[PATCH] Documentation/llvm: Add a section about supported architectures

[Nathan Chancellor]

The most common question around building the Linux kernel with clang is
"does it work?" and the answer has always been "it depends on your
architecture, configuration, and LLVM version" with no hard answers for
users wanting to experiment. LLVM support has significantly improved
over the past couple of years, resulting in more architectures and
configurations supported, and continuous integration has made it easier
to see what works and what does not.

Add a section that goes over what architectures are supported in the
current kernel version, how they should be built (with just clang or the
LLVM utilities as well), and the level of support they receive. This
will make it easier for people to try out building their kernel with
LLVM and reporting issues that come about from it.

Suggested-by: Miguel Ojeda <oj...@kernel.org>
Signed-off-by: Nathan Chancellor <natecha...@gmail.com>
---
Documentation/kbuild/llvm.rst | 44 +++++++++++++++++++++++++++++++++++
1 file changed, 44 insertions(+)

diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
index 21c847890d03..b18401d2ba82 100644
--- a/Documentation/kbuild/llvm.rst
+++ b/Documentation/kbuild/llvm.rst
@@ -63,6 +63,50 @@ They can be enabled individually. The full list of the parameters: ::
Currently, the integrated assembler is disabled by default. You can pass
``LLVM_IAS=1`` to enable it.

+Supported Architectures
+-----------------------
+
+LLVM does not target all of the architectures that Linux supports and
+just because a target is supported in LLVM does not mean that the kernel
+will build or work without any issues. Below is a general summary of
+architectures that currently work with ``CC=clang`` or ``LLVM=1``. Level
+of support corresponds to "S" values in the MAINTAINERS files. If an
+architecture is not present, it either means that LLVM does not target
+it or there are known issues. Using the latest stable version of LLVM or
+even the development tree will generally yield the best results.
+An architecture's ``defconfig`` is generally expected to work well,
+certain configurations may have problems that have not been uncovered
+yet. Bug reports are always welcome at the issue tracker below!
+
+.. list-table::
+ :widths: 10 10 10
+ :header-rows: 1
+
+ * - Architecture
+ - Level of support
+ - ``make`` command
+ * - arm
+ - Supported
+ - ``LLVM=1``
+ * - arm64
+ - Supported
+ - ``LLVM=1``
+ * - mips
+ - Maintained
+ - ``CC=clang``
+ * - powerpc
+ - Maintained
+ - ``CC=clang``
+ * - riscv
+ - Maintained
+ - ``CC=clang``
+ * - s390
+ - Maintained
+ - ``CC=clang``
+ * - x86
+ - Supported
+ - ``LLVM=1``
+
Getting Help
------------


base-commit: 7c53f6b671f4aba70ff15e1b05148b10d58c2837
--
2.30.0

[Nick Desaulniers]

Patch looks fine, but `make -j htmldocs` seems to be taking forever
for me so I can't render it. Is this a known issue?

$ make -j htmldocs
SPHINX htmldocs --> file:///android0/linux-next/Documentation/output
make[2]: Nothing to be done for 'html'.
WARNING: The kernel documentation build process
support for Sphinx v3.0 and above is brand new. Be prepared for
possible issues in the generated output.
Warning: The Sphinx 'sphinx_rtd_theme' HTML theme was not
found. Make sure you have the theme installed to produce pretty HTML
output. Falling back to the default theme.
enabling CJK for LaTeX builder
./drivers/usb/dwc3/core.h:1259: warning: Function parameter or member
'gadget_max_speed' not described in 'dwc3'
./drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h:340: warning:
Incorrect use of kernel-doc format: *
@active_vblank_irq_count
./drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h:379: warning:
Function parameter or member 'active_vblank_irq_count' not described
in 'amdgpu_display_manager'
./include/linux/rcupdate.h:884: warning: Excess function parameter
'ptr' description in 'kfree_rcu'
./include/linux/rcupdate.h:884: warning: Excess function parameter
'rhf' description in 'kfree_rcu'

<seems like no updates from here>

--
Thanks,
~Nick Desaulniers

[Nick Desaulniers]

On Wed, Jan 13, 2021 at 5:19 PM Nick Desaulniers
<ndesau...@google.com> wrote:
>
> Patch looks fine, but `make -j htmldocs` seems to be taking forever
> for me so I can't render it. Is this a known issue?
>
> $ make -j htmldocs

Just took a while, lots of warning spew (but not from this addition AFAICT).
Reviewed-by: Nick Desaulniers <ndesa...@google.com>

--
Thanks,
~Nick Desaulniers

[Nathan Chancellor]

On Wed, Jan 13, 2021 at 06:05:32PM -0800, Nick Desaulniers wrote:
> On Wed, Jan 13, 2021 at 5:19 PM Nick Desaulniers
> <ndesau...@google.com> wrote:
> >
> > Patch looks fine, but `make -j htmldocs` seems to be taking forever
> > for me so I can't render it. Is this a known issue?
> >
> > $ make -j htmldocs

A clean "make -skj24 htmldocs" takes me a little over three minutes or
so on my Ryzen 9 3900X. Just to give some perspective.

