source: vbox/trunk/src/VBox/Additions/3D/mesa/mesa-24.0.2/docs/drivers/freedreno.rst@ 103996

Freedreno
=========

Freedreno GLES and GL driver for Adreno 2xx-6xx GPUs.  It implements up to
OpenGL ES 3.2 and desktop OpenGL 4.5.

See the `Freedreno Wiki
<https://gitlab.freedesktop.org/freedreno/freedreno/-/wikis/home>`__ for more
details.

Turnip
------

Turnip is a Vulkan 1.3 driver for Adreno 6xx GPUs.

The current set of specific chip versions supported can be found in
:file:`src/freedreno/common/freedreno_devices.py`.  The current set of features
supported can be found rendered at `Mesa Matrix <https://mesamatrix.net/>`__.
There are no plans to port to a5xx or earlier GPUs.

Hardware architecture
---------------------

Adreno is a mostly tile-mode renderer, but with the option to bypass tiling
("gmem") and render directly to system memory ("sysmem").  It is UMA, using
mostly write combined memory but with the ability to map some buffers as cache
coherent with the CPU.

.. toctree::
   :glob:

   freedreno/hw/*

Hardware acronyms
^^^^^^^^^^^^^^^^^

.. glossary::

  Cluster
    A group of hardware registers, often with multiple copies to allow
    pipelining.  There is an M:N relationship between hardware blocks that do
    work and the clusters of registers for the state that hardware blocks use.

  CP
    Command Processor.  Reads the stream of state changes and draw commands
    generated by the driver.

  PFP
    Prefetch Parser.  Adreno 2xx-4xx CP component.

  ME
    Micro Engine. Adreno 2xx-4xx CP component after PFP, handles most PM4 commands.

  SQE
    a6xx+ replacement for PFP/ME.  This is the microcontroller that runs the
    microcode (loaded from Linux) which actually processes the command stream
    and writes to the hardware registers.  See `afuc
    <https://gitlab.freedesktop.org/mesa/mesa/-/blob/main/src/freedreno/afuc/README.rst>`__.

  ROQ
    DMA engine used by the SQE for reading memory, with some prefetch buffering.
    Mostly reads in the command stream, but also serves for
    ``CP_MEMCPY``/``CP_MEM_TO_REG`` and visibility stream reads.

  SP
    Shader Processor.  Unified, scalar shader engine.  One or more, depending on
    GPU and tier.

  TP
    Texture Processor.

  UCHE
    Unified L2 Cache.  32KB on A330, unclear how big now.

  CCU
    Color Cache Unit.

  VSC
    Visibility Stream Compressor

  PVS
    Primitive Visibility Stream

  FE
    Front End?  Index buffer and vertex attribute fetch cluster.  Includes PC,
    VFD, VPC.

  VFD
    Vertex Fetch and Decode

  VPC
    Varying/Position Cache?  Hardware block that stores shaded vertex data for
    primitive assembly.

  HLSQ
    High Level Sequencer.  Manages state for the SPs, batches up PS invocations
    between primitives, is involved in preemption.

  PC_VS
    Cluster where varyings are read from VPC and assembled into primitives to
    feed GRAS.

  VS
    Vertex Shader. Responsible for generating VS/GS/tess invocations

  GRAS
    Rasterizer. Responsible for generating PS invocations from primitives, also
    does LRZ

  PS
    Pixel Shader.

  RB
    Render Backend.  Performs both early and late Z testing, blending, and
    attachment stores of output of the PS.

  GMEM
    Roughly 128KB-1MB of memory on the GPU (SKU-dependent), used to store
    attachments during tiled rendering

  LRZ
    Low Resolution Z.  A low resolution area of the depth buffer that can be
    initialized during the binning pass to contain the worst-case (farthest) Z
    values in a block, and then used to early reject fragments during
    rasterization.

Cache hierarchy
^^^^^^^^^^^^^^^

The a6xx GPUs have two main caches: CCU and UCHE.

