

Each binding that follows maps a (vector of) varying slots to a (consecutive) coefficient registers. Some details about the varying (perspective interpolation, flat shading, point sprites) are configured here.

Coefficient registers may be ordered the same as the internal varying slots. However, this may be inconvenient for some APIs that require a separable shader model. For these APIs, the flexibility to mix-and-match slots and coefficient registers allows mixing shaders without shader variants. In that case, the bindings should be generated outside of the compiler. For simple APIs where the bindings are fixed and known at compile-time, the bindings could be generated within the compiler.

Fragment shader

In the fragment shader, coefficient registers, identified by the prefix cf followed by a decimal index, act as opaque handles to varyings. For flat shading, coefficient registers may be loaded into general registers with the ldcf instruction. For smooth shading, the coefficient register corresponding to the desired varying is passed as an argument to the “iterate” instruction iter in order to “iterate” (interpolate) a varying. As perspective correct interpolation also requires the W component of the fragment coordinate, the coefficient register for W is passed as a second argument. As an example, if there’s a single varying to interpolate, an instruction like iter r0, cf1, cf0 is used.

Iterator

To actually interpolate varyings, AGX provides fixed-function iteration hardware to multiply the specified coefficient registers with the required barycentrics, producing an interpolated value, hence the name “coefficient register”. This operation is purely mathematical and does not require any memory access, as the required coefficients are preloaded before the shader begins execution. That means the iterate instruction executes in constant time, does not signal a data fence, and does not require the shader to wait on a data fence before using the value.

Image layouts

AGX supports several image layouts, described here. To work with image layouts in the drivers, use the ail library, located in src/asahi/layout.

The simplest layout is strided linear. Pixels are stored in raster-order in memory with a software-controlled stride. Strided linear images are useful for working with modifier-unaware window systems, however performance will suffer. Strided linear images have numerous limitations:

  • Strides must be a multiple of 16 bytes.

  • Strides must be nonzero. For 1D images where the stride is logically irrelevant, ail will internally select the minimal stride.

  • Only 1D and 2D images may be linear. In particular, no 3D or cubemaps.

  • Array texture may not be linear. No 2D arrays or cubemap arrays.

  • 2D images must not be mipmapped.

  • Block-compressed formats and multisampled images are unsupported. Elements of a strided linear image are simply pixels.

With these limitations, addressing into a strided linear image is as simple as

\[\text{address} = (y \cdot \text{stride}) + (x \cdot \text{bytes per pixel})\]

In practice, this suffices for window system integration and little else.

The most common uncompressed layout is twiddled. The image is divided into power-of-two sized tiles. The tiles themselves are stored in raster-order. Within each tile, elements (pixels/blocks) are stored in Morton (Z) order.

The tile size used depends on both the image size and the block size of the image format. For large images, \(n \times n\) or \(2n \times n\) tiles are used (\(n\) power-of-two). \(n\) is such that each page contains exactly one tile. Only power-of-two block sizes are supported in hardware, ensuring such a tile size always exists. The hardware uses 16 KiB pages, so tile sizes are as follows:

Tile sizes for large images

Bytes per block

Tile size

1

128 x 128

2

128 x 64

4

64 x 64

8

64 x 32

16

32 x 32

The dimensions of large images are rounded up to be multiples of the tile size. In addition, non-power-of-two large images have extra padding tiles when mipmapping is used, see below.

That rounding would waste a great deal of memory for small images. If an image is smaller than this tile size, a smaller tile size is used to reduce the memory footprint. For small images, the tile size is \(m \times m\) where

\[m = 2^{\lceil \log_2( \min \{ \text{width}, \text{ height} \}) \rceil}\]

In other words, small images use the smallest square power-of-two tile such that the image’s minor axis fits in one tile.

For mipmapped images, tile sizes are determined independently for each level. Typically, the first levels of an image are “large” and the remaining levels are “small”. This scheme reduces the memory footprint of mipmapping, compared to a fixed tile size for the whole image. Each mip level are padded to fill at least one cache line (128 bytes), ensure no cache line contains multiple mip levels.

There is a wrinkle: the dimensions of large mip levels in tiles are determined by the dimensions of level 0. For power-of-two images, the two calculations are equivalent. However, they differ subtly for non-power-of-two images. To determine the number of tiles to allocate for level \(l\), the number of tiles for level 0 should be right-shifted by \(2l\). That appears to divide by \(2^l\) in both width and height, matching the definition of mipmapping, however it rounds down incorrectly. To compensate, the level contains one extra row, column, or both (with the corner) as required if any of the first \(l\) levels were rounded down. This hurt the memory footprint. However, it means non-power-of-two integer multiplication is only required for level 0. Calculating the sizes for subsequent levels requires only addition and bitwise math. That simplifies the hardware (but complicates software).

A 2D image consists of a full miptree (constructed as above) rounded up to the page size (16 KiB).

3D images consist simply of an array of 2D layers (constructed as above). That means cube maps, 2D arrays, cube map arrays, and 3D images all use the same layout. The only difference is the number of layers. Notably, 3D images (like GL_TEXTURE_3D) reserve space even for mip levels that do not exist logically. These extra levels pad out layers of 3D images to the size of the first layer, simplifying layout calculations for both software and hardware. Although the padding is logically unnecessary, it wastes little space compared to the sizes of large mipmapped 3D textures.

drm-shim (Linux only)

Mesa includes a library that mocks out the DRM UABI used by the Asahi driver stack, allowing the Mesa driver to run on non-M1 Linux hardware. This can be useful for exercising the compiler. To build, use options:

-Dgallium-drivers=asahi -Dtools=drm-shim

Then run an OpenGL workload with environment variable:

LD_PRELOAD=~/mesa/build/src/asahi/drm-shim/libasahi_noop_drm_shim.so

For example to compile a shader with shaderdb and print some statistics along with the IR:

~/shader-db$ AGX_MESA_DEBUG=shaders,shaderdb ASAHI_MESA_DEBUG=precompile LIBGL_DRIVERS_PATH=~/lib/dri/ LD_PRELOAD=~/mesa/build/src/asahi/drm-shim/libasahi_noop_drm_shim.so ./run shaders/glmark/1-12.shader_test

The drm-shim implementation for Asahi is located in src/asahi/drm-shim. The drm-shim implementation there should be updated as new UABI is added.