[PATCH v4 0/4] Don't create Python bytecode when building the kernel

[PATCH v4 0/4] Don't create Python bytecode when building the kernel

[Mauro Carvalho Chehab]

As reported by Andy, the Kernel build system runs kernel-doc script for DRM,
when W=1. Due to Python's normal behavior, its JIT compiler will create
a bytecode and store it under scripts/lib/*/__pycache__.  As one may be using
O= and even having the sources on a read-only mount point, disable its
creation during build time.

This is done by adding PYTHONDONTWRITEBYTECODE=1 on every place
where the script is called within Kbuild and when called via another script.
 
This only solves half of the issue though, as one may be manually running
the script by hand, without asking Python to not store any bytecode.
This should be OK, but afterwards, git status will list the __pycache__ as
not committed. To prevent that, add *.pyc to .gitignore.

This series contain 4 patches:

- patch 1 adjusts a variable that pass extra data to scripts/kerneldoc.py;
- patch 2moves scripts/kernel-doc location to the main makefile
  and exports it, as scripts/Makefile.build will need it;
- patch 3 disables __pycache__ generation and ensure that the entire Kbuild
  will use KERNELDOC var for the location of kernel-doc;
- patch 4 adds *.pyc at the list of object files to be ignored.

---

v4:
- placed *.pyc at the alphabetical order at the final patch

v3:
- move KERNELDOC to the main Makefile;
- get rid of the badly-named KERNELDOC_CONF var.

v2:
- added a .gitignore file;
- add PYTHONDONTWRITEBYTECODE=1 to the places where kernel-doc
  is called.

Mauro Carvalho Chehab (4):
  docs: Makefile: get rid of KERNELDOC_CONF env variable
  Makefile: move KERNELDOC macro to the main Makefile
  scripts/kernel-doc.py: don't create *.pyc files
  .gitignore: ignore Python compiled bytecode

 .gitignore                    | 1 +
 Documentation/Makefile        | 5 ++---
 Makefile                      | 5 +++++
 drivers/gpu/drm/Makefile      | 2 +-
 drivers/gpu/drm/i915/Makefile | 2 +-
 include/drm/Makefile          | 2 +-
 scripts/Makefile.build        | 2 +-
 scripts/find-unused-docs.sh   | 2 +-
 8 files changed, 13 insertions(+), 8 deletions(-)

-- 
2.49.0

[PATCH v4 1/4] docs: Makefile: get rid of KERNELDOC_CONF env variable

[Mauro Carvalho Chehab]

Despite its name, what's there is a set of Sphinx arguments that
are passed to sphinx/kerneldoc.py:

- kerneldoc_srctree: location of the source tree;
- kerneldoc_bin: external script to excecute kernel-doc

Drop it, and just place the values at the already-existing
ALLSPHINXOPTS variable.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 Documentation/Makefile | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/Documentation/Makefile b/Documentation/Makefile
index c022b97c487e..a006c7681412 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -61,8 +61,8 @@ endif #HAVE_LATEXMK
 PAPEROPT_a4     = -D latex_paper_size=a4
 PAPEROPT_letter = -D latex_paper_size=letter
 KERNELDOC       = $(srctree)/scripts/kernel-doc.py
-KERNELDOC_CONF  = -D kerneldoc_srctree=$(srctree) -D kerneldoc_bin=$(KERNELDOC)
-ALLSPHINXOPTS   =  $(KERNELDOC_CONF) $(PAPEROPT_$(PAPER)) $(SPHINXOPTS)
+ALLSPHINXOPTS   = -D kerneldoc_srctree=$(srctree) -D kerneldoc_bin=$(KERNELDOC)
+ALLSPHINXOPTS   += $(PAPEROPT_$(PAPER)) $(SPHINXOPTS)
 ifneq ($(wildcard $(srctree)/.config),)
 ifeq ($(CONFIG_RUST),y)
 	# Let Sphinx know we will include rustdoc
-- 
2.49.0

