Re: [PATCH v2 0/2] arm64: Cut rebuild time when changing CONFIG_BLK_DEV_INITRD

[Russell King - ARM Linux <linux@armlinux.org.uk>]

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@gmail.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).

-- 
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