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diff --git a/Documentation/dmaengine.txt b/Documentation/dmaengine.txt new file mode 100644 index 00000000..5a0cb1ef --- /dev/null +++ b/Documentation/dmaengine.txt @@ -0,0 +1,96 @@ + DMA Engine API Guide + ==================== + + Vinod Koul <vinod dot koul at intel.com> + +NOTE: For DMA Engine usage in async_tx please see: + Documentation/crypto/async-tx-api.txt + + +Below is a guide to device driver writers on how to use the Slave-DMA API of the +DMA Engine. This is applicable only for slave DMA usage only. + +The slave DMA usage consists of following steps +1. Allocate a DMA slave channel +2. Set slave and controller specific parameters +3. Get a descriptor for transaction +4. Submit the transaction and wait for callback notification + +1. Allocate a DMA slave channel +Channel allocation is slightly different in the slave DMA context, client +drivers typically need a channel from a particular DMA controller only and even +in some cases a specific channel is desired. To request a channel +dma_request_channel() API is used. + +Interface: +struct dma_chan *dma_request_channel(dma_cap_mask_t mask, + dma_filter_fn filter_fn, + void *filter_param); +where dma_filter_fn is defined as: +typedef bool (*dma_filter_fn)(struct dma_chan *chan, void *filter_param); + +When the optional 'filter_fn' parameter is set to NULL dma_request_channel +simply returns the first channel that satisfies the capability mask. Otherwise, +when the mask parameter is insufficient for specifying the necessary channel, +the filter_fn routine can be used to disposition the available channels in the +system. The filter_fn routine is called once for each free channel in the +system. Upon seeing a suitable channel filter_fn returns DMA_ACK which flags +that channel to be the return value from dma_request_channel. A channel +allocated via this interface is exclusive to the caller, until +dma_release_channel() is called. + +2. Set slave and controller specific parameters +Next step is always to pass some specific information to the DMA driver. Most of +the generic information which a slave DMA can use is in struct dma_slave_config. +It allows the clients to specify DMA direction, DMA addresses, bus widths, DMA +burst lengths etc. If some DMA controllers have more parameters to be sent then +they should try to embed struct dma_slave_config in their controller specific +structure. That gives flexibility to client to pass more parameters, if +required. + +Interface: +int dmaengine_slave_config(struct dma_chan *chan, + struct dma_slave_config *config) + +3. Get a descriptor for transaction +For slave usage the various modes of slave transfers supported by the +DMA-engine are: +slave_sg - DMA a list of scatter gather buffers from/to a peripheral +dma_cyclic - Perform a cyclic DMA operation from/to a peripheral till the + operation is explicitly stopped. +The non NULL return of this transfer API represents a "descriptor" for the given +transaction. + +Interface: +struct dma_async_tx_descriptor *(*chan->device->device_prep_dma_sg)( + struct dma_chan *chan, + struct scatterlist *dst_sg, unsigned int dst_nents, + struct scatterlist *src_sg, unsigned int src_nents, + unsigned long flags); +struct dma_async_tx_descriptor *(*chan->device->device_prep_dma_cyclic)( + struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len, + size_t period_len, enum dma_data_direction direction); + +4. Submit the transaction and wait for callback notification +To schedule the transaction to be scheduled by dma device, the "descriptor" +returned in above (3) needs to be submitted. +To tell the dma driver that a transaction is ready to be serviced, the +descriptor->submit() callback needs to be invoked. This chains the descriptor to +the pending queue. +The transactions in the pending queue can be activated by calling the +issue_pending API. If channel is idle then the first transaction in queue is +started and subsequent ones queued up. +On completion of the DMA operation the next in queue is submitted and a tasklet +triggered. The tasklet would then call the client driver completion callback +routine for notification, if set. +Interface: +void dma_async_issue_pending(struct dma_chan *chan); + +============================================================================== + +Additional usage notes for dma driver writers +1/ Although DMA engine specifies that completion callback routines cannot submit +any new operations, but typically for slave DMA subsequent transaction may not +be available for submit prior to callback routine being called. This requirement +is not a requirement for DMA-slave devices. But they should take care to drop +the spin-lock they might be holding before calling the callback routine |