UCHE (Unified L2 Cache) is the cache behind the vertex fetch, VSC writes,
texture L1, LRZ, and storage image accesses (``ldib``/``stib``).  Misses and
flushes access system memory.

The CCU is the separate cache used by 2D blits and sysmem render target access
(and also for resolves to system memory when in GMEM mode).  Its memory comes
from a carveout of GMEM controlled by ``RB_CCU_CNTL``, with a varying amount
reserved based on whether we're in a render pass using GMEM for attachment
storage, or we're doing sysmem rendering.  Cache entries have the attachment
number and layer mixed into the cache tag in some way, likely so that a
fragment's access is spread through the cache even if the attachments are the
same size and alignments in address space.  This means that the cache must be
flushed and invalidated between memory being used for one attachment and another
(notably depth vs color, but also MRT color).

The Texture Processors (TP) additionally have a small L1 cache (1KB on A330,
unclear how big now) before accessing UCHE. This cache is used for normal
sampling like ``sam``` and ``isam`` (and the compiler will make read-only
storage image access through it as well).  It is not coherent with UCHE (may get
stale results when you ``sam`` after ``stib``), but must get flushed per draw or
something because you don't need a manual invalidate between draws storing to an
image and draws sampling from a texture.

The command processor (CP) does not read from either of these caches, and
instead uses FIFOs in the ROQ to avoid stalls reading from system memory.

Draw states
^^^^^^^^^^^

Since the SQE is not a fast processor, and tiled rendering means that many draws
won't even be used in many bins, since a5xx state updates can be batched up into
"draw states" that point to a fragment of CP packets.  At draw time, if the draw
call is going to actually execute (some primitive is visible in the current
tile), the SQE goes through the ``GROUP_ID``\s and for any with an update since
the last time they were executed, it executes the corresponding fragment.

Starting with a6xx, states can be tagged with whether they should be executed
at draw time for any of sysmem, binning, or tile rendering.  This allows a
single command stream to be generated which can be executed in any of the modes,
unlike pre-a6xx where we had to generate separate command lists for the binning
and rendering phases.

Note that this means that the generated draw state has to always update all of
the state you have chosen to pack into that ``GROUP_ID``, since any of your
previous state changes in a previous draw state command may have been skipped.

Pipelining (a6xx+)
^^^^^^^^^^^^^^^^^^

Most CP commands write to registers.  In a6xx+, the registers are located in
clusters corresponding to the stage of the pipeline they are used from (see
``enum tu_stage`` for a list). To pipeline state updates and drawing, registers
generally have two copies ("contexts") in their cluster, so previous draws can
be working on the previous set of register state while the next draw's state is
being set up. You can find what registers go into which clusters by looking at
:command:`crashdec` output in the ``regs-name: CP_MEMPOOL`` section.

As SQE processes register writes in the command stream, it sends them into a
per-cluster queue stored in ``CP_MEMPOOL``.  This allows the pipeline stages to
process their stream of register updates and events independent of each other
(so even with just 2 contexts in a stage, earlier stages can proceed on to later
draws before later stages have caught up).

Each cluster has a per-context bit indicating that the context is done/free.
Register writes will stall on the context being done.

During a 3D draw command, SQE generates several internal events flow through the
pipeline:

- ``CP_EVENT_START`` clears the done bit for the context when written to the
  cluster
- ``PC_EVENT_CMD``/``PC_DRAW_CMD``/``HLSQ_EVENT_CMD``/``HLSQ_DRAW_CMD`` kick off
  the actual event/drawing.
- ``CONTEXT_DONE`` event completes after the event/draw is complete and sets the
  done flag.
- ``CP_EVENT_END`` waits for the done flag on the next context, then copies all
  the registers that were dirtied in this context to that one.

The 2D blit engine has its own ``CP_2D_EVENT_START``, ``CP_2D_EVENT_END``,
``CONTEXT_DONE_2D``, so 2D and 3D register contexts can do separate context
rollover.

