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

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Message-ID: <6e647f76-8523-09d3-9b72-d8d8abd213a4@gmail.com>
Date:   Wed, 24 Oct 2018 13:01:19 -0700
From:   Florian Fainelli <f.fainelli@...il.com>
To:     Rob Herring <robh@...nel.org>
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>,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
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        Linux-MIPS <linux-mips@...ux-mips.org>,
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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 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.

Thanks!
-- 
Florian