linux-kernel - Re: [PATCH v2] MIPS: Allow compressed images to be loaded at any address

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Date:   Mon, 13 Feb 2017 22:19:45 +0000
From:   James Hogan <james.hogan@...tec.com>
To:     Alban <albeu@...e.fr>
CC:     <linux-mips@...ux-mips.org>, Ralf Baechle <ralf@...ux-mips.org>,
        Jonas Gorski <jogo@...nwrt.org>,
        <linux-kernel@...r.kernel.org>,
        Matt Redfearn <Matt.Redfearn@...tec.com>
Subject: Re: [PATCH v2] MIPS: Allow compressed images to be loaded at any
 address

Hi Alban,

On Mon, Feb 13, 2017 at 09:38:08PM +0100, Alban wrote:
> From: Alban Bedel <albeu@...e.fr>
> 
> Compressed images (vmlinuz.bin) have to be loaded at a specific
> address that differ from the address normaly used for vmlinux.bin.
> This is because the decompressor just write its output at the address
> vmlinux.bin should be loaded at, and it shouldn't overwrite itself.
> This limitation mean that the bootloader must be configured differently
> when loading a vmlinux.bin or a vmlinuz.bin image, this is annoying
> and a source of error.
> 
> To workaround this we extend the compressed loader to cope with being
> loaded at (nearly) any address. During the early init a jump is used
> to compute the offset between the current address and the linked
> address, if they differ the whole image is first copied to the linked
> address before proceeding.
> 
> Some load address won't work, for example if there is an overlap with
> the range where vmlinuz.bin should be loaded. However for the typical
> case of using the vmlinux.bin address that won't be the case.
> 
> Signed-off-by: Alban Bedel <albeu@...e.fr>
> Suggested-by: Jonas Gorski <jonas.gorski@...il.com>
> ---
> Changelog:
> v2: * Rework the code as suggested by Jonas Gorski to autodetect the
>       load address and remove the need for a Kconfig option.
> ---
>  arch/mips/boot/compressed/head.S | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/arch/mips/boot/compressed/head.S b/arch/mips/boot/compressed/head.S
> index 409cb48..3c25a96 100644
> --- a/arch/mips/boot/compressed/head.S
> +++ b/arch/mips/boot/compressed/head.S
> @@ -25,6 +25,29 @@ start:
>  	move	s2, a2
>  	move	s3, a3
>  
> +	/* Get the offset between the current address and linked address */
> +	PTR_LA	t0, reloc_label
> +	bal	reloc_label
> +	 nop
> +reloc_label:
> +	subu	t0, ra, t0
> +
> +	/* If there is no offset no reloc is needed */
> +	beqz	t0, clear_bss
> +	 nop
> +
> +	/* Move the text, data section and DTB to the correct address */
> +	PTR_LA	a0, .text
> +	addu	a1, t0, a0
> +	PTR_LA	a2, _edata
> +copy_vmlinuz:
> +	lw	a3, 0(a1)
> +	sw	a3, 0(a0)
> +	addiu	a0, a0, 4
> +	bne	a2, a0, copy_vmlinuz
> +	 addiu	a1, a1, 4

Does this need to sync the icache and resolve the instruction hazard
before jumping into the newly written code?

E.g. on mips32/64 r2 and later you could I think "synci" at SYNCI_Step
intervals (as determined by RDHWR instruction), followed by a "sync" and
then using "jr.hb" instead of "jr" to clear the instruction hazard while
jumping to the newly written code.

That is roughly what arch/mips/kernel/relocate.c and
arch/mips/kernel/head.S do, but as mentioned that assumes MIPS32/64 r2+,
and at least 2 platforms selecting SYS_SUPPORTS_ZBOOT* also select
CPU_HAS_CPU_MIPS32_R1.

Cheers
James

> +
> +clear_bss:
>  	/* Clear BSS */
>  	PTR_LA	a0, _edata
>  	PTR_LA	a2, _end
> -- 
> 2.7.4
> 
> 

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