The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/Documentation/dmaengine.txt

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    1                         DMA Engine API Guide
    2                         ====================
    3 
    4                  Vinod Koul <vinod dot koul at intel.com>
    5 
    6 NOTE: For DMA Engine usage in async_tx please see:
    7         Documentation/crypto/async-tx-api.txt
    8 
    9 
   10 Below is a guide to device driver writers on how to use the Slave-DMA API of the
   11 DMA Engine. This is applicable only for slave DMA usage only.
   12 
   13 The slave DMA usage consists of following steps:
   14 1. Allocate a DMA slave channel
   15 2. Set slave and controller specific parameters
   16 3. Get a descriptor for transaction
   17 4. Submit the transaction
   18 5. Issue pending requests and wait for callback notification
   19 
   20 1. Allocate a DMA slave channel
   21 
   22    Channel allocation is slightly different in the slave DMA context,
   23    client drivers typically need a channel from a particular DMA
   24    controller only and even in some cases a specific channel is desired.
   25    To request a channel dma_request_channel() API is used.
   26 
   27    Interface:
   28         struct dma_chan *dma_request_channel(dma_cap_mask_t mask,
   29                         dma_filter_fn filter_fn,
   30                         void *filter_param);
   31    where dma_filter_fn is defined as:
   32         typedef bool (*dma_filter_fn)(struct dma_chan *chan, void *filter_param);
   33 
   34    The 'filter_fn' parameter is optional, but highly recommended for
   35    slave and cyclic channels as they typically need to obtain a specific
   36    DMA channel.
   37 
   38    When the optional 'filter_fn' parameter is NULL, dma_request_channel()
   39    simply returns the first channel that satisfies the capability mask.
   40 
   41    Otherwise, the 'filter_fn' routine will be called once for each free
   42    channel which has a capability in 'mask'.  'filter_fn' is expected to
   43    return 'true' when the desired DMA channel is found.
   44 
   45    A channel allocated via this interface is exclusive to the caller,
   46    until dma_release_channel() is called.
   47 
   48 2. Set slave and controller specific parameters
   49 
   50    Next step is always to pass some specific information to the DMA
   51    driver.  Most of the generic information which a slave DMA can use
   52    is in struct dma_slave_config.  This allows the clients to specify
   53    DMA direction, DMA addresses, bus widths, DMA burst lengths etc
   54    for the peripheral.
   55 
   56    If some DMA controllers have more parameters to be sent then they
   57    should try to embed struct dma_slave_config in their controller
   58    specific structure. That gives flexibility to client to pass more
   59    parameters, if required.
   60 
   61    Interface:
   62         int dmaengine_slave_config(struct dma_chan *chan,
   63                                   struct dma_slave_config *config)
   64 
   65    Please see the dma_slave_config structure definition in dmaengine.h
   66    for a detailed explanation of the struct members.  Please note
   67    that the 'direction' member will be going away as it duplicates the
   68    direction given in the prepare call.
   69 
   70 3. Get a descriptor for transaction
   71 
   72    For slave usage the various modes of slave transfers supported by the
   73    DMA-engine are:
   74 
   75    slave_sg     - DMA a list of scatter gather buffers from/to a peripheral
   76    dma_cyclic   - Perform a cyclic DMA operation from/to a peripheral till the
   77                   operation is explicitly stopped.
   78    interleaved_dma - This is common to Slave as well as M2M clients. For slave
   79                  address of devices' fifo could be already known to the driver.
   80                  Various types of operations could be expressed by setting
   81                  appropriate values to the 'dma_interleaved_template' members.
   82 
   83    A non-NULL return of this transfer API represents a "descriptor" for
   84    the given transaction.