Because the clusters proceed independently of each other even across draws, if
you need to synchronize an earlier cluster to the output of a later one, then
you will need to ``CP_WAIT_FOR_IDLE`` after flushing and invalidating any
necessary caches.

Also, note that some registers are not banked at all, and will require a
``CP_WAIT_FOR_IDLE`` for any previous usage of the register to complete.

In a2xx-a4xx, there weren't per-stage clusters, and instead there were two
register banks that were flipped between per draw.

Bindless/Bindful Descriptors (a6xx+)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Starting with a6xx++, cat5 (texture) and cat6 (image/ssbo/ubo) instructions are
extended to support bindless descriptors.

In the old bindful model, descriptors are separate for textures, samplers,
UBOs, and IBOs (combined descriptor for images and SSBOs), with separate
registers for the memory containing the array of descriptors, and/or different
``STATE_TYPE`` and ``STATE_BLOCK`` for ``CP_LOAD_STATE``/``_FRAG``/``_GEOM``
to pre-load the descriptors into cache.

- textures - per-shader-stage
   - registers: ``SP_xS_TEX_CONST``/``SP_xS_TEX_COUNT``
   - state-type: ``ST6_CONSTANTS``
   - state-block: ``SB6_xS_TEX``
- samplers - per-shader-stage
   - registers: ``SP_xS_TEX_SAMP``
   - state-type: ``ST6_SHADER``
   - state-block: ``SB6_xS_TEX``
- UBOs - per-shader-stage
   - registers: none
   - state-type: ``ST6_UBO``
   - state-block: ``SB6_xS_SHADER``
- IBOs - global acress shader 3d stages, separate for compute shader
   - registers: ``SP_IBO``/``SP_IBO_COUNT`` or ``SP_CS_IBO``/``SP_CS_IBO_COUNT``
   - state-type: ``ST6_SHADER``
   - state-block: ``ST6_IBO`` or ``ST6_CS_IBO`` for compute shaders
   - Note, unlike per-shader-stage descriptors, ``CP_LOAD_STATE6`` is used,
     as opposed to ``CP_LOAD_STATE6_GEOM`` or ``CP_LOAD_STATE6_FRAG``
     depending on shader stage.

.. note::
   For the per-shader-stage registers and state-blocks the ``xS`` notation
   refers to per-shader-stage names, ex. ``SP_FS_TEX_CONST`` or ``SB6_DS_TEX``

Textures and IBOs (images) use *basically* the same 64byte descriptor format
with some exceptions (for ex, for IBOs cubemaps are handles as 2d array).
SSBOs are just untyped buffers, but otherwise use the same descriptors and
instructions as images.  Samplers use a 16byte descriptor, and UBOs use an
8byte descriptor which packs the size in the upper 15 bits of the UBO address.

In the bindless model, descriptors are split into 5 descriptor sets, which are
global across shader stages (but as with bindful IBO descriptors, separate for
3d stages vs compute stage).  Each hw descriptor is an array of descriptors
of configurable size (each descriptor set can be configured for a descriptor
pitch of 8bytes or 64bytes).  Each descriptor can be of arbitrary format (ie.
UBOs/IBOs/textures/samplers interleaved), it's interpretation by the hw is
determined by the instruction that references the descriptor.  Each descriptor
set can contain at least 2^^16 descriptors.

The hw is configured with the base address of the descriptor set via an array
of "BINDLESS_BASE" registers, ie ``SP_BINDLESS_BASE[n]``/``HLSQ_BINDLESS_BASE[n]``
for 3d shader stages, or ``SP_CS_BINDLESS_BASE[n]``/``HLSQ_CS_BINDLESS_BASE[n]``
for compute shaders, with the descriptor pitch encoded in the low bits.
Which of the descriptor sets is referenced is encoded via three bits in the
instruction.  The address of the descriptor is calculated as::

   descriptor_addr = (BINDLESS_BASE[n] & ~0x3) +
                     (idx * 4 * (2 << BINDLESS_BASE[n] & 0x3))


.. note::
   Turnip reserves one descriptor set for internal use and exposes the other
   four for the application via the vulkan API.

