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

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Date:   Thu, 25 Oct 2018 10:51:34 +0100
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     Rob Herring <robh@...nel.org>,
        Linux-MIPS <linux-mips@...ux-mips.org>,
        linux-ia64@...r.kernel.org, SH-Linux <linux-sh@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>, devicetree@...r.kernel.org,
        sparclinux@...r.kernel.org, linux-riscv@...ts.infradead.org,
        "open list:GENERIC INCLUDE/ASM HEADER FILES" 
        <linux-arch@...r.kernel.org>, linux-s390@...r.kernel.org,
        Florian Fainelli <f.fainelli@...il.com>,
        linux-c6x-dev@...ux-c6x.org, linux-hexagon@...r.kernel.org,
        arcml <linux-snps-arc@...ts.infradead.org>,
        "moderated list:H8/300 ARCHITECTURE" 
        <uclinux-h8-devel@...ts.sourceforge.jp>,
        linux-xtensa@...ux-xtensa.org, Arnd Bergmann <arnd@...db.de>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-um@...ts.infradead.org, linux-m68k@...ts.linux-m68k.org,
        Openrisc <openrisc@...ts.librecores.org>,
        "moderated list:ARM/FREESCALE IMX / MXC ARM ARCHITECTURE" 
        <linux-arm-kernel@...ts.infradead.org>,
        linux-parisc@...r.kernel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        linux-alpha@...r.kernel.org, Olof Johansson <olof@...om.net>,
        nios2-dev@...ts.rocketboards.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v2 0/2] arm64: Cut rebuild time when changing
 CONFIG_BLK_DEV_INITRD

On Thu, Oct 25, 2018 at 10:38:34AM +0100, Mike Rapoport wrote:
> On Wed, Oct 24, 2018 at 02:55:17PM -0500, 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. :)
> > 
> > > 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.
> 
> I think arm64 does not have to redefine __early_init_dt_declare_initrd().
> Something like this might be just all we need (completely untested,
> probably it won't even compile):

The alternative solution would be to replace initrd_start/initrd_end
with physical address versions of these everywhere - that's what
we're passed from DT, it's what 32-bit ARM would prefer, and seemingly
what 64-bit ARM would also like as well.

Grepping for initrd_start in arch/*/mm shows that there's lots of
architectures that have virtual/physical conversions on these, and
a number that have obviously been derived from 32-bit ARM's approach
(with maintaining a phys_initrd_start variable to simplify things).

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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