   85 
   86    Interface:
   87         struct dma_async_tx_descriptor *(*chan->device->device_prep_slave_sg)(
   88                 struct dma_chan *chan, struct scatterlist *sgl,
   89                 unsigned int sg_len, enum dma_data_direction direction,
   90                 unsigned long flags);
   91 
   92         struct dma_async_tx_descriptor *(*chan->device->device_prep_dma_cyclic)(
   93                 struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
   94                 size_t period_len, enum dma_data_direction direction);
   95 
   96         struct dma_async_tx_descriptor *(*device_prep_interleaved_dma)(
   97                 struct dma_chan *chan, struct dma_interleaved_template *xt,
   98                 unsigned long flags);
   99 
  100    The peripheral driver is expected to have mapped the scatterlist for
  101    the DMA operation prior to calling device_prep_slave_sg, and must
  102    keep the scatterlist mapped until the DMA operation has completed.
  103    The scatterlist must be mapped using the DMA struct device.  So,
  104    normal setup should look like this:
  105 
  106         nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
  107         if (nr_sg == 0)
  108                 /* error */
  109 
  110         desc = chan->device->device_prep_slave_sg(chan, sgl, nr_sg,
  111                         direction, flags);
  112 
  113    Once a descriptor has been obtained, the callback information can be
  114    added and the descriptor must then be submitted.  Some DMA engine
  115    drivers may hold a spinlock between a successful preparation and
  116    submission so it is important that these two operations are closely
  117    paired.
  118 
  119    Note:
  120         Although the async_tx API specifies that completion callback
  121         routines cannot submit any new operations, this is not the
  122         case for slave/cyclic DMA.
  123 
  124         For slave DMA, the subsequent transaction may not be available
  125         for submission prior to callback function being invoked, so
  126         slave DMA callbacks are permitted to prepare and submit a new
  127         transaction.
  128 
  129         For cyclic DMA, a callback function may wish to terminate the
  130         DMA via dmaengine_terminate_all().
  131 
  132         Therefore, it is important that DMA engine drivers drop any
  133         locks before calling the callback function which may cause a
  134         deadlock.
  135 
  136         Note that callbacks will always be invoked from the DMA
  137         engines tasklet, never from interrupt context.
  138 
  139 4. Submit the transaction
  140 
  141    Once the descriptor has been prepared and the callback information
  142    added, it must be placed on the DMA engine drivers pending queue.
  143 
  144    Interface:
  145         dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
  146 
  147    This returns a cookie can be used to check the progress of DMA engine
  148    activity via other DMA engine calls not covered in this document.
  149 
  150    dmaengine_submit() will not start the DMA operation, it merely adds
  151    it to the pending queue.  For this, see step 5, dma_async_issue_pending.
  152 
  153 5. Issue pending DMA requests and wait for callback notification
  154 
  155    The transactions in the pending queue can be activated by calling the
  156    issue_pending API. If channel is idle then the first transaction in
  157    queue is started and subsequent ones queued up.
  158 
  159    On completion of each DMA operation, the next in queue is started and
  160    a tasklet triggered. The tasklet will then call the client driver
  161    completion callback routine for notification, if set.
  162 
  163    Interface:
  164         void dma_async_issue_pending(struct dma_chan *chan);
  165 
  166 Further APIs:
  167 
  168 1. int dmaengine_terminate_all(struct dma_chan *chan)
  169 
  170    This causes all activity for the DMA channel to be stopped, and may
  171    discard data in the DMA FIFO which hasn't been fully transferred.
  172    No callback functions will be called for any incomplete transfers.
  173 
  174 2. int dmaengine_pause(struct dma_chan *chan)
  175 
  176    This pauses activity on the DMA channel without data loss.
  177 
  178 3. int dmaengine_resume(struct dma_chan *chan)
  179 
  180    Resume a previously paused DMA channel.  It is invalid to resume a
  181    channel which is not currently paused.
  182 
  183 4. enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
  184         dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used)
  185 
  186    This can be used to check the status of the channel.  Please see
  187    the documentation in include/linux/dmaengine.h for a more complete
  188    description of this API.
  189 
  190    This can be used in conjunction with dma_async_is_complete() and
  191    the cookie returned from 'descriptor->submit()' to check for
  192    completion of a specific DMA transaction.
  193 
  194    Note:
  195         Not all DMA engine drivers can return reliable information for
  196         a running DMA channel.  It is recommended that DMA engine users
  197         pause or stop (via dmaengine_terminate_all) the channel before
  198         using this API.

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