Panfrost¶
The Panfrost driver stack includes an OpenGL ES implementation for Arm Mali GPUs based on the Midgard and Bifrost microarchitectures. It is conformant on Mali-G52 and Mali-G57 but non-conformant on other GPUs. The following hardware is currently supported:
Product |
Architecture |
OpenGL ES |
OpenGL |
---|---|---|---|
Mali T620 |
Midgard (v4) |
2.0 |
2.1 |
Mali T720 |
Midgard (v4) |
2.0 |
2.1 |
Mali T760 |
Midgard (v5) |
3.1 |
3.1 |
Mali T820 |
Midgard (v5) |
3.1 |
3.1 |
Mali T830 |
Midgard (v5) |
3.1 |
3.1 |
Mali T860 |
Midgard (v5) |
3.1 |
3.1 |
Mali T880 |
Midgard (v5) |
3.1 |
3.1 |
Mali G72 |
Bifrost (v6) |
3.1 |
3.1 |
Mali G31 |
Bifrost (v7) |
3.1 |
3.1 |
Mali G51 |
Bifrost (v7) |
3.1 |
3.1 |
Mali G52 |
Bifrost (v7) |
3.1 |
3.1 |
Mali G76 |
Bifrost (v7) |
3.1 |
3.1 |
Mali G57 |
Valhall (v9) |
3.1 |
3.1 |
Other Midgard and Bifrost chips (T604, G71) are not yet supported.
Older Mali chips based on the Utgard architecture (Mali 400, Mali 450) are supported in the Lima driver, not Panfrost. Lima is also available in Mesa.
Other graphics APIs (Vulkan, OpenCL) are not supported at this time.
Building¶
Panfrost’s OpenGL support is a Gallium driver. Since Mali GPUs are 3D-only and
do not include a display controller, Mesa uses kmsro to support display
controllers paired with Mali GPUs. If your board with a Panfrost supported GPU
has a display controller with mainline Linux support not supported by kmsro,
it’s easy to add support, see the commit cff7de4bb597e9
as an example.
LLVM is not required by Panfrost’s compilers. LLVM support in Mesa can safely be disabled for most OpenGL ES users with Panfrost.
Build like meson . build/ -Dvulkan-drivers=
-Dgallium-drivers=panfrost -Dllvm=disabled
for a build directory
build
.
For general information on building Mesa, read the install documentation.
Chat¶
Panfrost developers and users hang out on IRC at #panfrost
on OFTC. Note
that registering and authenticating with NickServ
is required to prevent
spam. Join the chat.
Compressed texture support¶
In the driver, Panfrost supports ASTC, ETC, and all BCn formats (e.g. RGTC,
S3TC, etc.) However, Panfrost depends on the hardware to support these formats
efficiently. All supported Mali architectures support these formats, but not
every system-on-chip with a Mali GPU support all these formats. Many lower-end
systems lack support for some BCn formats, which can cause problems when playing
desktop games with Panfrost. To check whether this issue applies to your
system-on-chip, Panfrost includes a panfrost_texfeatures
tool to query
supported formats.
To use this tool, include the option -Dtools=panfrost
when configuring Mesa.
Then inside your Mesa build directory, the tool is located at
src/panfrost/tools/panfrost_texfeatures
. Copy it to your target device,
set as executable as necessary, and run on the target device. A table of
supported formats will be printed to standard output.
drm-shim¶
Panfrost implements drm-shim
, stubbing out the Panfrost kernel interface.
Use cases for this functionality include:
Future hardware bring up
Running shader-db on non-Mali workstations
Reproducing compiler (and some driver) bugs without Mali hardware
Although Mali hardware is usually paired with an Arm CPU, Panfrost is portable C code and should work on any Linux machine. In particular, you can test the compiler on shader-db on an Intel desktop.
To build Mesa with Panfrost drm-shim, configure Meson with
-Dgallium-drivers=panfrost
and -Dtools=drm-shim
. See the above
building section for a full invocation. The drm-shim binary will be built to
build/src/panfrost/drm-shim/libpanfrost_noop_drm_shim.so
.
To use, set the LD_PRELOAD
environment variable to the drm-shim binary. It
may also be necessary to set LIBGL_DRIVERS_PATH
to the location where Mesa
was installed.