[PATCH v4 2/4] Makefile: move KERNELDOC macro to the main Makefile

[Mauro Carvalho Chehab]

As kernel-doc script is used not only on Documentation, but
also on scripts and drivers/drm Makefiles, move it to the
main makefile, as otherwise sub-makefiles may not have it.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 Documentation/Makefile | 1 -
 Makefile               | 5 +++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/Documentation/Makefile b/Documentation/Makefile
index a006c7681412..8c1f6a3dfc44 100644
--- a/Documentation/Makefile
+++ b/Documentation/Makefile
@@ -60,7 +60,6 @@ endif #HAVE_LATEXMK
 # Internal variables.
 PAPEROPT_a4     = -D latex_paper_size=a4
 PAPEROPT_letter = -D latex_paper_size=letter
-KERNELDOC       = $(srctree)/scripts/kernel-doc.py
 ALLSPHINXOPTS   = -D kerneldoc_srctree=$(srctree) -D kerneldoc_bin=$(KERNELDOC)
 ALLSPHINXOPTS   += $(PAPEROPT_$(PAPER)) $(SPHINXOPTS)
 ifneq ($(wildcard $(srctree)/.config),)
diff --git a/Makefile b/Makefile
index 38689a0c3605..2a05988740a9 100644
--- a/Makefile
+++ b/Makefile
@@ -458,6 +458,11 @@ endif
 HOSTRUSTC = rustc
 HOSTPKG_CONFIG	= pkg-config
 
+# the KERNELDOC macro needs to be exported, as scripts/Makefile.build
+# has a logic to call it
+KERNELDOC       = $(srctree)/scripts/kernel-doc.py
+export KERNELDOC
+
 KBUILD_USERHOSTCFLAGS := -Wall -Wmissing-prototypes -Wstrict-prototypes \
 			 -O2 -fomit-frame-pointer -std=gnu11
 KBUILD_USERCFLAGS  := $(KBUILD_USERHOSTCFLAGS) $(USERCFLAGS)
-- 
2.49.0

[PATCH v4 3/4] scripts/kernel-doc.py: don't create *.pyc files

[Mauro Carvalho Chehab]

As reported by Andy, kernel-doc.py is creating a __pycache__
directory at build time.

Disable creation of __pycache__ for the libraries used by
kernel-doc.py, when excecuted via the build system or via
scripts/find-unused-docs.sh.

Reported-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Closes: https://lore.kernel.org/linux-doc/Z_zYXAJcTD-c3xTe@black.fi.intel.com/
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
---
 drivers/gpu/drm/Makefile      | 2 +-
 drivers/gpu/drm/i915/Makefile | 2 +-
 include/drm/Makefile          | 2 +-
 scripts/Makefile.build        | 2 +-
 scripts/find-unused-docs.sh   | 2 +-
 5 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
index ed54a546bbe2..d21d0cd2c752 100644
--- a/drivers/gpu/drm/Makefile
+++ b/drivers/gpu/drm/Makefile
@@ -236,7 +236,7 @@ always-$(CONFIG_DRM_HEADER_TEST) += \
 quiet_cmd_hdrtest = HDRTEST $(patsubst %.hdrtest,%.h,$@)
       cmd_hdrtest = \
 		$(CC) $(c_flags) -fsyntax-only -x c /dev/null -include $< -include $<; \
-		$(srctree)/scripts/kernel-doc -none $(if $(CONFIG_WERROR)$(CONFIG_DRM_WERROR),-Werror) $<; \
+		 PYTHONDONTWRITEBYTECODE=1 $(KERNELDOC) -none $(if $(CONFIG_WERROR)$(CONFIG_DRM_WERROR),-Werror) $<; \
 		touch $@
 
