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

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Message-ID: <20250820152316.1012757-1-apatel@ventanamicro.com>
Date: Wed, 20 Aug 2025 20:53:16 +0530
From: Anup Patel <apatel@...tanamicro.com>
To: Sunil V L <sunilvl@...tanamicro.com>,
	"Rafael J . Wysocki" <rafael@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	Paul Walmsley <paul.walmsley@...ive.com>
Cc: 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,
	Anup Patel <apatel@...tanamicro.com>
Subject: [PATCH] RISC-V: Define pgprot_dmacoherent() for non-coherent devices

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
+
 /*
  * 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