linux-kernel - Re: [PATCH] riscv: Add pgprot

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Message-ID: <CAK9=C2UrYg+sHsDqkmz7f7KnaREU7hXF4rbON002cnAti_pDXg@mail.gmail.com>
Date: Sun, 12 Oct 2025 21:21:11 +0530
From: Anup Patel <apatel@...tanamicro.com>
To: guoren@...nel.org
Cc: samuel.holland@...ive.com, david@...hat.com, yongxuan.wang@...ive.com, 
	cuiyunhui@...edance.com, luxu.kernel@...edance.com, paul.walmsley@...ive.com, 
	aou@...s.berkeley.edu, alex@...ti.fr, palmer@...belt.com, 
	linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: Add pgprot_dmacoherent definition

On Sat, Oct 11, 2025 at 9:28 PM <guoren@...nel.org> wrote:
>
> From: "Guo Ren (Alibaba DAMO Academy)" <guoren@...nel.org>
>
> RISC-V Svpbmt Standard Extension for Page-Based Memory Types
> defines three modes:
>
>  Mode | Value | Requested Memory Attributes
>  PMA  |   0   | None
>  NC   |   1   | Non-cacheable, idempotent, weakly-ordered (RVWMO),
>       |       | main memory
>  IO   |   2   | Non-cacheable, non-idempotent, strongly-ordered
>       |       | (I/O ordering), I/O
>
> The pgprot_dmacoherent default uses the IO memory attribute if there
> is no asm definition, but IO is not for main memory according to
> Svpbmt rules.
>
> This commit corrects pgprot_dmacoherent with the NC memory attribute,
> which satisfies performance improvement and prevents using the IO
> attribute to access main memory.
>
> Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@...nel.org>

I had sent the same patch on Aug 20 and you had provided
Tested-by to that patch.

If you had concerns with my patch then you could have provided
comments but you choose to hijack it and change authorship.

Regards,
Anup

> ---
>  arch/riscv/include/asm/pgtable.h | 9 +++++++++
>  1 file changed, 9 insertions(+)
>
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 29e994a9afb6..2a84479de81b 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -654,6 +654,15 @@ static inline pgprot_t pgprot_writecombine(pgprot_t _prot)
>         return __pgprot(prot);
>  }
>
> +/*
> + * DMA allocations for non-coherent devices use what the RISC-V architecture
> + * call "Non-Cacheable" memory attribute, which permits idempotent, weakly-ordered
> + * (RVWMO), main memory. This is different from "I/O" memory attribute which is
> + * intended for MMIO access with Non-cacheable, non-idempotent, strongly-ordered
> + * (I/O ordering), I/O attributes.
> + */
> +#define pgprot_dmacoherent pgprot_writecombine
> +
>  /*
>   * Both Svade and Svadu control the hardware behavior when the PTE A/D bits need to be set. By
>   * default the M-mode firmware enables the hardware updating scheme when only Svadu is present in
> --
> 2.40.1
>
>