Software Architecture
---------------------

Freedreno and Turnip use a shared core for shader compiler, image layout, and
register and command stream definitions.  They implement separate state
management and command stream generation.

.. toctree::
   :glob:

   freedreno/*

GPU devcoredump
^^^^^^^^^^^^^^^^^^

A kernel message from DRM of "gpu fault" can mean any sort of error reported by
the GPU (including its internal hang detection).  If a fault in GPU address
space happened, you should expect to find a message from the iommu, with the
faulting address and a hardware unit involved:

.. code-block:: console

  *** gpu fault: ttbr0=000000001c941000 iova=000000010066a000 dir=READ type=TRANSLATION source=TP|VFD (0,0,0,1)

On a GPU fault or hang, a GPU core dump is taken by the DRM driver and saved to
``/sys/devices/virtual/devcoredump/**/data``.  You can cp that file to a
:file:`crash.devcore` to save it, otherwise the kernel will expire it
eventually. Echo 1 to the file to free the core early, as another core won't be
taken until then.

Once you have your core file, you can use :command:`crashdec -f crash.devcore`
to decode it.  The output will have ``ESTIMATED CRASH LOCATION`` where we
estimate the CP to have stopped.  Note that it is expected that this will be
some distance past whatever state triggered the fault, given GPU pipelining, and
will often be at some ``CP_REG_TO_MEM`` (which waits on previous WFIs) or
``CP_WAIT_FOR_ME`` (which waits for all register writes to land) or similar
event. You can try running the workload with ``TU_DEBUG=flushall`` or
``FD_MESA_DEBUG=flush`` to try to close in on the failing commands.

You can also find what commands were queued up to each cluster in the
``regs-name: CP_MEMPOOL`` section.

If ``ESTIMATED CRASH LOCATION`` doesn't exist you could find ``CP_SQE_STAT``,
though going here is the last resort and likely won't be helpful.

.. code-block::

  indexed-registers:
    - regs-name: CP_SQE_STAT
      dwords: 51
         PC: 00d7                                <-------------
        PKT: CP_LOAD_STATE6_FRAG
        $01: 70348003           $11: 00000000
        $02: 20000000           $12: 00000022

The ``PC`` value is an instruction address in the current firmware.
You would need to disassemble the firmware (/lib/firmware/qcom/aXXX_sqe.fw) via:

.. code-block:: console

  afuc-disasm -v a650_sqe.fw > a650_sqe.fw.disasm

Now you should search for PC value in the disassembly, e.g.:

.. code-block::

  l018: 00d1: 08dd0001  add $addr, $06, 0x0001
        00d2: 981ff806  mov $data, $data
        00d3: 8a080001  mov $08, 0x0001 << 16
        00d4: 3108ffff  or $08, $08, 0xffff
        00d5: 9be8f805  and $data, $data, $08
        00d6: 9806e806  mov $addr, $06
        00d7: 9803f806  mov $data, $03           <------------- HERE
        00d8: d8000000  waitin
        00d9: 981f0806  mov $01, $data


Command Stream Capture
^^^^^^^^^^^^^^^^^^^^^^

During Mesa development, it's often useful to look at the command streams we
send to the kernel.  Mesa itself doesn't implement a way to stream them out
(though it maybe should!).  Instead, we have an interface for the kernel to
capture all submitted command streams:

.. code-block:: console

  cat /sys/kernel/debug/dri/0/rd > cmdstream &

By default, command stream capture does not capture texture/vertex/etc. data.
You can enable capturing all the BOs with:

.. code-block:: console

  echo Y > /sys/module/msm/parameters/rd_full

Note that, since all command streams get captured, it is easy to run the system
out of memory doing this, so you probably don't want to enable it during play of
a heavyweight game.  Instead, to capture a command stream within a game, you
probably want to cause a crash in the GPU during a frame of interest so that a
single GPU core dump is generated.  Emitting ``0xdeadbeef`` in the CS should be
enough to cause a fault.

