linux-kernel - Re: [PATCH] MIPS: Allow compressed images to be loaded at the usual address

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Message-ID: <20170213093736.1ee183f3@tock>
Date:   Mon, 13 Feb 2017 09:37:36 +0100
From:   Alban <albeu@...e.fr>
To:     Jonas Gorski <jonas.gorski@...il.com>
Cc:     Aban Bedel <albeu@...e.fr>,
        MIPS Mailing List <linux-mips@...ux-mips.org>,
        Ralf Baechle <ralf@...ux-mips.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] MIPS: Allow compressed images to be loaded at the usual
 address

On Thu, 9 Feb 2017 13:22:37 +0100
Jonas Gorski <jonas.gorski@...il.com> wrote:

> Hi,
> 
> On 5 February 2017 at 21:21, Alban <albeu@...e.fr> wrote:
> > From: Alban Bedel <albeu@...e.fr>
> >
> > Normally compressed images have to be loaded at a different address to
> > allow the decompressor to run. This add an option to let vmlinuz copy
> > itself to the correct address from the normal vmlinux address.
> >
> > Signed-off-by: Alban Bedel <albeu@...e.fr>
> > ---
> >  arch/mips/Kconfig                |  8 ++++++++
> >  arch/mips/boot/compressed/head.S | 13 +++++++++++++
> >  2 files changed, 21 insertions(+)
> >
> > diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> > index b3c5bde..8074fc5 100644
> > --- a/arch/mips/Kconfig
> > +++ b/arch/mips/Kconfig
> > @@ -2961,6 +2961,14 @@ choice
> >                 bool "Extend builtin kernel arguments with bootloader arguments"
> >  endchoice
> >
> > +config ZBOOT_VMLINUZ_AT_VMLINUX_LOAD_ADDRESS
> > +       bool "Load compressed images at the same address as uncompressed"
> > +       depends on SYS_SUPPORTS_ZBOOT
> > +       help
> > +         vmlinux and vmlinuz normally have different load addresses, with
> > +         this option vmlinuz expect to be loaded at the same address as
> > +         vmlinux.
> > +
> >  endmenu  
> 
> Okay, it took me a while to understand the intention of this change. I
> thought it was for supporting the case that VMLINUZ_LOAD_ADDRESS ==
> VMLINUX_LOAD_ADDRESS, but it is indented for  VMLINUZ_LOAD_ADDRESS !=
> VMLINUX_LOAD_ADDRESS, but still being loaded at VMLINUX_LOAD_ADDRESS.
> 
> So I guess that this can only happen with vmlinuz.bin, as vmlinux's
> ELF header will cause it to be loaded at the expected address (for
> sane bootloaders at least).

Yes, this is for bootloaders that use raw images. Having to configure
different load addresses for compressed and uncompressed images was just
too annoying.

> >  config LOCKDEP_SUPPORT
> > diff --git a/arch/mips/boot/compressed/head.S b/arch/mips/boot/compressed/head.S
> > index 409cb48..a215171 100644
> > --- a/arch/mips/boot/compressed/head.S
> > +++ b/arch/mips/boot/compressed/head.S
> > @@ -25,6 +25,19 @@ start:
> >         move    s2, a2
> >         move    s3, a3
> >
> > +#ifdef CONFIG_ZBOOT_VMLINUZ_AT_VMLINUX_LOAD_ADDRESS  
> 
> With a bit of BAL trickery you could easily detect this at runtime and
> then conditionally copy without requiring any additional config
> symbols. Then you aren't limited to being executed from
> VMLINUX_LOAD_ADDRESS.

Could you expand a bit on what you mean with "BAL trickery"? I hoped
that it would be possible to auto detect the current running address,
but as I know very little about MIPS assembly I didn't found how that
could be done.

Alban

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