> Just took a while, lots of warning spew (but not from this addition AFAICT).
> Reviewed-by: Nick Desaulniers <ndesa...@google.com>

Thanks for the review!

Cheers,
Nathan

[Sedat Dilek]

On Thu, Jan 14, 2021 at 1:35 AM Nathan Chancellor
<natecha...@gmail.com> wrote:
>
> The most common question around building the Linux kernel with clang is
> "does it work?" and the answer has always been "it depends on your
> architecture, configuration, and LLVM version" with no hard answers for
> users wanting to experiment. LLVM support has significantly improved
> over the past couple of years, resulting in more architectures and
> configurations supported, and continuous integration has made it easier
> to see what works and what does not.
>
> Add a section that goes over what architectures are supported in the
> current kernel version, how they should be built (with just clang or the
> LLVM utilities as well), and the level of support they receive. This
> will make it easier for people to try out building their kernel with
> LLVM and reporting issues that come about from it.
>

Thanks, this was overdue and is definitely helpful for users and developers.

For x86 64bit:

Reviewed-by: Sedat Dilek <sedat...@gmail.com>

Together with "[PATCH] kbuild: check the minimum compiler version in
Kconfig" this looks very good to me.

/o\
- Sedat -

[1] https://marc.info/?t=161059811000001&r=1&w=2

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[Miguel Ojeda]

On Thu, Jan 14, 2021 at 3:23 AM Nathan Chancellor
<natecha...@gmail.com> wrote:
>
> A clean "make -skj24 htmldocs" takes me a little over three minutes or
> so on my Ryzen 9 3900X. Just to give some perspective.

Oh, wow, that's something...

Thanks a lot for adding this:

Reviewed-by: Miguel Ojeda <oj...@kernel.org>

Cheers,
Miguel

[Jonathan Corbet]

On Wed, 13 Jan 2021 17:19:59 -0800
Nick Desaulniers <ndesau...@google.com> wrote:

> Patch looks fine, but `make -j htmldocs` seems to be taking forever
> for me so I can't render it. Is this a known issue?
>
> $ make -j htmldocs
> SPHINX htmldocs --> file:///android0/linux-next/Documentation/output
> make[2]: Nothing to be done for 'html'.
> WARNING: The kernel documentation build process
> support for Sphinx v3.0 and above is brand new. Be prepared for
> possible issues in the generated output.

Sphinx 3.x is *way* slower to build the docs, alas. One of many
"improvements" we got with that change.

jon

[Nick Desaulniers]

Oh, man, yeah. Noticeably. I had sworn I had run `make htmldocs`
before. Have you had a chance to report this regression upstream?
--
Thanks,
~Nick Desaulniers

[Nathan Chancellor]

On Wed, Jan 13, 2021 at 05:34:47PM -0700, Nathan Chancellor wrote:
> The most common question around building the Linux kernel with clang is
> "does it work?" and the answer has always been "it depends on your
> architecture, configuration, and LLVM version" with no hard answers for
> users wanting to experiment. LLVM support has significantly improved
> over the past couple of years, resulting in more architectures and
> configurations supported, and continuous integration has made it easier
> to see what works and what does not.
>
> Add a section that goes over what architectures are supported in the
> current kernel version, how they should be built (with just clang or the
> LLVM utilities as well), and the level of support they receive. This
> will make it easier for people to try out building their kernel with
> LLVM and reporting issues that come about from it.
>
> Suggested-by: Miguel Ojeda <oj...@kernel.org>
> Signed-off-by: Nathan Chancellor <natecha...@gmail.com>

Jonathan, did you need anything else from me on this, or does Masahiro
need to pick this up?

Cheers,
Nathan

[Masahiro Yamada]

Now I applied to linux-kbuild.
Thanks for the reminder.



--
Best Regards
Masahiro Yamada