Re: [RFC 2/3] MIPS: boot: realize _start in C

[Sascha Hauer <s.hauer@pengutronix.de>]

On Sat, Nov 17, 2012 at 04:10:38PM +0400, Antony Pavlov wrote:
> The pbl will be incharge of the lowlevel init and
> barebox relocation. So we can skip it in barebox itself
> and use simple _start.
> 
> Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
> ---
>  arch/mips/boot/Makefile     |    1 -
>  arch/mips/boot/main_entry.c |   18 ++++++
>  arch/mips/boot/start.S      |  142 -------------------------------------------
>  3 files changed, 18 insertions(+), 143 deletions(-)
>  delete mode 100644 arch/mips/boot/start.S
> 
> diff --git a/arch/mips/boot/Makefile b/arch/mips/boot/Makefile
> index 6b093f1..4f11a48 100644
> --- a/arch/mips/boot/Makefile
> +++ b/arch/mips/boot/Makefile
> @@ -1,4 +1,3 @@
> -obj-y += start.o
>  obj-y += main_entry.o
>  
>  pbl-y += start-pbl.o main_entry-pbl.o
> diff --git a/arch/mips/boot/main_entry.c b/arch/mips/boot/main_entry.c
> index 0a33c45..422d40b 100644
> --- a/arch/mips/boot/main_entry.c
> +++ b/arch/mips/boot/main_entry.c
> @@ -22,6 +22,8 @@
>  #include <asm/cpu-features.h>
>  #include <asm/mipsregs.h>
>  #include <asm/addrspace.h>
> +#include <asm/regdef.h>
> +#include <asm-generic/memory_layout.h>
>  
>  extern void start_barebox(void);
>  extern void handle_reserved(void);
> @@ -92,3 +94,19 @@ void main_entry(void)
>  
>  	start_barebox();
>  }
> +
> +void __naked __section(.text_entry) _start(void)
> +{
> +	u32 r;
> +
> +	/* setup the stack */
> +	r = STACK_BASE + STACK_SIZE - 16;
> +
> +	__asm__ __volatile__(
> +		"move\t$sp, %0\n\t"
> +		: /* no outputs */
> +		: "r" (r)
> +	);
> +
> +	main_entry();

I wonder that the C function skips several things the Assembler version
does. Is this done in the pbl now? Does this patch make the pbl
mandatory? We haven't done this on Arm, mostly because we were not
confident enough that it works on every board. That may be different on
mips, but should be mentioned in the commit log. But maybe I am
misreading the patches...