By default, drm-shim mocks a Mali-G52 system. To select a specific Mali GPU,
set the PAN_GPU_ID
environment variable to the desired GPU ID:
Product |
Architecture |
GPU ID |
---|---|---|
Mali-T720 |
Midgard (v4) |
720 |
Mali-T860 |
Midgard (v5) |
860 |
Mali-G72 |
Bifrost (v6) |
6221 |
Mali-G52 |
Bifrost (v7) |
7212 |
Mali-G57 |
Valhall (v9) |
9093 |
Additional GPU IDs are enumerated in the panfrost_model_list
list in
src/panfrost/lib/pan_props.c
.
As an example: assuming Mesa is installed to a local path ~/lib
and Mesa’s
build directory is ~/mesa/build
, a shader can be compiled for Mali-G52 as:
~/shader-db$ BIFROST_MESA_DEBUG=shaders LIBGL_DRIVERS_PATH=~/lib/dri/ LD_PRELOAD=~/mesa/build/src/panfrost/drm-shim/libpanfrost_noop_drm_shim.so PAN_GPU_ID=7212 ./run shaders/glmark/1-1.shader_test
The same shader can be compiled for Mali-T720 as:
~/shader-db$ MIDGARD_MESA_DEBUG=shaders LIBGL_DRIVERS_PATH=~/lib/dri/ LD_PRELOAD=~/mesa/build/src/panfrost/drm-shim/libpanfrost_noop_drm_shim.so PAN_GPU_ID=720 ./run shaders/glmark/1-1.shader_test
These examples set the compilers’ shaders
debug flags to dump the optimized
NIR, backend IR after instruction selection, backend IR after register
allocation and scheduling, and a disassembly of the final compiled binary.
As another example, this invocation runs a single dEQP test “on” Mali-G52,
pretty-printing GPU data structures and disassembling all shaders
(PAN_MESA_DEBUG=trace
) as well as dumping raw GPU memory
(PAN_MESA_DEBUG=dump
). The EGL_PLATFORM=surfaceless
environment variable
and various flags to dEQP mimic the surfaceless environment that our
continuous integration (CI) uses. This eliminates window system dependencies,
although it requires a specially built CTS:
~/VK-GL-CTS/build/external/openglcts/modules$ PAN_MESA_DEBUG=trace,dump LIBGL_DRIVERS_PATH=~/lib/dri/ LD_PRELOAD=~/mesa/build/src/panfrost/drm-shim/libpanfrost_noop_drm_shim.so PAN_GPU_ID=7212 EGL_PLATFORM=surfaceless ./glcts --deqp-surface-type=pbuffer --deqp-gl-config-name=rgba8888d24s8ms0 --deqp-surface-width=256 --deqp-surface-height=256 -n dEQP-GLES31.functional.shaders.builtin_functions.common.abs.float_highp_compute
U-interleaved tiling¶
Panfrost supports u-interleaved tiling. U-interleaved tiling is
indicated by the DRM_FORMAT_MOD_ARM_16X16_BLOCK_U_INTERLEAVED
modifier.
The tiling reorders whole pixels (blocks). It does not compress or modify the pixels themselves, so it can be used for any image format. Internally, images are divided into tiles. Tiles occur in source order, but pixels (blocks) within each tile are reordered according to a space-filling curve.
For regular formats, 16x16 tiles are used. This harmonizes with the default tile size for binning and CRCs (transaction elimination). It also means a single line (16 pixels) at 4 bytes per pixel equals a single 64-byte cache line.
For formats that are already block compressed (S3TC, RGTC, etc), 4x4 tiles are used, where entire blocks are reorder. Most of these formats compress 4x4 blocks, so this gives an effective 16x16 tiling. This justifies the tile size intuitively, though it’s not a rule: ASTC may uses larger blocks.
Within a tile, the X and Y bits are interleaved (like Morton order), but with a twist: adjacent bit pairs are XORed. The reason to add XORs is not obvious. Visually, addresses take the form:
| y3 | (x3 ^ y3) | y2 | (y2 ^ x2) | y1 | (y1 ^ x1) | y0 | (y0 ^ x0) |
Reference routines to encode/decode u-interleaved images are available in
src/panfrost/shared/test/test-tiling.cpp
, which documents the space-filling
curve. This reference implementation is used to unit test the optimized
implementation used in production. The optimized implementation is available in
src/panfrost/shared/pan_tiling.c
.
Although these routines are part of Panfrost, they are also used by Lima, as Arm introduced the format with Utgard. It is the only tiling supported on Utgard. On Mali-T760 and newer, Arm Framebuffer Compression (AFBC) is more efficient and should be used instead where possible. However, not all formats are compressible, so u-interleaved tiling remains an important fallback on Panfrost.