linux-kernel - Re: [PATCH] RISC-V: Define pgprot_dmacoherent() for non-coherent devices

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Message-ID: <aOtFpju/42kVkBsx@gmail.com>
Date: Sun, 12 Oct 2025 02:07:34 -0400
From: Guo Ren <guoren@...nel.org>
To: Anup Patel <apatel@...tanamicro.com>
Cc: Sunil V L <sunilvl@...tanamicro.com>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Alexandre Ghiti <alex@...ti.fr>,
	Atish Patra <atish.patra@...ux.dev>,
	Andrew Jones <ajones@...tanamicro.com>,
	Anup Patel <anup@...infault.org>, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RISC-V: Define pgprot_dmacoherent() for non-coherent
 devices

On Wed, Aug 20, 2025 at 08:53:16PM +0530, Anup Patel wrote:
> The pgprot_dmacoherent() is used when allocating memory for
> non-coherent devices and by default pgprot_dmacoherent() is
> same as pgprot_noncached() unless architecture overrides it.
> 
> Currently, there is no pgprot_dmacoherent() definition for
> RISC-V hence non-coherent device memory is being mapped as
> IO thereby making CPU access to such memory slow.
> 
> Define pgprot_dmacoherent() to be same as pgprot_writecombine()
> for RISC-V so that CPU access non-coherent device memory as
> NOCACHE which is better than accessing it as IO.
> 
> Fixes: ff689fd21cb1 ("riscv: add RISC-V Svpbmt extension support")
> Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> ---
>  arch/riscv/include/asm/pgtable.h | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/pgtable.h b/arch/riscv/include/asm/pgtable.h
> index 91697fbf1f90..00d8bdaf1e8d 100644
> --- a/arch/riscv/include/asm/pgtable.h
> +++ b/arch/riscv/include/asm/pgtable.h
> @@ -653,6 +653,8 @@ static inline pgprot_t pgprot_writecombine(pgprot_t _prot)
>  	return __pgprot(prot);
>  }
>  
> +#define pgprot_dmacoherent pgprot_writecombine
I missed this patch and sent out a duplicate one [1]. Maybe the comments
from [1] could be appended to this one.

Tested-by: Guo Ren (Alibaba DAMO Academy) <guoren@...nel.org>

> +
>  /*
>   * 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
> -- 
> 2.43.0
> 
> 
> _______________________________________________
> linux-riscv mailing list
> linux-riscv@...ts.infradead.org
> http://lists.infradead.org/mailman/listinfo/linux-riscv
>

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