Documentation/dmaengine.txt

   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 explaination 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
  79    A non-NULL return of this transfer API represents a "descriptor" for
  80    the given transaction.
  81
  82    Interface:
  83         struct dma_async_tx_descriptor *(*chan->device->device_prep_slave_sg)(
  84                 struct dma_chan *chan, struct scatterlist *sgl,
  85                 unsigned int sg_len, enum dma_data_direction direction,
  86                 unsigned long flags);
  87
  88         struct dma_async_tx_descriptor *(*chan->device->device_prep_dma_cyclic)(
  89                 struct dma_chan *chan, dma_addr_t buf_addr, size_t buf_len,
  90                 size_t period_len, enum dma_data_direction direction);
  91
  92    The peripheral driver is expected to have mapped the scatterlist for
  93    the DMA operation prior to calling device_prep_slave_sg, and must
  94    keep the scatterlist mapped until the DMA operation has completed.
  95    The scatterlist must be mapped using the DMA struct device.  So,
  96    normal setup should look like this:
  97
  98         nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
  99         if (nr_sg == 0)
 100                 /* error */
 101
 102         desc = chan->device->device_prep_slave_sg(chan, sgl, nr_sg,
 103                         direction, flags);
 104
 105    Once a descriptor has been obtained, the callback information can be
 106    added and the descriptor must then be submitted.  Some DMA engine
 107    drivers may hold a spinlock between a successful preparation and
 108    submission so it is important that these two operations are closely
 109    paired.
 110
 111    Note:
 112         Although the async_tx API specifies that completion callback
 113         routines cannot submit any new operations, this is not the
 114         case for slave/cyclic DMA.
 115
 116         For slave DMA, the subsequent transaction may not be available
 117         for submission prior to callback function being invoked, so
 118         slave DMA callbacks are permitted to prepare and submit a new
 119         transaction.
 120
 121         For cyclic DMA, a callback function may wish to terminate the
 122         DMA via dmaengine_terminate_all().
 123
 124         Therefore, it is important that DMA engine drivers drop any
 125         locks before calling the callback function which may cause a
 126         deadlock.
 127
 128         Note that callbacks will always be invoked from the DMA
 129         engines tasklet, never from interrupt context.
 130
 131 4. Submit the transaction
 132
 133    Once the descriptor has been prepared and the callback information
 134    added, it must be placed on the DMA engine drivers pending queue.
 135
 136    Interface:
 137         dma_cookie_t dmaengine_submit(struct dma_async_tx_descriptor *desc)
 138
 139    This returns a cookie can be used to check the progress of DMA engine
 140    activity via other DMA engine calls not covered in this document.
 141
 142    dmaengine_submit() will not start the DMA operation, it merely adds
 143    it to the pending queue.  For this, see step 5, dma_async_issue_pending.
 144
 145 5. Issue pending DMA requests and wait for callback notification
 146
 147    The transactions in the pending queue can be activated by calling the
 148    issue_pending API. If channel is idle then the first transaction in
 149    queue is started and subsequent ones queued up.
 150
 151    On completion of each DMA operation, the next in queue is started and
 152    a tasklet triggered. The tasklet will then call the client driver
 153    completion callback routine for notification, if set.
 154
 155    Interface:
 156         void dma_async_issue_pending(struct dma_chan *chan);
 157
 158 Further APIs:
 159
 160 1. int dmaengine_terminate_all(struct dma_chan *chan)
 161
 162    This causes all activity for the DMA channel to be stopped, and may
 163    discard data in the DMA FIFO which hasn't been fully transferred.
 164    No callback functions will be called for any incomplete transfers.
 165
 166 2. int dmaengine_pause(struct dma_chan *chan)
 167
 168    This pauses activity on the DMA channel without data loss.
 169
 170 3. int dmaengine_resume(struct dma_chan *chan)
 171
 172    Resume a previously paused DMA channel.  It is invalid to resume a
 173    channel which is not currently paused.
 174
 175 4. enum dma_status dma_async_is_tx_complete(struct dma_chan *chan,
 176         dma_cookie_t cookie, dma_cookie_t *last, dma_cookie_t *used)
 177
 178    This can be used to check the status of the channel.  Please see
 179    the documentation in include/linux/dmaengine.h for a more complete
 180    description of this API.
 181
 182    This can be used in conjunction with dma_async_is_complete() and
 183    the cookie returned from 'descriptor->submit()' to check for
 184    completion of a specific DMA transaction.
 185
 186    Note:
 187         Not all DMA engine drivers can return reliable information for
 188         a running DMA channel.  It is recommended that DMA engine users
 189         pause or stop (via dmaengine_terminate_all) the channel before
 190         using this API.