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Diffstat (limited to 'Documentation/core-api/dma-api-howto.rst')
| -rw-r--r-- | Documentation/core-api/dma-api-howto.rst | 24 |
1 files changed, 22 insertions, 2 deletions
diff --git a/Documentation/core-api/dma-api-howto.rst b/Documentation/core-api/dma-api-howto.rst index e8a55f9d61..0bf31b6c43 100644 --- a/Documentation/core-api/dma-api-howto.rst +++ b/Documentation/core-api/dma-api-howto.rst @@ -203,13 +203,33 @@ setting the DMA mask fails. In this manner, if a user of your driver reports that performance is bad or that the device is not even detected, you can ask them for the kernel messages to find out exactly why. -The standard 64-bit addressing device would do something like this:: +The 24-bit addressing device would do something like this:: - if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) { + if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(24))) { dev_warn(dev, "mydev: No suitable DMA available\n"); goto ignore_this_device; } +The standard 64-bit addressing device would do something like this:: + + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64)) + +dma_set_mask_and_coherent() never return fail when DMA_BIT_MASK(64). Typical +error code like:: + + /* Wrong code */ + if (dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64))) + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32)) + +dma_set_mask_and_coherent() will never return failure when bigger than 32. +So typical code like:: + + /* Recommended code */ + if (support_64bit) + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(64)); + else + dma_set_mask_and_coherent(dev, DMA_BIT_MASK(32)); + If the device only supports 32-bit addressing for descriptors in the coherent allocations, but supports full 64-bits for streaming mappings it would look like this:: |