docs: dma-api: document __dma_from_device_group_begin()/end()

Document the __dma_from_device_group_begin()/end() annotations.

Message-ID: <01ea88055ded4d70cac70ba557680fd5fa7d9ff5.1767601130.git.mst@redhat.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Petr Tesarik <ptesarik@suse.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Author: mstsirkin

Documentation/core-api/dma-api-howto.rst

@@ -146,6 +146,58 @@ 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_group_begin/end annotations
+=============================================
+
+As explained previously, when a structure contains a DMA_FROM_DEVICE /
+DMA_BIDIRECTIONAL 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_group_begin(GROUP)/__dma_from_device_group_end(GROUP)``
+macros ensure proper alignment to prevent this::
+
+	struct my_device {
+		spinlock_t lock1;
+		__dma_from_device_group_begin();
+		char dma_buffer1[16];
+		char dma_buffer2[16];
+		__dma_from_device_group_end();
+		spinlock_t lock2;
+	};
+
+To isolate a DMA buffer from adjacent fields, use
+``__dma_from_device_group_begin(GROUP)`` before the first DMA buffer
+field and ``__dma_from_device_group_end(GROUP)`` after the last DMA
+buffer field (with the same GROUP name). This protects both the head
+and tail of the buffer from cache line sharing.
+
+The GROUP parameter is an optional identifier that names the DMA buffer group
+(in case you have several in the same structure)::
+
+	struct my_device {
+		spinlock_t lock1;
+		__dma_from_device_group_begin(buffer1);
+		char dma_buffer1[16];
+		__dma_from_device_group_end(buffer1);
+		spinlock_t lock2;
+		__dma_from_device_group_begin(buffer2);
+		char dma_buffer2[16];
+		__dma_from_device_group_end(buffer2);
+	};
+
+On cache-coherent platforms these macros expand to zero-length array markers.
+On non-coherent platforms, they also ensure the minimal DMA alignment, which
+can be as large as 128 bytes.
+
+.. note::
+
+        It is allowed (though somewhat fragile) to include extra fields, not
+        intended for DMA from the device, within the group (in order to pack the
+        structure tightly) - but only as long as the CPU does not write these
+        fields while any fields in the group are mapped for DMA_FROM_DEVICE or
+        DMA_BIDIRECTIONAL.
+
 DMA addressing capabilities
 ===========================