 $(obj)/%.hdrtest: $(src)/%.h FORCE
diff --git a/drivers/gpu/drm/i915/Makefile b/drivers/gpu/drm/i915/Makefile
index ed05b131ed3a..ab6b89a163e7 100644
--- a/drivers/gpu/drm/i915/Makefile
+++ b/drivers/gpu/drm/i915/Makefile
@@ -408,7 +408,7 @@ obj-$(CONFIG_DRM_I915_GVT_KVMGT) += kvmgt.o
 #
 # Enable locally for CONFIG_DRM_I915_WERROR=y. See also scripts/Makefile.build
 ifdef CONFIG_DRM_I915_WERROR
-    cmd_checkdoc = $(srctree)/scripts/kernel-doc -none -Werror $<
+    cmd_checkdoc = PYTHONDONTWRITEBYTECODE=1 $(KERNELDOC) -none -Werror $<
 endif
 
 # header test
diff --git a/include/drm/Makefile b/include/drm/Makefile
index a7bd15d2803e..1df6962556ef 100644
--- a/include/drm/Makefile
+++ b/include/drm/Makefile
@@ -11,7 +11,7 @@ always-$(CONFIG_DRM_HEADER_TEST) += \
 quiet_cmd_hdrtest = HDRTEST $(patsubst %.hdrtest,%.h,$@)
       cmd_hdrtest = \
 		$(CC) $(c_flags) -fsyntax-only -x c /dev/null -include $< -include $<; \
-		$(srctree)/scripts/kernel-doc -none $(if $(CONFIG_WERROR)$(CONFIG_DRM_WERROR),-Werror) $<; \
+		PYTHONDONTWRITEBYTECODE=1 $(KERNELDOC) -none $(if $(CONFIG_WERROR)$(CONFIG_DRM_WERROR),-Werror) $<; \
 		touch $@
 
 $(obj)/%.hdrtest: $(src)/%.h FORCE
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 13dcd86e74ca..884dc86ce04e 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -83,7 +83,7 @@ else ifeq ($(KBUILD_CHECKSRC),2)
 endif
 
 ifneq ($(KBUILD_EXTRA_WARN),)
-  cmd_checkdoc = $(srctree)/scripts/kernel-doc -none $(KDOCFLAGS) \
+  cmd_checkdoc = PYTHONDONTWRITEBYTECODE=1 $(KERNELDOC) -none $(KDOCFLAGS) \
         $(if $(findstring 2, $(KBUILD_EXTRA_WARN)), -Wall) \
         $<
 endif
diff --git a/scripts/find-unused-docs.sh b/scripts/find-unused-docs.sh
index ee6a50e33aba..d6d397fbf917 100755
--- a/scripts/find-unused-docs.sh
+++ b/scripts/find-unused-docs.sh
@@ -54,7 +54,7 @@ for file in `find $1 -name '*.c'`; do
 	if [[ ${FILES_INCLUDED[$file]+_} ]]; then
 	continue;
 	fi
-	str=$(scripts/kernel-doc -export "$file" 2>/dev/null)
+	str=$(PYTHONDONTWRITEBYTECODE=1 scripts/kernel-doc -export "$file" 2>/dev/null)
 	if [[ -n "$str" ]]; then
 	echo "$file"
 	fi
-- 
2.49.0

Re: [PATCH v4 3/4] scripts/kernel-doc.py: don't create *.pyc files

[Nicolas Schier]

