dmaengine: Document dmaengine_prep_dma_memset

Document this function to make clear the expected behavior of the
'value' parameter. It was intended to match the behavior of POSIX memset
as laid out here:

https://lore.kernel.org/dmaengine/YejrA5ZWZ3lTRO%2F1@matsya/Signed-off-by: Ben Walker <benjamin.walker@intel.com>
Link: https://lore.kernel.org/r/20220301182551.883474-2-benjamin.walker@intel.comSigned-off-by: Vinod Koul <vkoul@kernel.org>

Documentation/driver-api/dmaengine/provider.rst

@@ -206,6 +206,12 @@ Currently, the types available are:
   - The device is able to perform parity check using RAID6 P+Q
     algorithm against a memory buffer.
 
+- DMA_MEMSET
+
+  - The device is able to fill memory with the provided pattern
+
+  - The pattern is treated as a single byte signed value.
+
 - DMA_INTERRUPT
 
   - The device is able to trigger a dummy transfer that will

include/linux/dmaengine.h

@@ -1030,6 +1030,14 @@ static inline struct dma_async_tx_descriptor *dmaengine_prep_interleaved_dma(
 	return chan->device->device_prep_interleaved_dma(chan, xt, flags);
 }
 
+/**
+ * dmaengine_prep_dma_memset() - Prepare a DMA memset descriptor.
+ * @chan: The channel to be used for this descriptor
+ * @dest: Address of buffer to be set
+ * @value: Treated as a single byte value that fills the destination buffer
+ * @len: The total size of dest
+ * @flags: DMA engine flags
+ */
 static inline struct dma_async_tx_descriptor *dmaengine_prep_dma_memset(
 		struct dma_chan *chan, dma_addr_t dest, int value, size_t len,
 		unsigned long flags)