[U-Boot] [PATCH 1/2] sunxi: Use Thumb2 and move stack to gain more SRAM space in FEL mode

[Siarhei Siamashka <siarhei.siamashka@gmail.com>]

On Fri, 18 Jul 2014 20:09:44 +0300
Siarhei Siamashka <siarhei.siamashka@gmail.com> wrote:

> The Allwinner SoCs support a special FEL boot mode, which can be activated
> by users via a button press (or other means). In the FEL mode, the BROM
> implements a custom FEL protocol over USB, which allows to upload code to
> the device and run it. This protocol had been reverse engineered and
> documented by Henrik Nordstr?m:
> 
>     http://lists.phcomp.co.uk/pipermail/arm-netbook/2012-June/004341.html
> 
> Because the BROM code is using some parts of the SRAM for itself, only a
> few areas are available for use in u-boot. Currently the SPL is loaded
> into the "0x2000-0x5cff Free for program use" area and the stack pointer
> is at the end of this area. This is barely enough to fit just the current
> SPL and leaves almost no headroom for the future code.
> 
> This patch enables the use of a more compact Thumb2 mode for compiling the
> FEL SPL binary. And also relocates the stack to another "0x8000-0xbfff Free
> for program use" area.

Self review.

Maybe instead of adding the stack relocation hacks, a better idea
would be to just change the usb-boot script to load the SPL to 0x8000
address instead of 0x2000? The relevant usb-boot code is here:

    https://github.com/linux-sunxi/sunxi-tools/blob/e2a3f16e36f6/usb-boot#L73

In this case the stack would remain in the 0x2000-0x5cff area, and the
code/data would use 0x8000-0xbfff. However backwards compatibility with
the existing sunxi-tools becomes an issue. Does anyone have any opinion?

> Additionally, the BSS segment is cleared.
> 
> Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
> ---
>  arch/arm/cpu/armv7/sunxi/Makefile           |  1 +
>  arch/arm/cpu/armv7/sunxi/start_fel.S        | 42 +++++++++++++++++++++++++++++
>  arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds |  4 +--
>  include/configs/sunxi-common.h              |  2 --
>  4 files changed, 45 insertions(+), 4 deletions(-)
>  create mode 100644 arch/arm/cpu/armv7/sunxi/start_fel.S
> 
> diff --git a/arch/arm/cpu/armv7/sunxi/Makefile b/arch/arm/cpu/armv7/sunxi/Makefile
> index a64bfa1..b3eff98 100644
> --- a/arch/arm/cpu/armv7/sunxi/Makefile
> +++ b/arch/arm/cpu/armv7/sunxi/Makefile
> @@ -21,5 +21,6 @@ ifdef CONFIG_SPL_BUILD
>  obj-$(CONFIG_SUN7I)	+= dram.o
>  ifdef CONFIG_SPL_FEL
>  obj-y	+= start.o
> +extra-y += start_fel.o
>  endif
>  endif
> diff --git a/arch/arm/cpu/armv7/sunxi/start_fel.S b/arch/arm/cpu/armv7/sunxi/start_fel.S
> new file mode 100644
> index 0000000..2789fd9
> --- /dev/null
> +++ b/arch/arm/cpu/armv7/sunxi/start_fel.S
> @@ -0,0 +1,42 @@
> +/*
> + * Copyright (c) 2014 Siarhei Siamashka <siarhei.siamashka@gmail.com>
> + *
> + * SPDX-License-Identifier:	GPL-2.0+
> + */
> +
> +.syntax unified
> +.text
> +.arm
> +.arch armv7a
> +.p2align 2
> +
> +.globl _start_fel
> +.globl s_init
> +.globl __bss_start
> +.globl __bss_end
> +
> +_start_fel:
> +	/* Relocate stack to the 0x8000-0xBFFF area */
> +	mov	r0, #0xC000
> +	str	sp, [r0, #-4]!
> +	str	lr, [r0, #-4]!
> +	adr	lr, _exit_fel /* Return back to '_exit_fel' */
> +	mov	sp, r0
> +
> +	/* Erase the BSS segment */
> +	ldr	r0, =__bss_start
> +	ldr	r1, =__bss_end
> +	mov	r2, #0
> +0:	cmp	r0, r1
> +	strbne	r2, [r0], #1
> +	bne	0b
> +
> +	/* Pass control to the 's_init()' function */
> +	b	s_init
> +
> +_exit_fel:
> +	/* Relocate stack back and return */
> +	mov	r0, #0xC000
> +	ldr	sp, [r0, #-4]!
> +	ldr	lr, [r0, #-4]!
> +	bx	lr
> diff --git a/arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds b/arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds
> index 364e35c..418c2fc 100644
> --- a/arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds
> +++ b/arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds
> @@ -6,7 +6,7 @@
>   */
>  OUTPUT_FORMAT("elf32-littlearm", "elf32-littlearm", "elf32-littlearm")
>  OUTPUT_ARCH(arm)
> -ENTRY(s_init)
> +ENTRY(_start_fel)
>  SECTIONS
>  {
>  	. = 0x00002000;
> @@ -14,7 +14,7 @@ SECTIONS
>  	. = ALIGN(4);
>  	.text :
>  	{
> -		*(.text.s_init)
> +		arch/arm/cpu/armv7/sunxi/start_fel.o	(.text)
>  		*(.text*)
>  	}
>  
> diff --git a/include/configs/sunxi-common.h b/include/configs/sunxi-common.h
> index 5d72d62..4b980e9 100644
> --- a/include/configs/sunxi-common.h
> +++ b/include/configs/sunxi-common.h
> @@ -18,10 +18,8 @@
>   */
>  #define CONFIG_SUNXI		/* sunxi family */
>  #ifdef CONFIG_SPL_BUILD
> -#ifndef CONFIG_SPL_FEL
>  #define CONFIG_SYS_THUMB_BUILD	/* Thumbs mode to save space in SPL */
>  #endif
> -#endif
>  
>  #include <asm/arch/cpu.h>	/* get chip and board defs */
>  

-- 
Best regards,
Siarhei Siamashka