Re: [PATCH v2 00/12] arm64: Kconfig: Update ARCH_EXYNOS select configs

[Will McVicker <willmcvicker@google.com>]

On Fri, Oct 1, 2021 at 10:16 AM Olof Johansson <olof@lixom.net> wrote:
>
> On Fri, Oct 1, 2021 at 9:51 AM Will McVicker <willmcvicker@google.com> wrote:
> >
> > On Fri, Oct 1, 2021 at 9:00 AM Olof Johansson <olof@lixom.net> wrote:
> > >
> > > On Fri, Oct 1, 2021 at 4:59 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote:
> > > >
> > > > Hi Olof,
> > > >
> > > > On Fri, Oct 1, 2021 at 7:36 AM Olof Johansson <olof@lixom.net> wrote:
> > > > > A much more valuable approach would be to work towards being able to
> > > > > free up memory by un-probed drivers at the end of boot. That would
> > > > > possibly benefit all platforms on all architectures.
> > > >
> > > > We used to have such a functionality in arch/ppc (not arch/powerpc!),
> > > > where code/data could be tagged __prep, __chrp, or __pmac, to put it
> > > > in a special section, and to be freed with initdata when unused.  It
> > > > was removed in v2.6.15[1], as the savings weren't worth the hassle.
> > > > In a more fragmented space like arm the memory lost due to alignment
> > > > of the sections would be even more substantial.
> > >
> > > Yeah, the balance between per-platform code size and overall kernel
> > > code size shifted over time to a point where it wasn't as meaningful
> > > on ppc.
> > >
> > > > Another problem is to know when is the end of the boot, especially
> > > > with deferred probing.
> > >
> > > Most of this code either has a module_init() or an initcall that
> > > actually registers the drivers and/or probes for the platform and does
> > > the work.
> > >
> > > This means you can have a late equivalent hook/initcall that
> > > determines whether this path ended up being probed/used. If it wasn't,
> > > you can then unregister and flag the corresponding memory to be freed
> > > at the end, and would take out the heuristics and guessing on needing
> > > to do it automatically from the code path that's doing said freeing.
> > >
> > >
> > > -Olof
> >
> > First off, I appreciate the constructive conversations and I
> > understand the ask here. So I'd like to close the "we don't want this"
> > and "this isn't possible" conversation. We have already proven
> > downstream that it is in fact possible to modularize these drivers on
> > other SoCs (mentioned earlier if you missed it) and I'd like to direct
> > the conversation towards verifying/testing here instead of negatively
> > arguing about how SoC vendors aren't upstreaming their drivers. I
> > think everyone understands that, but unfortunately I have no control
> > over that even though I would love everyone to work upstream directly.
> >
> > I am fine with forcing these drivers to always be enabled in some form
> > upstream even though it doesn't really make much sense for a generic
> > kernel that will run on Qualcomm, Exynos, Mediatek, (you name it) SoC
> > devices. I thought about how to do this yesterday and wasn't able to
> > come up with a proper solution that didn't always force this driver to
> > be a module when CONFIG_MODULES is enabled.
>
> This line of reasoning: "I couldn't think of a better option" made us
> merge a userspace ABI some time ago that within a few months was
> replaced with a better solution. In that case it was the kernel
> headers bundling with a build (extending the vmlinux size by a lot),
> that seemed to have no concerns about binary growth. Not all that far
> after that went in, the BPF folks came up with a solid solution for
> CO-RE by introducing BTF, etc.
>
> So, the argument "I can't think of a better solution" is a local
> maxima that we shouldn't settle for if there's a likely better global
> maxima available with a bit more time/effort. If we say "this problem
> is worth solving but this doesn't seem to be the solution we want to
> go for" we might actually be better off long-term.
>
>
> -Olof

If the answer is, "we don't have a solution for that" then that's
fine. I'm not an expert at Kconfig configurations and am asking if
there is already a way to handle this. To me it sounded like there
might be a solution already due to this policy of "we don't allow
disabling drivers that are essential." I'll wait for Krysztof to
respond (or someone who has a potential solution) before I dig into
this deeper.

--Will