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

From: Guo Ren <guoren@kernel.org>
To: Anup Patel <apatel@ventanamicro.com>
Cc: Alexandre Ghiti <alex@ghiti.fr>,
	Atish Patra <atish.patra@linux.dev>,
	Anup Patel <anup@brainfault.org>,
	"Rafael J . Wysocki" <rafael@kernel.org>,
	linux-kernel@vger.kernel.org, Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	linux-riscv@lists.infradead.org,
	Andrew Jones <ajones@ventanamicro.com>
Subject: Re: [PATCH] RISC-V: Define pgprot_dmacoherent() for non-coherent devices
Date: Sun, 12 Oct 2025 02:07:34 -0400	[thread overview]
Message-ID: <aOtFpju/42kVkBsx@gmail.com> (raw)
In-Reply-To: <20250820152316.1012757-1-apatel@ventanamicro.com>

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@ventanamicro.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@kernel.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
> 
> 
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> 

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