On Thu, Apr 24, 2025 at 08:16:23AM +0800 Mauro Carvalho Chehab wrote:
> As reported by Andy, kernel-doc.py is creating a __pycache__
> directory at build time.
> 
> Disable creation of __pycache__ for the libraries used by
> kernel-doc.py, when excecuted via the build system or via
> scripts/find-unused-docs.sh.
> 
> Reported-by: Andy Shevchenko <andriy.shevchenko@intel.com>
> Closes: https://lore.kernel.org/linux-doc/Z_zYXAJcTD-c3xTe@black.fi.intel.com/
> Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
> Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> ---
>  drivers/gpu/drm/Makefile      | 2 +-
>  drivers/gpu/drm/i915/Makefile | 2 +-
>  include/drm/Makefile          | 2 +-
>  scripts/Makefile.build        | 2 +-
>  scripts/find-unused-docs.sh   | 2 +-
>  5 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> index ed54a546bbe2..d21d0cd2c752 100644
> --- a/drivers/gpu/drm/Makefile
> +++ b/drivers/gpu/drm/Makefile
> @@ -236,7 +236,7 @@ always-$(CONFIG_DRM_HEADER_TEST) += \
>  quiet_cmd_hdrtest = HDRTEST $(patsubst %.hdrtest,%.h,$@)
>        cmd_hdrtest = \
>  		$(CC) $(c_flags) -fsyntax-only -x c /dev/null -include $< -include $<; \
> -		$(srctree)/scripts/kernel-doc -none $(if $(CONFIG_WERROR)$(CONFIG_DRM_WERROR),-Werror) $<; \
> +		 PYTHONDONTWRITEBYTECODE=1 $(KERNELDOC) -none $(if $(CONFIG_WERROR)$(CONFIG_DRM_WERROR),-Werror) $<; \

Did someone check if we could add

  sys.dont_write_bytecode = True

to the script itself instead of cluttering PYTHONDONTWRITEBYTECODE
everywhere [1]?

Kind regards,
Nicolas


[1]: https://docs.python.org/3/library/sys.html#sys.dont_write_bytecode

Re: [PATCH v4 3/4] scripts/kernel-doc.py: don't create *.pyc files

[Mauro Carvalho Chehab]

Em Fri, 25 Apr 2025 20:46:41 +0200
Nicolas Schier <nicolas@fjasle.eu> escreveu:

> On Thu, Apr 24, 2025 at 08:16:23AM +0800 Mauro Carvalho Chehab wrote:
> > As reported by Andy, kernel-doc.py is creating a __pycache__
> > directory at build time.
> > 
> > Disable creation of __pycache__ for the libraries used by
> > kernel-doc.py, when excecuted via the build system or via
> > scripts/find-unused-docs.sh.
> > 
> > Reported-by: Andy Shevchenko <andriy.shevchenko@intel.com>
> > Closes: https://lore.kernel.org/linux-doc/Z_zYXAJcTD-c3xTe@black.fi.intel.com/
> > Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
> > Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > ---
> >  drivers/gpu/drm/Makefile      | 2 +-
> >  drivers/gpu/drm/i915/Makefile | 2 +-
> >  include/drm/Makefile          | 2 +-
> >  scripts/Makefile.build        | 2 +-
> >  scripts/find-unused-docs.sh   | 2 +-
> >  5 files changed, 5 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/Makefile b/drivers/gpu/drm/Makefile
> > index ed54a546bbe2..d21d0cd2c752 100644
> > --- a/drivers/gpu/drm/Makefile
> > +++ b/drivers/gpu/drm/Makefile
> > @@ -236,7 +236,7 @@ always-$(CONFIG_DRM_HEADER_TEST) += \
> >  quiet_cmd_hdrtest = HDRTEST $(patsubst %.hdrtest,%.h,$@)
> >        cmd_hdrtest = \
> >  		$(CC) $(c_flags) -fsyntax-only -x c /dev/null -include $< -include $<; \
> > -		$(srctree)/scripts/kernel-doc -none $(if $(CONFIG_WERROR)$(CONFIG_DRM_WERROR),-Werror) $<; \
> > +		 PYTHONDONTWRITEBYTECODE=1 $(KERNELDOC) -none $(if $(CONFIG_WERROR)$(CONFIG_DRM_WERROR),-Werror) $<; \  
> 
> Did someone check if we could add
> 
>   sys.dont_write_bytecode = True
> 
> to the script itself instead of cluttering PYTHONDONTWRITEBYTECODE
> everywhere [1]?

Nice to know that we can set it at the script level. Yet, this is is
meant to be a temporary solution, as IMO the best is to set
PYTHONCACHEPREFIX to match the directory on O=, when it is used.

