linux-kernel - Re: [PATCH 10/15] riscv: read the hart ID from mhartid on boot

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Date:   Fri, 18 Oct 2019 08:31:01 +0530
From:   Anup Patel <anup@...infault.org>
To:     Christoph Hellwig <hch@....de>
Cc:     Palmer Dabbelt <palmer@...ive.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Atish Patra <atish.patra@....com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/15] riscv: read the hart ID from mhartid on boot

On Thu, Oct 17, 2019 at 11:08 PM Christoph Hellwig <hch@....de> wrote:
>
> From: Damien Le Moal <Damien.LeMoal@....com>
>
> When in M-Mode, we can use the mhartid CSR to get the ID of the running
> HART. Doing so, direct M-Mode boot without firmware is possible.
>
> Signed-off-by: Damien Le Moal <damien.lemoal@....com>
> Signed-off-by: Christoph Hellwig <hch@....de>
> Reviewed-by: Atish Patra <atish.patra@....com>
> ---
>  arch/riscv/include/asm/csr.h | 1 +
>  arch/riscv/kernel/head.S     | 8 ++++++++
>  2 files changed, 9 insertions(+)
>
> diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
> index 0dae5c361f29..d0b5113e1a54 100644
> --- a/arch/riscv/include/asm/csr.h
> +++ b/arch/riscv/include/asm/csr.h
> @@ -81,6 +81,7 @@
>  #define SIE_SEIE               (_AC(0x1, UL) << IRQ_S_EXT)
>
>  /* symbolic CSR names: */
> +#define CSR_MHARTID            0xf14
>  #define CSR_MSTATUS            0x300
>  #define CSR_MIE                        0x304
>  #define CSR_MTVEC              0x305
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 679e63d29edb..583784cb3a32 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -50,6 +50,14 @@ _start_kernel:
>         csrw CSR_XIE, zero
>         csrw CSR_XIP, zero
>
> +#ifdef CONFIG_RISCV_M_MODE
> +       /*
> +        * The hartid in a0 is expected later on, and we have no firmware
> +        * to hand it to us.
> +        */
> +       csrr a0, CSR_MHARTID
> +#endif
> +
>         /* Load the global pointer */
>  .option push
>  .option norelax
> --
> 2.20.1
>
>
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv

LGTM.

Reviewed-by: Anup Patel <anup@...infault.org>

Regards,
Anup

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