[U-Boot] [PATCH 1/4] sunxi: Make FEL mode usable again

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

On Fri, 30 Jan 2015 13:58:46 +0200
Siarhei Siamashka <siarhei.siamashka@gmail.com> wrote:

> The commit f630974ccb3ce93e9607a3354e9acb266a8b7e95
> 'sunxi: Move SPL s_init() code to board_init_f()'
> broke the FEL boot mode.
> 
> This patch moves the DRAM initialization back to s_init() and
> introduces an assembly entry point for FEL in order to provide
> guaranteed initialization of the gdata pointer (r9). The assembly
> entry point is also needed to ensure that the SPL code starts
> executing in ARM mode.
> 
> Because the sunxi board_init_f() does not contain anything that
> is not already done by the default board_init_f(), it is removed
> too.
> 
> Signed-off-by: Siarhei Siamashka <siarhei.siamashka@gmail.com>
> ---
>  arch/arm/cpu/armv7/sunxi/Makefile           |  1 +
>  arch/arm/cpu/armv7/sunxi/board.c            | 26 ++++++++++----------------
>  arch/arm/cpu/armv7/sunxi/start_fel.S        | 16 ++++++++++++++++
>  arch/arm/cpu/armv7/sunxi/u-boot-spl-fel.lds |  3 ++-
>  4 files changed, 29 insertions(+), 17 deletions(-)
>  create mode 100644 arch/arm/cpu/armv7/sunxi/start_fel.S

[...]

> +++ b/arch/arm/cpu/armv7/sunxi/start_fel.S
> @@ -0,0 +1,16 @@
> +/*
> + * Entry point of the FEL mode SPL.
> + *
> + * Copyright (c) 2015 Siarhei Siamashka <siarhei.siamashka@gmail.com>
> + *
> + * SPDX-License-Identifier:	GPL-2.0+
> + */
> +
> +#include <asm-offsets.h>
> +#include <config.h>
> +#include <linux/linkage.h>
> +
> +ENTRY(_start_fel)
> +	ldr	r9, =gdata
> +	b	s_init
> +ENDPROC(_start_fel)

In fact, we probably need to save/restore the r9 register and do it as:

    push    {r9, lr}
    ldr     r9, =gdata
    bl      s_init
    pop     {r9, pc}

And maybe save some other registers, depending on the calling
conventions expected by the FEL code in BROM.

As a side note, corrupting r9 mimics the old u-boot sunxi behaviour.
And it used not to cause any visible problems so far, at least
when working with the BROM code in the current Allwinner SoCs.

Also as I see it, the ".bss" sections is supposed to be in DRAM,
and cleared only after the DRAM is initialized. This violates the
C standard a little bit and enforces some sort of u-boot specific 
coding tricks. Such as explicitly placing gdata in the ".data"
section instead of ".bss". This is ugly, but probably justified.

I'll submit a fixed v2 version of this patch later, but will first
wait for additional comments from the other people.

-- 
Best regards,
Siarhei Siamashka