Re: [PATCH v1 0/4] arm64: Kconfig: Update ARCH_EXYNOS select configs

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Saravana,

On Mon, Sep 27, 2021 at 8:07 PM Saravana Kannan <saravanak@google.com> wrote:
> On Mon, Sep 27, 2021 at 1:08 AM Krzysztof Kozlowski
> <krzysztof.kozlowski@canonical.com> wrote:
> > On 25/09/2021 04:17, Saravana Kannan wrote:
> > > On Tue, Sep 21, 2021 at 1:25 AM Krzysztof Kozlowski
> > > <krzysztof.kozlowski@canonical.com> wrote:
> > >> On 21/09/2021 10:11, Lee Jones wrote:
> > >>> On Tue, 21 Sep 2021, Krzysztof Kozlowski wrote:
> > >>>> On 20/09/2021 21:03, Will McVicker wrote:
> > >>>>> This patch series tries to address the issue of ARCH_EXYNOS force selecting
> > >>>>> a handful of drivers without allowing the vendor to override any of the
> > >>>>> default configs. This takes away from the flexibilty of compiling a generic
> > >>>>> kernel with exynos kernel modules. For example, it doesn't allow vendors to
> > >>>>> modularize these drivers out of the core kernel in order to share a generic
> > >>>>> kernel image across multiple devices that require device-specific kernel
> > >>>>> modules.
> > >>>>
> > >>>> You do not address the issue in these patches. The problem you describe
> > >>>> is that drivers are not modules and you are not changing them into modules.
> > >>>
> > >>> The wording is unfortunate.  The reason for this change doesn't have
> > >>> much to do with kernel modules.
> > >>>
> > >>> Let's go back in time 18 months or so when Greg KH submitted this [0]
> > >>> patch, which you Acked.  Greg was trying to solve the problem of not
> > >>> having to enable ARCH_EXYNOS on kernels which are designed to be
> > >>> platform agnostic (sometimes called Generic Kernels).  For some reason
> > >>> SERIAL_SAMSUNG is the only symbol with these dependencies, so the
> > >>> solution seemed simple and straight forward at the time.
> > >>>
> > >>> However, For sound reasons Geert NACKed the patch.
> > >>>
> > >>> Quoting from [1] he says:
> > >>>
> > >>>   "A generic kernel will include Samsung SoC support, hence
> > >>>   PLAT_SAMSUNG or ARCH_EXYNOS will be enabled."
> > >>
> > >> Yes, it's correct reasoning. There is also one more use-case -
> > >> non-upstreamed (out of tree) platform which wants to use Exynos-specific
> > >> drivers. Something like was happening with Apple M1 except that it got
> > >> upstreamed and we do not care much about out-of-tree.
> > >>
> > >>>
> > >>> However, since the entry for ARCH_EXYNOS *insists* on building-in a
> > >>> bunch of other symbols (via 'select') which will be unused in most
> > >>> cases, this is not a currently acceptable approach for many Generic
> > >>> Kernels due to size constraints.
> > >>
> > >> In the mainline kernel there is no such use case. If you want to have
> > >> Exynos-whatever-driver (e.g. SERIAL_SAMSUNG or S3C RTC), you should
> > >> select ARCH_EXYNOS because otherwise it does not make any sense. Zero
> > >> sense. Such kernel won't work.
> > >>
> > >> It makes sense only if there is some other work, hidden here, where
> > >> someone might want to have SERIAL_SAMSUNG or S3C RTC without
> > >> ARCH_EXYNOS. Although GKI is not that work because GKI kernel will
> > >> select ARCH_EXYNOS. It must select ARCH_EXYNOS if it wants to support
> > >> Exynos platforms.
> > >>
> > >> Therefore I expect first to bring this "some other work, hidden here" to
> > >> broader audience, so we can review its use case.
> > >>
> > >>>
> > >>> What this patch does is migrates those symbols from being 'select'ed
> > >>> (always built-in with no recourse) to 'default y'.  Where the former
> > >>> cannot be over-ridden, but the latter can be via a vendor's
> > >>> defconfig/fragment.
> > >>
> > >> It cannot be overridden by vendor fragment because options are not
> > >> visible. You cannot change them.
> > >>
> > >> The patch does nothing in this regard (making them selectable/possible
> > >> to disable), which is why I complained.
> > >>
> > >>>
> > >>> I doubt many (any?) of these symbols can be converted to kernel
> > >>> modules anyway, as they are required very early on in the boot
> > >>> sequence.
> > >>
> > >> True, some could, some not. Also some platforms are set up via
> > >> bootloader, so actually could "survive" till module is loaded from some
> > >> initrd.
> > >
> > > I was trying to chime in, but the discussion got spread out across all
> > > the patches. Since the cover letter seems to have everyone, I thought
> > > I'd reply here. Hope you don't mind. I'll try to respond/chime in on
> > > the various topics that were raised across the patches.
> > >
> > > Yes, the next patch series would To/Cc folks correctly. William simply
> > > forgot to use the --to-cover and --cc-cover options when using git
> > > send-email.
> > >
> > > I agree with you that it doesn't make sense to have ARCH_EXYNOS
> > > enabled but to have all the clock drivers exynos compiled out. Then
> > > one obviously can't boot an exynos platform using that kernel.
> >
> > If downstream kernel does not use any upstream platforms (e.g.
> > Exynos5433 or Exynos7) and has its own drivers for everything, then
> > downstream does not even need ARCH_EXYNOS. Just disable it.
>
> As Geert pointed out in another reply, that prevents the use of
> earlyconsole on an exynos SoC + fully modular generic kernel. Are we
> okay with removing the ARCH_EXYNOS dependency on the early console
> driver now?

IMHO not in upstream, as there is no upstream use yet for not having
the dependencies.

Even if there was, I think it is good to have dependencies like
ARCH_EXYNOS, as they let us partition the (19000, as Arnd said recently)
Kconfig symbols into better manageable groups.  Without these, we cannot
do better than "depends on ARM || ARM64 || COMPILE_TEST".

Greg says that's what defconfig files are for, but the arm64 policy is
to have a single defconfig file only.  But thanks to the ARCH_* symbol,
you can take arm64 defconfig, disable the ARCH_* symbols not applicable
to your platform, and have a good start for a config file tailored to
your platform. Note that works for the arm multi_v*_defconfigs, too.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds