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