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Date:   Mon, 29 Apr 2019 16:40:36 -0700 (PDT)
From:   Palmer Dabbelt <palmer@...ive.com>
To:     atish.patra@....com
CC:     linux-kernel@...r.kernel.org, atish.patra@....com,
        aou@...s.berkeley.edu, anup@...infault.org,
        linux-riscv@...ts.infradead.org, zong@...estech.com
Subject:     Re: [PATCH] RISC-V: Add an Image header that boot loader can parse.

On Tue, 23 Apr 2019 16:25:06 PDT (-0700), atish.patra@....com wrote:
> Currently, last stage boot loaders such as U-Boot can accept only
> uImage which is an unnecessary additional step in automating boot flows.
>
> Add a simple image header that boot loaders can parse and directly
> load kernel flat Image. The existing booting methods will continue to
> work as it is.
>
> Tested on both QEMU and HiFive Unleashed using OpenSBI + U-Boot + Linux.
>
> Signed-off-by: Atish Patra <atish.patra@....com>
> ---
>  arch/riscv/include/asm/image.h | 32 ++++++++++++++++++++++++++++++++
>  arch/riscv/kernel/head.S       | 28 ++++++++++++++++++++++++++++
>  2 files changed, 60 insertions(+)
>  create mode 100644 arch/riscv/include/asm/image.h
>
> diff --git a/arch/riscv/include/asm/image.h b/arch/riscv/include/asm/image.h
> new file mode 100644
> index 000000000000..76a7e0d4068a
> --- /dev/null
> +++ b/arch/riscv/include/asm/image.h
> @@ -0,0 +1,32 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef __ASM_IMAGE_H
> +#define __ASM_IMAGE_H
> +
> +#define RISCV_IMAGE_MAGIC	"RISCV"
> +
> +#ifndef __ASSEMBLY__
> +/*
> + * struct riscv_image_header - riscv kernel image header
> + *
> + * @code0:		Executable code
> + * @code1:		Executable code
> + * @text_offset:	Image load offset
> + * @image_size:		Effective Image size
> + * @reserved:		reserved
> + * @magic:		Magic number
> + * @reserved:		reserved
> + */
> +
> +struct riscv_image_header {
> +	u32 code0;
> +	u32 code1;
> +	u64 text_offset;
> +	u64 image_size;
> +	u64 res1;
> +	u64 magic;
> +	u32 res2;
> +	u32 res3;
> +};

I don't want to invent our own file format.  Is there a reason we can't just
use something standard?  Off the top of my head I can think of ELF files and
multiboot.

> +#endif /* __ASSEMBLY__ */
> +#endif /* __ASM_IMAGE_H */
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index fe884cd69abd..154647395601 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -19,9 +19,37 @@
>  #include <asm/thread_info.h>
>  #include <asm/page.h>
>  #include <asm/csr.h>
> +#include <asm/image.h>
>
>  __INIT
>  ENTRY(_start)
> +	/*
> +	 * Image header expected by Linux boot-loaders. The image header data
> +	 * structure is described in asm/image.h.
> +	 * Do not modify it without modifying the structure and all bootloaders
> +	 * that expects this header format!!
> +	 */
> +	/* jump to start kernel */
> +	j _start_kernel
> +	/* reserved */
> +	.word 0
> +	.balign 8
> +#if __riscv_xlen == 64
> +	/* Image load offset(2MB) from start of RAM */
> +	.dword 0x200000
> +#else
> +	/* Image load offset(4MB) from start of RAM */
> +	.dword 0x400000
> +#endif
> +	/* Effective size of kernel image */
> +	.dword _end - _start
> +	.dword 0
> +	.asciz RISCV_IMAGE_MAGIC
> +	.word 0
> +	.word 0
> +
> +.global _start_kernel
> +_start_kernel:
>  	/* Mask all interrupts */
>  	csrw sie, zero

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