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 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.