mesa/docs/drivers/kosmickrisp/workarounds.rst

KosmicKrisp workarounds
#######################

This file documents the relevant issues found in either Metal, the MSL
compiler or any other component we have no control over that needed to be
worked around to accomplish Vulkan conformance.

All workarounds must be documented here and no code comment info should be
provided other than the name ``KK_WORKAROUND_#``.

Once a workaround was removed from the code, the code comment will be
removed but the documentation here will be kept.

Template
========

Use the following template to create documentation for a new workaround:

.. code-block::

   KK_WORKAROUND_#
   ---------------
   | macOS version:
   | Metal ticket:
   | Metal ticket status:
   | CTS test failure/crash:
   | Comments:
   | Log:

``macOS version`` needs to have the OS version with which it was found.

``Metal ticket`` needs to be either the Metal ticket number with the GitLab
handle of the user that reported the ticket or ``Unreported``.

``Metal ticket status`` needs to be either ``Fixed in macOS # (Build hash)``,

``Waiting resolution`` or empty if unreported. If Apple reported that the issue
was fixed, but no user has verified the fix, append ``[Untested]``.
``CTS test failure/crash`` (remove ``failure`` or ``crash`` based on test
behavior) needs to be the name of the test or test family the issue can be
reproduced with.

``Comments`` needs to include as much information on the issue and how the
workaround fixes it.

``Log`` needs to have the dates (yyyy-mm-dd, the only correct date format) with
info on what was updated.

Workarounds
===========

KK_WORKAROUND_7
---------------
| macOS version: 26.0.1
| Metal ticket: Not reported
| Metal ticket status:
| CTS test failure: ``dEQP-VK.renderpasses.renderpass2.depth_stencil_resolve.image_2d_*testing_stencil_samplemask``
| Comments:

Metal seems to ignore sample_mask out for cases for the stencil attachment
where we have no color attachments, a multisample depth_stencil attachment
with at least 2 samples and a fragment shader that only writes the depth and
a static sample_mask out.

My conclusion is that they may try to prematurely optimize by doing early
fragment testing disregarding completely the sample_mask out and applying
the value to all stencil samples.

The failing tests do something along the lines of, start render pass with
depth_stencil cleared to 0.0f and 0 respectively, if depth test passes set
stencil to 1. Sample mask out is 1 (first sample). Draw framebuffer size square
with 0.5f depth. End render pass storing values. Start render pass loading the
previous output but if depth passes stencil will be set to 255. Sample mask out
is 2 (second sample). Draw framebuffer size square with 0.5f depth.

In a similar fashion to 2 workarounds below this one, we do a conditional
discard at the end discarding fragments not covered by the coverage_mask.

| Log:
| 2026-02-05: Workaround implemented

KK_WORKAROUND_6
---------------
| macOS version: 26.0.1
| Metal ticket: Not reported
| Metal ticket status:
| CTS test failure: ``dEQP-VK.spirv_assembly.instruction.*.float16.opcompositeinsert.*``
| Comments:

Metal does not respect its own Memory Coherency Model (MSL spec 4.8). From
the spec:
``By default, memory in the device address space has threadgroup coherence.``

If we have a single thread compute dispatch so that we do (simplified version):

.. code-block:: c

   for (...) {
      value = ssbo_data[0]; // ssbo_data is a device buffer
      ...
      ssbo_data[0] = new_value;
   }

``ssbo_data[0]`` will not correctly store/load the values so the value
written in iteration 0, will not be available in iteration 1. The workaround
to this issue is marking the device memory pointer through which the memory
is accessed as coherent so that the value is stored and loaded correctly.
Hopefully this does not affect performance much.

| Log:
| 2025-12-08: Workaround implemented and reported to Apple

KK_WORKAROUND_5
---------------
| macOS version: 26.0.1
| Metal ticket: Not reported
| Metal ticket status:
| CTS test failure: ``dEQP-VK.fragment_operations.early_fragment.discard_no_early_fragment_tests_depth``
| Comments:

Fragment shaders that have side effects (like writing to a buffer) will be
prematurely discarded if there is a ``discard_fragment`` that will always
execute. To work around this, we just make the discard "optional" by moving
it inside a run time conditional that will always be true (such as is the
fragment a helper?). This tricks the MSL compiler into not optimizing it into
a premature discard.

| Log:
| 2025-12-01: Workaround implemented

KK_WORKAROUND_4
---------------
| macOS version: 26.0.1
| Metal ticket: FB21124215 (@aitor)
| Metal ticket status: Waiting resolution
| CTS test failure: ``dEQP-VK.draw.renderpass.shader_invocation.helper_invocation*`` and few others
| Comments:

``simd_is_helper_thread()`` will always return true if the shader was started
as a non-helper thread, even after ``discard_fragment()`` is called. The
workaround is to have a variable tracking this state and update it when the
fragment is discarded. This issue is present in M1 and M2 chips.

| Log:
| 2025-11-22: Workaround implemented and reported to Apple

KK_WORKAROUND_3
---------------
| macOS version: 15.4.x
| Metal ticket: FB20113490 (@aitor)
| Metal ticket status: Waiting resolution
| CTS test failure: ``dEQP-VK.subgroups.ballot_other.*.subgroupballotfindlsb``
| Comments:

``simd_is_first`` does not seem to behave as documented in the MSL
specification. The following code snippet misbehaves:

.. code-block:: c

   if (simd_is_first())
      temp = 3u;
   else
      temp = simd_ballot(true); /* <- This will return all active threads... */

The way to fix this is by changing the conditional to:

.. code-block:: c

   if (simd_is_first() && (ulong)simd_ballot(true))
      temp = 3u;
   else
      temp = (ulong)simd_ballot(true);

| Log:
| 2025-09-09: Workaround implemented and reported to Apple

KK_WORKAROUND_2
---------------
| macOS version: 15.4.x
| Metal ticket: FB21065475 (@aitor)
| Metal ticket status: Waiting resolution
| CTS test crash: ``dEQP-VK.graphicsfuzz.cov-nested-loops-never-change-array-element-one`` and ``dEQP-VK.graphicsfuzz.disc-and-add-in-func-in-loop``
| Comments:

We need to loop to infinite since MSL compiler crashes if we have something
like (simplified version):

.. code-block:: c

   while (true) {
      if (some_conditional) {
         break_loop = true;
      } else {
         break_loop = false;
      }
      if (break_loop) {
         break;
      }
   }

The issue I believe is that ``some_conditional`` wouldn't change the value no
matter in which iteration we are (something like fetching the same value from
a buffer) and the MSL compiler doesn't seem to like that much to the point it
crashes.

The implemented solution is to change the ``while(true)`` loop with
``for (uint64_t no_crash = 0u; no_crash < UINT64_MAX; ++no_crash)``, which
tricks the MSL compiler into believing we are not doing an infinite loop
(wink wink).

| Log:
| 2025-09-08: Workaround implemented

KK_WORKAROUND_1
---------------
| macOS version: 15.4.x
| Metal ticket: FB17604106 (@aitor)
| Metal ticket status: [Untested] Fixed in macOS 26 Beta (25A5279m)
| CTS test crash: ``dEQP-VK.glsl.indexing.tmp_array.vec3_dynamic_loop_write_dynamic_loop_read_fragment``
| Comments:

Uninitialized local scratch variable causes the MSL compiler to crash.
Initialize scratch to avoid issue.

| Log:
| 2025-05-14: Workaround implemented and reported to Apple
| 2025-06-14: Apple reported back saying it is now fixed in macOS 26 Beta (Build 25A5279m)