Sascha


> +}
> diff --git a/arch/mips/boot/start.S b/arch/mips/boot/start.S
> deleted file mode 100644
> index 7e2ae5e..0000000
> --- a/arch/mips/boot/start.S
> +++ /dev/null
> @@ -1,142 +0,0 @@
> -/*
> - * Startup Code for MIPS CPU
> - *
> - * Copyright (C) 2011 Antony Pavlov <antonynpavlov@gmail.com>
> - * Used code copyrighted (C) 2009 by Shinya Kuribayashi <skuribay@pobox.com>
> - *
> - * This file is part of barebox.
> - * See file CREDITS for list of people who contributed to this project.
> - *
> - * This program is free software; you can redistribute it and/or modify
> - * it under the terms of the GNU General Public License version 2
> - * as published by the Free Software Foundation.
> - *
> - * This program is distributed in the hope that it will be useful,
> - * but WITHOUT ANY WARRANTY; without even the implied warranty of
> - * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> - * GNU General Public License for more details.
> - *
> - */
> -
> -#include <asm/regdef.h>
> -#include <asm/mipsregs.h>
> -#include <asm/asm.h>
> -#include <asm-generic/memory_layout.h>
> -#include <generated/compile.h>
> -#include <generated/utsrelease.h>
> -
> -	/*
> -	 * ADR macro instruction (inspired by ARM)
> -	 *
> -	 * ARM architecture doesn't have PC-relative jump instruction
> -	 * like MIPS' B/BAL insns.  When ARM makes PC-relative jumps,
> -	 * it uses ADR insn.  ADR is used to get a destination address
> -	 * of 'label' against current PC.  With this, ARM can safely
> -	 * make PC-relative jumps.
> -	 */
> -	.macro	ADR rd label temp
> -	.set	push
> -	.set	noreorder
> -	move	\temp, ra			# preserve ra beforehand
> -	bal	_pc
> -	 nop
> -_pc:	addiu	\rd, ra, \label - _pc		# label is assumed to be
> -	move	ra, \temp			# within pc +/- 32KB
> -	.set	pop
> -	.endm
> -
> -	.set noreorder
> -	.text
> -	.section ".text_bare_init"
> -	.align 4
> -
> -EXPORT(_start)
> -
> -	b	__start
> -	 nop
> -
> -	.org	0x10
> -	.ascii	"barebox " UTS_RELEASE " " UTS_VERSION
> -	.byte	0
> -
> -	.align 4
> -__start:
> -	/* disable watchpoints */
> -	mtc0	zero, CP0_WATCHLO
> -	mtc0	zero, CP0_WATCHHI
> -
> -	/* disable interrupts */
> -	mfc0	k0, CP0_STATUS
> -	li	k1, ~ST0_IE
> -	and	k0, k1
> -	mtc0	k0, CP0_STATUS
> -
> -	/* copy barebox to link location */
> -	ADR	a0, _start, t1	/* a0 <- pc-relative position of _start */
> -
> -	la	a1, _start	/* link (RAM) _start address */
> -
> -	beq	a0, a1, stack_setup
> -	 nop
> -
> -	la	t0, _start
> -	la	t1, __bss_start
> -	subu	t2, t1, t0	/* t2 <- size of barebox */
> -	addu	a2, a0, t2	/* a2 <- source end address */
> -
> -#define LONGSIZE	4
> -
> -copy_loop:
> -	/* copy from source address [a0] */
> -	lw		t4, LONGSIZE * 0(a0)
> -	lw		t5, LONGSIZE * 1(a0)
> -	lw		t6, LONGSIZE * 2(a0)
> -	lw		t7, LONGSIZE * 3(a0)
> -	/* copy fo target address [a1] */
> -	sw		t4, LONGSIZE * 0(a1)
> -	sw		t5, LONGSIZE * 1(a1)
> -	sw		t6, LONGSIZE * 2(a1)
> -	sw		t7, LONGSIZE * 3(a1)
> -	addi		a0, LONGSIZE * 4
> -	subu		t3, a0, a2
> -	blez		t3, copy_loop
> -	addi		a1, LONGSIZE * 4
> -
> -	/*
> -	 * Dominic Sweetman, See MIPS Run, Morgan Kaufmann, 2nd edition, 2006
> -	 *
> -	 * 11.2.2 Stack Argument Structure in o32
> -	 * ...
> -	 * At the point where a function is called, sp must be
> -	 * eight-byte-aligned, matching the alignment of the largest
> -	 * basic types -- a long long integer or a floating-point double.
> -	 * The eight-byte alignment is not required by 32-bit MIPS integer
> -	 * hardware, but it's essential for compatibility with CPUs with
> -	 * 64-bit registers, and thus part of the rules. Subroutines fit
> -	 * in with this by always adjusting the stack pointer by a multiple
> -	 * of eight.
> -	 * ...
> -	 * SGI's n32 and n64 standards call for the stack to be maintained
> -	 * with 16-byte alignment.
> -	 *
> -	 */
> -
> -#if (STACK_BASE + STACK_SIZE) % 16 != 0
> -#error stack pointer must be 16-byte-aligned
> -#endif
> -
> -stack_setup:
> -	la	sp, STACK_BASE + STACK_SIZE
> -
> -	/* reserve four 32-bit argument slots */
> -	addiu	sp, -16
> -
> -	la	v0, main_entry
> -	jal     v0
> -	 nop
> -
> -	/* No return */
> -
> -__error:
> -	b __error
> -	 nop
> -- 
> 1.7.10.4
> 
> 
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