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    SubjectRe: [PATCH] MIPS: Allow compressed images to be loaded at the usual address
    Hi,

    On 13 February 2017 at 09:37, Alban <albeu@free.fr> wrote:
    > On Thu, 9 Feb 2017 13:22:37 +0100
    > Jonas Gorski <jonas.gorski@gmail.com> wrote:
    >
    >> Hi,
    >>
    >> On 5 February 2017 at 21:21, Alban <albeu@free.fr> wrote:
    >> > From: Alban Bedel <albeu@free.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@free.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.

    With BAL (branch and link) you can do a pc-relative jump, and the
    current address will be stored in $ra. comparing it with the expected
    address will give you the offset by which you were loaded. To quote
    the lzma-loader from OpenWrt/Lede[1]:

    la t0, __reloc_label # get linked address of label
    bal __reloc_label # branch and link to label to
    nop # get actual address
    __reloc_label:
    subu t0, ra, t0 # get reloc_delta
    beqz t0, __reloc_done # if delta is 0 we are in the
    right place
    nop

    /* Copy our code to the right place */
    la t1, _code_start # get linked address of _code_start
    la t2, _code_end # get linked address of _code_end
    addu t0, t0, t1 # calculate actual address of
    _code_start

    __reloc_copy:
    ...
    __reloc_done:
    ...


    Regards
    Jonas

    [1] https://git.lede-project.org/?p=source.git;a=blob;f=target/linux/ar71xx/image/lzma-loader/src/head.S;h=47a7c9bd6300ad92e6a0d426c5f44bc0f3e7e85f;hb=HEAD#l49

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