Capturing Hang RD
+++++++++++++++++

Devcore file doesn't contain all submitted command streams, only the hanging one.
Additionally it is geared towards analyzing the GPU state at the moment of the crash.

Alternatively, it's possible to obtain the whole submission with all command
streams via ``/sys/kernel/debug/dri/0/hangrd``:

.. code-block:: console

  sudo cat /sys/kernel/debug/dri/0/hangrd > logfile.rd // Do the cat _before_ the expected hang

The format of hangrd is the same as in ordinary command stream capture.
``rd_full`` also has the same effect on it.

Replaying Command Stream
^^^^^^^^^^^^^^^^^^^^^^^^

`replay` tool allows capturing and replaying ``rd`` to reproduce GPU faults.
Especially useful for transient GPU issues since it has much higher chances to
reproduce them.

Dumping rendering results or even just memory is currently unsupported.

- Replaying command streams requires kernel with ``MSM_INFO_SET_IOVA`` support.
- Requires ``rd`` capture to have full snapshots of the memory (``rd_full`` is enabled).

Replaying is done via `replay` tool:

.. code-block:: console

  ./replay test_replay.rd

More examples:

.. code-block:: console

  ./replay --first=start_submit_n --last=last_submit_n test_replay.rd

.. code-block:: console

  ./replay --override=0 --generator=./generate_rd test_replay.rd

Editing Command Stream (a6xx+)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

While replaying a fault is useful in itself, modifying the capture to
understand what causes the fault could be even more useful.

``rddecompiler`` decompiles a single cmdstream from ``rd`` into compilable C source.
Given the address space bounds the generated program creates a new ``rd`` which
could be used to override cmdstream with 'replay'. Generated ``rd`` is not replayable
on its own and depends on buffers provided by the source ``rd``.

C source could be compiled using rdcompiler-meson.build as an example.

The workflow would look like this:

1. Find the cmdstream № you want to edit;
2. Decompile it:

.. code-block:: console

  ./rddecompiler -s %cmd_stream_n% example.rd > generate_rd.c

3. Edit the command stream;
4. Compile it back, see rdcompiler-meson.build for the instructions;
5. Plug the generator into cmdstream replay:

.. code-block:: console

  ./replay --override=%cmd_stream_№% --generator=~/generate_rd

6. Repeat 3-5.

GPU Hang Debugging
^^^^^^^^^^^^^^^^^^

Not a guide for how to do it but mostly an enumeration of methods.

Useful ``TU_DEBUG`` (for Turnip) options to narrow down the hang cause:

``sysmem``, ``gmem``, ``nobin``, ``forcebin``, ``noubwc``, ``nolrz``, ``flushall``, ``syncdraw``, ``rast_order``

Useful ``FD_MESA_DEBUG`` (for Freedreno) options:

``sysmem``, ``gmem``, ``nobin``, ``noubwc``, ``nolrz``, ``notile``, ``dclear``, ``ddraw``, ``flush``, ``inorder``, ``noblit``

Useful ``IR3_SHADER_DEBUG`` options:

``nouboopt``, ``spillall``, ``nopreamble``, ``nofp16``

Use Graphics Flight Recorder to narrow down the place which hangs,
use our own breadcrumbs implementation in case of unrecoverable hangs.

In case of faults use RenderDoc to find the problematic command. If it's
a draw call, edit shader in RenderDoc to find whether it culprit is a shader.
If yes, bisect it.

If editing the shader messes the assembly too much and the issue becomes unreproducible
try editing the assembly itself via ``IR3_SHADER_OVERRIDE_PATH``.

If fault or hang is transient try capturing an ``rd`` and replay it. If issue
is reproduced - bisect the GPU packets until the culprit is found.

Do the above if culprit is not a shader.

The hang recovery mechanism in Kernel is not perfect, in case of unrecoverable
hangs check whether the kernel is up to date and look for unmerged patches
which could improve the recovery.

