linux-kernel - Re: [PATCH v2 0/2] arm64: Cut rebuild time when changing CONFIG_BLK_DEV

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Message-ID: <CAL_JsqKC6+8gobdk1uhMLLdMUHEO8D0c3m6+F_3y3NTU_KidOw@mail.gmail.com>
Date:   Wed, 24 Oct 2018 16:25:47 -0500
From:   Rob Herring <robh@...nel.org>
To:     Florian Fainelli <f.fainelli@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Olof Johansson <olof@...om.net>, linux-alpha@...r.kernel.org,
        arcml <linux-snps-arc@...ts.infradead.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        linux-c6x-dev@...ux-c6x.org,
        "moderated list:H8/300 ARCHITECTURE" 
        <uclinux-h8-devel@...ts.sourceforge.jp>,
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        "open list:GENERIC INCLUDE/ASM HEADER FILES" 
        <linux-arch@...r.kernel.org>
Subject: Re: [PATCH v2 0/2] arm64: Cut rebuild time when changing CONFIG_BLK_DEV_INITRD

On Wed, Oct 24, 2018 at 3:01 PM Florian Fainelli <f.fainelli@...il.com> wrote:
>
> On 10/24/18 12:55 PM, Rob Herring wrote:
> > On Wed, Oct 24, 2018 at 2:33 PM Florian Fainelli <f.fainelli@...il.com> wrote:
> >>
> >> Hi all,
> >>
> >> While investigating why ARM64 required a ton of objects to be rebuilt
> >> when toggling CONFIG_DEV_BLK_INITRD, it became clear that this was
> >> because we define __early_init_dt_declare_initrd() differently and we do
> >> that in arch/arm64/include/asm/memory.h which gets included by a fair
> >> amount of other header files, and translation units as well.
> >
> > I scratch my head sometimes as to why some config options rebuild so
> > much stuff. One down, ? to go. :)
> >
>
> This one was by far the most invasive one due to its include chain, but
> yes, there would be many more that could be optimized.
>
> >> Changing the value of CONFIG_DEV_BLK_INITRD is a common thing with build
> >> systems that generate two kernels: one with the initramfs and one
> >> without. buildroot is one of these build systems, OpenWrt is also
> >> another one that does this.
> >>
> >> This patch series proposes adding an empty initrd.h to satisfy the need
> >> for drivers/of/fdt.c to unconditionally include that file, and moves the
> >> custom __early_init_dt_declare_initrd() definition away from
> >> asm/memory.h
> >>
> >> This cuts the number of objects rebuilds from 1920 down to 26, so a
> >> factor 73 approximately.
> >>
> >> Apologies for the long CC list, please let me know how you would go
> >> about merging that and if another approach would be preferable, e.g:
> >> introducing a CONFIG_ARCH_INITRD_BELOW_START_OK Kconfig option or
> >> something like that.
> >
> > There may be a better way as of 4.20 because bootmem is now gone and
> > only memblock is used. This should unify what each arch needs to do
> > with initrd early. We need the physical address early for memblock
> > reserving. Then later on we need the virtual address to access the
> > initrd. Perhaps we should just change initrd_start and initrd_end to
> > physical addresses (or add 2 new variables would be less invasive and
> > allow for different translation than __va()). The sanity checks and
> > memblock reserve could also perhaps be moved to a common location.
> >
> > Alternatively, given arm64 is the only oddball, I'd be fine with an
> > "if (IS_ENABLED(CONFIG_ARM64))" condition in the default
> > __early_init_dt_declare_initrd as long as we have a path to removing
> > it like the above option.
>
> OK, let me cook a patch doing that and meanwhile I will look at how much
> work is involved to implement the above option you outlined, which also
> sounds entirely reasonable.

BTW, I would suspect that initrd_below_start_ok being 1 is not okay
for most arches. I'm not sure how that would work. min_low_pfn is
typically based on the start of memory. arm64 is not even setting it.

Rob

> --
> Florian