Thanks,
Mauro

[PATCH v4 4/4] .gitignore: ignore Python compiled bytecode

[Mauro Carvalho Chehab]

While the building system doesn't create any Python JIT bytecode,
if one manually runs kernel-doc.py or get_abi.py, Python will,
by default, create a bytecode and store it under scripts/lib/*.

This is normal, and not controlled by the Kernel itself. So,
add *.pyc as an extension to be ignored.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
---
 .gitignore | 1 +
 1 file changed, 1 insertion(+)

diff --git a/.gitignore b/.gitignore
index f2f63e47fb88..bf5ee6e01cd4 100644
--- a/.gitignore
+++ b/.gitignore
@@ -40,6 +40,7 @@
 *.o
 *.o.*
 *.patch
+*.pyc
 *.rmeta
 *.rpm
 *.rsi
-- 
2.49.0

Re: [PATCH v4 4/4] .gitignore: ignore Python compiled bytecode

[Miguel Ojeda]

On Thu, Apr 24, 2025 at 2:17 AM Mauro Carvalho Chehab
<mchehab+huawei@kernel.org> wrote:
>
> While the building system doesn't create any Python JIT bytecode,
> if one manually runs kernel-doc.py or get_abi.py, Python will,
> by default, create a bytecode and store it under scripts/lib/*.
>
> This is normal, and not controlled by the Kernel itself. So,
> add *.pyc as an extension to be ignored.
>
> Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Looks sorted now, thanks!

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

Cheers,
Miguel

Re: [PATCH v4 0/4] Don't create Python bytecode when building the kernel

[Jonathan Corbet]

Mauro Carvalho Chehab <mchehab+huawei@kernel.org> writes:

> As reported by Andy, the Kernel build system runs kernel-doc script for DRM,
> when W=1. Due to Python's normal behavior, its JIT compiler will create
> a bytecode and store it under scripts/lib/*/__pycache__.  As one may be using
> O= and even having the sources on a read-only mount point, disable its
> creation during build time.
>
> This is done by adding PYTHONDONTWRITEBYTECODE=1 on every place
> where the script is called within Kbuild and when called via another script.
>  
> This only solves half of the issue though, as one may be manually running
> the script by hand, without asking Python to not store any bytecode.
> This should be OK, but afterwards, git status will list the __pycache__ as
> not committed. To prevent that, add *.pyc to .gitignore.
>
> This series contain 4 patches:
>
> - patch 1 adjusts a variable that pass extra data to scripts/kerneldoc.py;
> - patch 2moves scripts/kernel-doc location to the main makefile
>   and exports it, as scripts/Makefile.build will need it;
> - patch 3 disables __pycache__ generation and ensure that the entire Kbuild
>   will use KERNELDOC var for the location of kernel-doc;
> - patch 4 adds *.pyc at the list of object files to be ignored.

I've applied the set, thanks.

jon

Re: [PATCH v4 0/4] Don't create Python bytecode when building the kernel

[Akira Yokosawa]

Hi Andy,

Responding to Mauro's cover-letter of v4 at:

    https://lore.kernel.org/cover.1745453655.git.mchehab+huawei@kernel.org/

, which did not CC'd to you.

On Thu, 24 Apr 2025 08:16:20 +0800, Mauro Carvalho Chehab wrote:
> As reported by Andy, the Kernel build system runs kernel-doc script for DRM,
> when W=1. Due to Python's normal behavior, its JIT compiler will create
> a bytecode and store it under scripts/lib/*/__pycache__.  As one may be using
> O= and even having the sources on a read-only mount point, disable its
> creation during build time.
> 
> This is done by adding PYTHONDONTWRITEBYTECODE=1 on every place
> where the script is called within Kbuild and when called via another script.
>  
> This only solves half of the issue though, as one may be manually running
> the script by hand, without asking Python to not store any bytecode.
> This should be OK, but afterwards, git status will list the __pycache__ as
> not committed. To prevent that, add *.pyc to .gitignore.
> 
> This series contain 4 patches:
> 
> - patch 1 adjusts a variable that pass extra data to scripts/kerneldoc.py;
> - patch 2moves scripts/kernel-doc location to the main makefile
>   and exports it, as scripts/Makefile.build will need it;
> - patch 3 disables __pycache__ generation and ensure that the entire Kbuild
>   will use KERNELDOC var for the location of kernel-doc;
> - patch 4 adds *.pyc at the list of object files to be ignored.

