linux-kernel - Re: [PATCH v2 00/12] arm64: Kconfig: Update ARCH

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Date:   Fri, 1 Oct 2021 08:59:57 -0700
From:   Olof Johansson <olof@...om.net>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Saravana Kannan <saravanak@...gle.com>,
        Will McVicker <willmcvicker@...gle.com>,
        Krzysztof Kozlowski <krzysztof.kozlowski@...onical.com>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Michael Turquette <mturquette@...libre.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Sylwester Nawrocki <s.nawrocki@...sung.com>,
        Tomasz Figa <tomasz.figa@...il.com>,
        Chanwoo Choi <cw00.choi@...sung.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alessandro Zummo <a.zummo@...ertech.it>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        John Stultz <john.stultz@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Lee Jones <lee.jones@...aro.org>,
        "Cc: Android Kernel" <kernel-team@...roid.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-samsung-soc <linux-samsung-soc@...r.kernel.org>,
        linux-clk <linux-clk@...r.kernel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-rtc@...r.kernel.org, Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v2 00/12] arm64: Kconfig: Update ARCH_EXYNOS select configs

On Fri, Oct 1, 2021 at 4:59 AM Geert Uytterhoeven <geert@...ux-m68k.org> wrote:
>
> Hi Olof,
>
> On Fri, Oct 1, 2021 at 7:36 AM Olof Johansson <olof@...om.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