linux-kernel - [PATCH RFC 02/13] docs: dma-api: document __dma_align

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Message-ID: <192a335d783a2e54f539dc5ff81bf3207dafa88f.1767089672.git.mst@redhat.com>
Date: Tue, 30 Dec 2025 05:15:49 -0500
From: "Michael S. Tsirkin" <mst@...hat.com>
To: linux-kernel@...r.kernel.org
Cc: Cong Wang <xiyou.wangcong@...il.com>, Jonathan Corbet <corbet@....net>,
	Olivia Mackall <olivia@...enic.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Jason Wang <jasowang@...hat.com>,
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	Stefan Hajnoczi <stefanha@...hat.com>,
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Subject: [PATCH RFC 02/13] docs: dma-api: document __dma_align_begin/end

Document the __dma_align_begin/__dma_align_end annotations
introduced by the previous patch.

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
---
 Documentation/core-api/dma-api-howto.rst | 42 ++++++++++++++++++++++++
 1 file changed, 42 insertions(+)

diff --git a/Documentation/core-api/dma-api-howto.rst b/Documentation/core-api/dma-api-howto.rst
index 96fce2a9aa90..99eda4c5c8e7 100644
--- a/Documentation/core-api/dma-api-howto.rst
+++ b/Documentation/core-api/dma-api-howto.rst
@@ -146,6 +146,48 @@ What about block I/O and networking buffers?  The block I/O and
 networking subsystems make sure that the buffers they use are valid
 for you to DMA from/to.
 
+__dma_from_device_aligned_begin/end annotations
+===============================================
+
+As explained previously, when a structure contains a DMA_FROM_DEVICE buffer
+(device writes to memory) alongside fields that the CPU writes to, cache line
+sharing between the DMA buffer and CPU-written fields can cause data corruption
+on CPUs with DMA-incoherent caches.
+
+The ``__dma_from_device_aligned_begin/__dma_from_device_aligned_end``
+annotations ensure proper alignment to prevent this::
+
+	struct my_device {
+		spinlock_t lock1;
+		__dma_from_device_aligned_begin char dma_buffer1[16];
+		char dma_buffer2[16];
+		__dma_from_device_aligned_end spinlock_t lock2;
+	};
+
+On cache-coherent platforms these macros expand to nothing. On non-coherent
+platforms, they ensure the minimal DMA alignment, which can be as large as 128
+bytes.
+
+.. note::
+
+	To isolate a DMA buffer from adjacent fields, you must apply
+	``__dma_from_device_aligned_begin`` to the first DMA buffer field
+	**and additionally** apply ``__dma_from_device_aligned_end`` to the
+	**next** field in the structure, **beyond** the DMA buffer (as opposed
+	to the last field of the DMA buffer!).  This protects both the head and
+	tail of the buffer from cache line sharing.
+
+	When the DMA buffer is the **last field** in the structure, just
+	``__dma_from_device_aligned_begin`` is enough - the compiler's struct
+	padding protects the tail::
+
+		struct my_device {
+			spinlock_t lock;
+			struct mutex mlock;
+			__dma_from_device_aligned_begin char dma_buffer1[16];
+			char dma_buffer2[16];
+		};
+
 DMA addressing capabilities
 ===========================
 
-- 
MST