I see Jon has merged them all, but responding here anyway.

In https://lore.kernel.org/Z_zYXAJcTD-c3xTe@black.fi.intel.com/, you said:

> This started well, until it becomes a scripts/lib/kdoc.
> So, it makes the `make O=...` builds dirty *). Please make sure this doesn't leave
> "disgusting turd" )as said by Linus) in the clean tree.
>
>*) it creates that __pycache__ disaster. And no, .gitignore IS NOT a solution.w

Andy, I don't agree with your words "__pycache__ disaster" and 
".gitignore IS NOT a solution".

Running "find . -name ".gitignore" -exec grep -nH --null -F -e ".pyc" \{\} +"
under today's Linus master returns this:

-------------------------------------------------------------
./scripts/gdb/linux/.gitignore:2:*.pyc
./drivers/comedi/drivers/ni_routing/tools/.gitignore:3:*.pyc
./tools/perf/.gitignore:32:*.pyc
./tools/testing/selftests/tc-testing/.gitignore:3:*.pyc
./Documentation/.gitignore:3:*.pyc
-------------------------------------------------------------

, and they have been working perfectly.

Having seen your response at https://lore.kernel.org/aAoERIArkvj497ns@smile.fi.intel.com/ :

> I tried before, but I admit, that I have missed something. It was a mess
> in that case. Now I probably can't repeat as I don't remember what was
> the environment and settings I had that time. I'm really glad to see it
> is working this way!

, I'm guessing you had a traumatic experience caused by python's bytecode
caching in the past.  Do you still believe ".gitignore IS NOT a solution"?

From my viewpoint, applying only 4/4 of this series is the right thing to do.

Bothering with might-become-incompatilbe-in-the-future python environment
variables in kernel Makefiles looks over-engineering to me.
Also, as Mauro says in 3/4, it is incomplete in that it does not cover
the cases where those scripts are invoked outside of kernel build.
And it will interfere with existing developers who want the benefit of
bytecode caching.

I'm not precluding the possibility of incoherent bytecode cache; for example
by using a shared kernel source tree among several developers, and only
one of them (owner) has a write permission of it.  In that case, said
owner might update the tree without running relevant python scripts.

I don't know if python can notice outdated cache and disregard it.

In such a situation, setting PYTHONPYCACHEPREFIX as an environment
variable should help, for sure, but only in such special cases.

Andy, what do you say if I ask reverts of 1/4, 2/4/, and 3/4?

Regards,
Akira

Re: [PATCH v4 0/4] Don't create Python bytecode when building the kernel

[Mauro Carvalho Chehab]

Hi Akira,

Em Sat, 26 Apr 2025 11:39:05 +0900
Akira Yokosawa <akiyks@gmail.com> escreveu:

> Bothering with might-become-incompatilbe-in-the-future python environment
> variables in kernel Makefiles looks over-engineering to me.
> Also, as Mauro says in 3/4, it is incomplete in that it does not cover
> the cases where those scripts are invoked outside of kernel build.
> And it will interfere with existing developers who want the benefit of
> bytecode caching.
> 
> I'm not precluding the possibility of incoherent bytecode cache; for example
> by using a shared kernel source tree among several developers, and only
> one of them (owner) has a write permission of it.  In that case, said
> owner might update the tree without running relevant python scripts.
> 
> I don't know if python can notice outdated cache and disregard it.
> 
> In such a situation, setting PYTHONPYCACHEPREFIX as an environment
> variable should help, for sure, but only in such special cases.
> 
> Andy, what do you say if I ask reverts of 1/4, 2/4/, and 3/4?

