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Date: Tue, 9 Apr 2024 14:00:55 -0300
From: Arnaldo Carvalho de Melo <acme@kernel.org>
To: Namhyung Kim <namhyung@kernel.org>
Cc: Jani Nikula <jani.nikula@linux.intel.com>,
	Ingo Molnar <mingo@kernel.org>, Ian Rogers <irogers@google.com>,
	Kan Liang <kan.liang@linux.intel.com>, Jiri Olsa <jolsa@kernel.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-perf-users@vger.kernel.org,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Maxime Ripard <mripard@kernel.org>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	David Airlie <airlied@gmail.com>, Daniel Vetter <daniel@ffwll.ch>,
	dri-devel@lists.freedesktop.org
Subject: Re: [PATCH 1/9] tools/include: Sync uapi/drm/i915_drm.h with the
 kernel sources
Message-ID: <ZhV0R1lNRrcOYKkV@x1>
References: <20240408185520.1550865-1-namhyung@kernel.org>
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 <874jcb9etk.fsf@intel.com>
 <ZhTwPLliHXKPAJUQ@gmail.com>
 <87pluy972m.fsf@intel.com>
 <CAM9d7cizO8j3tLjxiVKh4qw3dOzqKmUKZg8BZwaCF=ChmbX81A@mail.gmail.com>
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On Tue, Apr 09, 2024 at 08:58:55AM -0700, Namhyung Kim wrote:
> Hello,
> 
> On Tue, Apr 9, 2024 at 3:14 AM Jani Nikula <jani.nikula@linux.intel.com> wrote:
> >
> > On Tue, 09 Apr 2024, Ingo Molnar <mingo@kernel.org> wrote:
> > > * Jani Nikula <jani.nikula@linux.intel.com> wrote:
> > >
> > >> On Mon, 08 Apr 2024, Namhyung Kim <namhyung@kernel.org> wrote:
> > >> > To pick up changes from:
> > >> >
> > >> >    b112364867499 ("drm/i915: Add GuC submission interface version query")
> > >> >    5cf0fbf763741 ("drm/i915: Add some boring kerneldoc")
> > >> >
> > >> > This should be used to beautify DRM syscall arguments and it addresses
> > >> > these tools/perf build warnings:
> > >> >
> > >> >   Warning: Kernel ABI header differences:
> > >> >     diff -u tools/include/uapi/drm/i915_drm.h include/uapi/drm/i915_drm.h
> > >>
> > >> All these years and I never realized there are header copies
> > >> there. But... why copies?
> > >
> > > It's better than all the alternatives we tried so far:
> > >
> > >  - Symbolic links and direct #includes: this was the original approach but
> > >    was pushed back on from the kernel side, when tooling modified the
> > >    headers and broke them accidentally for kernel builds.
> > >
> > >  - Duplicate self-defined ABI headers like glibc: double the maintenance
> > >    burden, double the chance for mistakes, plus there's no tech-driven
> > >    notification mechanism to look at new kernel side changes.
> > >
> > > What we are doing now is a third option:
> > >
> > >  - A software-enforced copy-on-write mechanism of kernel headers to
> > >    tooling, driven by non-fatal warnings on the tooling side build when
> > >    kernel headers get modified:
> > >
> > >     Warning: Kernel ABI header differences:
> > >       diff -u tools/include/uapi/drm/i915_drm.h include/uapi/drm/i915_drm.h
> > >       diff -u tools/include/uapi/linux/fs.h include/uapi/linux/fs.h
> > >       diff -u tools/include/uapi/linux/kvm.h include/uapi/linux/kvm.h
> > >       ...
> > >
> > >    The tooling policy is to always pick up the kernel side headers as-is,
> > >    and integate them into the tooling build. The warnings above serve as a
> > >    notification to tooling maintainers that there's changes on the kernel
> > >    side.
> > >
> > > We've been using this for many years now, and it might seem hacky, but
> > > works surprisingly well.
> > >
> > > Does this make sense to you?
> >
> > Yes, although there are probably pieces of the puzzle I'm missing.
> > Thanks for the explanation! (That might work almost as-is copied to
> > tools/include/uapi/README. ;)
> >
> > It's also kind of funny to find this kind of back alleys of the kernel
> > repo I've never wandered to before.
> 
> I have some explanation in the cover letter of the series but I forgot
> to mention that in each commit message.  Probably I can update the
> explanation with Ingo's reply.

I think we can combine Ingo's with the reply I used and you adopted and
continue to have it when sending the update messages, probably keep it
in the cover letter (the combined text) and add a link to each
individual update:

"Please see tools/include/README.kernel-copies."

The recommendation that developers shouldn't update the copy seems
important to have there as well.

- Arnaldo