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Date:   Sat, 19 Dec 2020 00:35:25 +0900
From:   Johannes Thumshirn <>
To:     Jonathan Corbet <>
        Johannes Thumshirn <>,
        Hannes Duerr <>
Subject: [PATCH] Documentation: document dma device use for mcb

Hannes reported a problem with setting up dma transfers on a mcb device.
The problem boiled down to the use of a wrong 'device' for the dma

Document how to setup dma transfers for a IP core on a mcb carrier.

Reported-by: Hannes Duerr <>
Signed-off-by: Johannes Thumshirn <>
 Documentation/driver-api/men-chameleon-bus.rst | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/Documentation/driver-api/men-chameleon-bus.rst b/Documentation/driver-api/men-chameleon-bus.rst
index 1b1f048aa748..6f0b9ee47595 100644
--- a/Documentation/driver-api/men-chameleon-bus.rst
+++ b/Documentation/driver-api/men-chameleon-bus.rst
@@ -18,6 +18,7 @@ MEN Chameleon Bus
        4.1 The driver structure
        4.2 Probing and attaching
        4.3 Initializing the driver
+       4.4 Using DMA
@@ -173,3 +174,14 @@ module at the MCB core::
 The module_mcb_driver() macro can be used to reduce the above code::
+Using DMA
+To make use of the kernel's DMA-API's function, you will need to use the
+carrier device's 'struct device'. Fortunately 'struct mcb_device' embeds a
+pointer (->dma_dev) to the carrier's device for DMA purposes::
+        ret = dma_set_mask_and_coherent(&mdev->dma_dev, DMA_BIT_MASK(dma_bits));
+        if (rc)
+                /* Handle errors */

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