forked from Minki/linux
223 lines
8.8 KiB
Plaintext
223 lines
8.8 KiB
Plaintext
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_chan() API is used.
|
|
|
|
Interface:
|
|
struct dma_chan *dma_request_chan(struct device *dev, const char *name);
|
|
|
|
Which will find and return the 'name' DMA channel associated with the 'dev'
|
|
device. The association is done via DT, ACPI or board file based
|
|
dma_slave_map matching table.
|
|
|
|
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 *dmaengine_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 *dmaengine_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 *dmaengine_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 dmaengine_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.
|
|
If a mapping needs to be synchronized later, dma_sync_*_for_*() must be
|
|
called using the DMA struct device, too.
|
|
So, normal setup should look like this:
|
|
|
|
nr_sg = dma_map_sg(chan->device->dev, sgl, sg_len);
|
|
if (nr_sg == 0)
|
|
/* error */
|
|
|
|
desc = dmaengine_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_async().
|
|
|
|
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_sync(struct dma_chan *chan)
|
|
int dmaengine_terminate_async(struct dma_chan *chan)
|
|
int dmaengine_terminate_all(struct dma_chan *chan) /* DEPRECATED */
|
|
|
|
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.
|
|
|
|
Two variants of this function are available.
|
|
|
|
dmaengine_terminate_async() might not wait until the DMA has been fully
|
|
stopped or until any running complete callbacks have finished. But it is
|
|
possible to call dmaengine_terminate_async() from atomic context or from
|
|
within a complete callback. dmaengine_synchronize() must be called before it
|
|
is safe to free the memory accessed by the DMA transfer or free resources
|
|
accessed from within the complete callback.
|
|
|
|
dmaengine_terminate_sync() will wait for the transfer and any running
|
|
complete callbacks to finish before it returns. But the function must not be
|
|
called from atomic context or from within a complete callback.
|
|
|
|
dmaengine_terminate_all() is deprecated and should not be used in new code.
|
|
|
|
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 dmaengine_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.
|
|
|
|
5. void dmaengine_synchronize(struct dma_chan *chan)
|
|
|
|
Synchronize the termination of the DMA channel to the current context.
|
|
|
|
This function should be used after dmaengine_terminate_async() to synchronize
|
|
the termination of the DMA channel to the current context. The function will
|
|
wait for the transfer and any running complete callbacks to finish before it
|
|
returns.
|
|
|
|
If dmaengine_terminate_async() is used to stop the DMA channel this function
|
|
must be called before it is safe to free memory accessed by previously
|
|
submitted descriptors or to free any resources accessed within the complete
|
|
callback of previously submitted descriptors.
|
|
|
|
The behavior of this function is undefined if dma_async_issue_pending() has
|
|
been called between dmaengine_terminate_async() and this function.
|