From: Will Deacon <will@kernel.org>
To: linux-arm-kernel@lists.infradead.org
Cc: Will Deacon <will@kernel.org>,
	Steffen Eiden <seiden@linux.ibm.com>,
	Andreas Grapentin <gra@linux.ibm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Marc Zyngier <maz@kernel.org>
Subject: [PATCH] arm64: mm: Treat all devices as dma-coherent when CLIDR_EL1.LoC == 0
Date: Thu, 23 Jul 2026 14:03:21 +0100
Message-ID: <20260723130322.3179-1-will@kernel.org>
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On systems where CLIDR_EL1.LoC == 0, no cache maintenance is required
when cleaning or invalidating to the Point of Coherency and therefore
all DMA agents can be treated as coherent.

Extend arch_setup_dma_ops() to take CLIDR_EL1.LoC into account when
setting the DMA ops for a device, emitting a warning message if the
firmware advertises a non-coherent device on a fully coherent system.

Cc: Steffen Eiden <seiden@linux.ibm.com>
Cc: Andreas Grapentin <gra@linux.ibm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
---
 arch/arm64/mm/dma-mapping.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/arch/arm64/mm/dma-mapping.c b/arch/arm64/mm/dma-mapping.c
index 994b7b36e2b9..7f6f2f40dba2 100644
--- a/arch/arm64/mm/dma-mapping.c
+++ b/arch/arm64/mm/dma-mapping.c
@@ -42,6 +42,11 @@ void arch_setup_dma_ops(struct device *dev, bool coherent)
 {
 	int cls = cache_line_size_of_cpu();
 
+	if (!coherent && !CLIDR_LOC(read_sysreg(clidr_el1))) {
+		dev_warn(dev, "CLIDR_EL1.LoC == 0, treating as coherent\n");
+		coherent = true;
+	}
+
 	WARN_TAINT(!coherent && cls > ARCH_DMA_MINALIGN,
 		   TAINT_CPU_OUT_OF_SPEC,
 		   "%s %s: ARCH_DMA_MINALIGN smaller than CTR_EL0.CWG (%d < %d)",
-- 
2.55.0.229.g6434b31f56-goog