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- 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
-5. Issue pending requests 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);
-
- The 'filter_fn' parameter is optional, but highly recommended for
- slave and cyclic channels as they typically need to obtain a specific
- DMA channel.
-
- When the optional 'filter_fn' parameter is NULL, dma_request_channel()
- simply returns the first channel that satisfies the capability mask.
-
- Otherwise, the 'filter_fn' routine will be called once for each free
- channel which has a capability in 'mask'. 'filter_fn' is expected to
- return 'true' when the desired DMA channel is found.
-
- 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. This allows the clients to specify
- DMA direction, DMA addresses, bus widths, DMA burst lengths etc
- for the peripheral.
-
- 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)
-
- Please see the dma_slave_config structure definition in dmaengine.h
- for a detailed explanation of the struct members. Please note
- that the 'direction' member will be going away as it duplicates the
- direction given in the prepare call.
-
-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.
- interleaved_dma - This is common to Slave as well as M2M clients. For slave
- address of devices' fifo could be already known to the driver.
- Various types of operations could be expressed by setting
- appropriate values to the 'dma_interleaved_template' members.
-
- A non-NULL return of this transfer API represents a "descriptor" for
- the given transaction.
-
- Interface:
- struct dma_async_tx_descriptor *(*chan->device->device_prep_slave_sg)(
- struct dma_chan *chan, struct scatterlist *sgl,
- unsigned int sg_len, enum dma_data_direction direction,
- 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);
-
- struct dma_async_tx_descriptor *(*device_prep_interleaved_dma)(
- struct dma_chan *chan, struct dma_interleaved_template *xt,
- unsigned long flags);
-
- The peripheral driver is expected to have mapped the scatterlist for
- the DMA operation prior to calling device_prep_slave_sg, and must
- keep the scatterlist mapped until the DMA operation has completed.
- The scatterlist must be mapped using the DMA struct device. So,
- normal setup should look like this:
-
- nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
- if (nr_sg == 0)
- /* error */
-
- desc = chan->device->device_prep_slave_sg(chan, sgl, nr_sg,
- direction, flags);
-
- Once a descriptor has been obtained, the callback information can be
- added and the descriptor must then be submitted. Some DMA engine
- drivers may hold a spinlock between a successful preparation and
- submission so it is important that these two operations are closely
- paired.
-
- Note:
- Although the async_tx API specifies that completion callback
- routines cannot submit any new operations, this is not the
- case for slave/cyclic DMA.
-
- For slave DMA, the subsequent transaction may not be available
- for submission prior to callback function being invoked, so
- slave DMA callbacks are permitted to prepare and submit a new
- transaction.
-
- For cyclic DMA, a callback function may wish to terminate the
- DMA via dmaengine_terminate_all().
-
- Therefore, it is important that DMA engine drivers drop any
- locks before calling the callback function which may cause a
- deadlock.
-
- Note that callbacks will always be invoked from the DMA
- engines tasklet, never from interrupt context.
-
-4. Submit the transaction
-
- Once the descriptor has been prepared and the callback information
- added, it must be placed on the DMA engine drivers pending queue.
-
- Interface:
- dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
-
- This returns a cookie can be used to check the progress of DMA engine
- activity via other DMA engine calls not covered in this document.
-
- dmaengine_submit() will not start the DMA operation, it merely adds
- it to the pending queue. For this, see step 5, dma_async_issue_pending.
-
-5. Issue pending DMA requests and wait for callback notification
-
- 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 each DMA operation, the next in queue is started and
- a tasklet triggered. The tasklet will then call the client driver
- completion callback routine for notification, if set.
-
- Interface:
- void dma_async_issue_pending(struct dma_chan *chan);
-
-Further APIs:
-
-1. int dmaengine_terminate_all(struct dma_chan *chan)
-
- This causes all activity for the DMA channel to be stopped, and may
- discard data in the DMA FIFO which hasn't been fully transferred.
- No callback functions will be called for any incomplete transfers.
-
-2. int dmaengine_pause(struct dma_chan *chan)
-
- This pauses activity on the DMA channel without data loss.
-
-3. int dmaengine_resume(struct dma_chan *chan)
-
- Resume a previously paused DMA channel. It is invalid to resume a
- channel which is not currently paused.
-
-4. enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
- dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used)
-
- This can be used to check the status of the channel. Please see
- the documentation in include/linux/dmaengine.h for a more complete
- description of this API.
-
- This can be used in conjunction with dma_async_is_complete() and
- the cookie returned from 'descriptor->submit()' to check for
- completion of a specific DMA transaction.
-
- Note:
- Not all DMA engine drivers can return reliable information for
- a running DMA channel. It is recommended that DMA engine users
- pause or stop (via dmaengine_terminate_all) the channel before
- using this API.