GPU Breadcrumbs
+++++++++++++++

Breadcrumbs described below are available only in Turnip.

Freedreno has simpler breadcrumbs, in debug build writes breadcrumbs
into ``CP_SCRATCH_REG[6]`` and per-tile breadcrumbs into ``CP_SCRATCH_REG[7]``,
in this way they are available in the devcoredump. TODO: generalize Tunip's
breadcrumbs implementation.

This is a simple implementations of breadcrumbs tracking of GPU progress
intended to be a last resort when debugging unrecoverable hangs.
For best results use Vulkan traces to have a predictable place of hang.

For ordinary hangs as a more user-friendly solution use GFR
"Graphics Flight Recorder".

Or breadcrumbs implementation aims to handle cases where nothing can be done
after the hang. In-driver breadcrumbs also allow more precise tracking since
we could target a single GPU packet.

While breadcrumbs support gmem, try to reproduce the hang in a sysmem mode
because it would require much less breadcrumb writes and syncs.

Breadcrumbs settings:

.. code-block:: console

  TU_BREADCRUMBS=%IP%:%PORT%,break=%BREAKPOINT%:%BREAKPOINT_HITS%

``BREAKPOINT``
  The breadcrumb starting from which we require explicit ack.
``BREAKPOINT_HITS``
  How many times breakpoint should be reached for break to occur.
  Necessary for a gmem mode and re-usable cmdbuffers in both of which
  the same cmdstream could be executed several times.

A typical work flow would be:

- Start listening for breadcrumbs on a remote host:

.. code-block:: console

   nc -lvup $PORT | stdbuf -o0 xxd -pc -c 4 | awk -Wposix '{printf("%u:%u\n", "0x" $0, a[$0]++)}'

- Start capturing command stream;
- Replay the hanging trace with:

.. code-block:: console

   TU_BREADCRUMBS=$IP:$PORT,break=-1:0

- Increase hangcheck period:

.. code-block:: console

   echo -n 60000 > /sys/kernel/debug/dri/0/hangcheck_period_ms

- After GPU hang note the last breadcrumb and relaunch trace with:

.. code-block:: console

   TU_BREADCRUMBS=%IP%:%PORT%,break=%LAST_BREADCRUMB%:%HITS%

- After the breakpoint is reached each breadcrumb would require
  explicit ack from the user. This way it's possible to find
  the last packet which didn't hang.

- Find the packet in the decoded cmdstream.

Debugging random failures
^^^^^^^^^^^^^^^^^^^^^^^^^

In most cases random GPU faults and rendering artifacts are caused by some kind
of undifined behaviour that falls under the following categories:

- Usage of a stale reg value;
- Usage of stale memory (e.g. expecting it to be zeroed when it is not);
- Lack of the proper synchronization.

Finding instances of stale reg reads
++++++++++++++++++++++++++++++++++++

Turnip has a debug option to stomp the registers with invalid values to catch
the cases where stale data is read.

.. code-block:: console

  MESA_VK_ABORT_ON_DEVICE_LOSS=1 \
  TU_DEBUG_STALE_REGS_RANGE=0x00000c00,0x0000be01 \
  TU_DEBUG_STALE_REGS_FLAGS=cmdbuf,renderpass \
  ./app

.. envvar:: TU_DEBUG_STALE_REGS_RANGE

  the reg range in which registers would be stomped. Add ``inverse`` to the
  flags in order for this range to specify which registers NOT to stomp.

.. envvar:: TU_DEBUG_STALE_REGS_FLAGS

  ``cmdbuf``
    stomp registers at the start of each command buffer.
  ``renderpass``
    stomp registers before each renderpass.
  ``inverse``
    changes `TU_DEBUG_STALE_REGS_RANGE` meaning to
    "regs that should NOT be stomped".

The best way to pinpoint the reg which causes a failure is to bisect the regs
range. In case when a fail is caused by combination of several registers
the `inverse` flag may be set to find the reg which prevents the failure.