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

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Message-ID: <aOtR39pl5xjyYHn1@gmail.com>
Date: Sun, 12 Oct 2025 02:59:43 -0400
From: Guo Ren <guoren@...nel.org>
To: Anup Patel <apatel@...tanamicro.com>
Cc: Alexandre Ghiti <alex@...ti.fr>, Atish Patra <atish.patra@...ux.dev>,
	Anup Patel <anup@...infault.org>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	linux-kernel@...r.kernel.org, Palmer Dabbelt <palmer@...belt.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	linux-riscv@...ts.infradead.org,
	Andrew Jones <ajones@...tanamicro.com>
Subject: Re: [PATCH] RISC-V: Define pgprot_dmacoherent() for non-coherent
 devices

On Sun, Oct 12, 2025 at 02:07:34AM -0400, Guo Ren wrote:
> 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.
[1]: https://lore.kernel.org/linux-riscv/20251011155746.1558731-1-guoren@kernel.org/

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

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