Patches 1 and 2 are, IMO, needed anyway, as they fix a problem:
KERNELDOC environment is not used consistently.

Now, patch 3 is the one that may require more thinking.

I agree with Andy that, when O=<dir> is used, nothing shall be
written to source dir.

There are a couple of reasons for that:

1. source dir may be read only;
2. one may want to do cross compilation and use multiple output
   directories, one for each version;
3. the source dir could be mapped via NFS to multiple machines
   with different architectures.

For (3), it could mean that multiple machines may have different
Python versions, so, sharing the Python bytecode from source dir doesn't
sound a good idea. Also, I'm not sure if the pyc from different archs
would be identical.

With that, there are two options:

a. disable cache;
b. set PYTHONCACHEPREFIX.

We're currently doing (a). I guess everybody agrees that this is
is not ideal.

So, ideally, we should move to (b). For Spinx, the easiest solution
is just to place it under Documentation/output, but this is not
generic enough: ideally, we should revert patch 3 and set
PYTHONCACHEPREFIX when O is used. Eventually, we can apply my
patch for Documentation/output, while we craft such logic.

Regards,
Mauro

Re: [PATCH v4 0/4] Don't create Python bytecode when building the kernel

[Andy Shevchenko]

On Sat, Apr 26, 2025 at 08:57:08PM +0800, Mauro Carvalho Chehab wrote:
> Em Sat, 26 Apr 2025 11:39:05 +0900
> Akira Yokosawa <akiyks@gmail.com> escreveu:
> 
> > Bothering with might-become-incompatilbe-in-the-future python environment
> > variables in kernel Makefiles looks over-engineering to me.
> > Also, as Mauro says in 3/4, it is incomplete in that it does not cover
> > the cases where those scripts are invoked outside of kernel build.
> > And it will interfere with existing developers who want the benefit of
> > bytecode caching.
> > 
> > I'm not precluding the possibility of incoherent bytecode cache; for example
> > by using a shared kernel source tree among several developers, and only
> > one of them (owner) has a write permission of it.  In that case, said
> > owner might update the tree without running relevant python scripts.
> > 
> > I don't know if python can notice outdated cache and disregard it.
> > 
> > In such a situation, setting PYTHONPYCACHEPREFIX as an environment
> > variable should help, for sure, but only in such special cases.
> > 
> > Andy, what do you say if I ask reverts of 1/4, 2/4/, and 3/4?
> 
> Patches 1 and 2 are, IMO, needed anyway, as they fix a problem:
> KERNELDOC environment is not used consistently.
> 
> Now, patch 3 is the one that may require more thinking.
> 
> I agree with Andy that, when O=<dir> is used, nothing shall be
> written to source dir.
> 
> There are a couple of reasons for that:
> 
> 1. source dir may be read only;
> 2. one may want to do cross compilation and use multiple output
>    directories, one for each version;
> 3. the source dir could be mapped via NFS to multiple machines
>    with different architectures.
> 
> For (3), it could mean that multiple machines may have different
> Python versions, so, sharing the Python bytecode from source dir doesn't
> sound a good idea. Also, I'm not sure if the pyc from different archs
> would be identical.
> 
> With that, there are two options:
> 
> a. disable cache;
> b. set PYTHONCACHEPREFIX.

Thanks, Mauro, for replying. I'm with you on all of it.

> We're currently doing (a). I guess everybody agrees that this is
> is not ideal.

Yes, I also prefer to have cache working if it's possible. The only BUT here is
that users should not suffer from it.

> So, ideally, we should move to (b). For Spinx, the easiest solution
> is just to place it under Documentation/output, but this is not
> generic enough: ideally, we should revert patch 3 and set
> PYTHONCACHEPREFIX when O is used. Eventually, we can apply my
> patch for Documentation/output, while we craft such logic.

-- 
With Best Regards,
Andy Shevchenko