Linux dma engine

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Linux DMA Engine

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What to Expect
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DMA Controllers
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Types of DMA Transfers
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Linux DMA Engine API
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Steps for DMA Transfer

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DMA Controllers
Descriptor Buffer
DMA Controller
Device Controller
FIFO
Request
Line

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Contiguous Memory DMA
Device Controller
RAM
FIFO

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Scatter Gather DMA
Device Controller
RAM
FIFO

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Scatter Gather Descriptors
Source
Destination
Size
Configuration
Next
Source
Destination
Size
Configuration
Next
Source
Destination
Size
Configuration
Next
Interrupt
Interrupt Interrupt

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Cyclic Transfers
Source
Destination
Size
Configuration
Next
Source
Destination
Size
Configuration
Next
Source
Destination
Size
Configuration
Next
Interrupt
Interrupt Interrupt

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DMA Controller with Peripherals
RAM
SPI
Controller
Audio
Controller
Network
Controller
Buffer Buffer Buffer
DMA Controller
FIFO FIFO FIFO

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Linux DMA Engine
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The DMA Engine driver works as a layer on the top of
SoC (e.g. TI) specific DMA driver using the slave DMA
API
●
Drivers using the DMA Engine are referred to as a
clients

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Linux DMA Engine...
Kernel
SoC DMA Driver
DMA Engine Driver
Device Specific Driver
Applicaton

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Steps in DMA Transfer
Allocate a
DMA slave
channel
Set Slave &
controller
specific
params
Get a
Descriptor for
the
transaction
Submit the
transaction to
queue it in the
DMA
Start the
Transaction
Wait for it to
complete

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DMA Channel
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Header: include/linux/dmaengine.h
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Data Structure:
struct dma_chan {
struct dma_device;
dma_cookie_t cookie;
chan_id;
client_count;
table_count;
}

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Allocate a DMA Channel
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Header: include/linux/dmaengine.h
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APIs:
dma_caps_set(transaction_type, mask);
struct dma_chan
*dma_request_slave_channel(struct device *dev,
const char *name);

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Allocate a DMA Channel Example
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Set up the capabilities for the channel that will
be requested
dma_cap_mask_t mask;
dma_cap_zero(mask);
dma_cap_set(DMA_SLAVE | DMA_PRIVATE,
mask);
Request the DMA channel from the DMA
engine
chan = dma_request_channel(mask, NULL, NULL);
Release the DMA channel
dma_release_channel(chan);

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Set DMA Slave & Controller Params
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DMA slave channel runtime configuration
Data Structure:
struct dma_slave_config {
enum dma_transfer_direction direction;
dma_addr_t src_addr;
dma_addr_t dst_addr;
enum dma_slave_buswidth src_addr_width;
enum dma_slave_buswidth dst_addr_width;
u32 src_maxburst;
u32 dst_maxburst;
bool device_fc;
unsigned int slave_id;
};
API: dmaengine_slave_config(dma_channel, dma_slave_config *);

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Set Controller Params Examples
struct dma_slave_config cfg;
cfg.src_addr = OMAP2_MCSPI_RX0;
cfg.dst_addr = OMAP2_MCSPI_TX0;
cfg.src_addr_width = width;
cfg.dst_addr_width = width;
cfg.src_maxburst = burst;
cfg.dst_maxburst = burst;
dmaengine_slave_config(dma_channel, cfg);

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Prepare the Descriptor
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Descriptor is used to describe a DMA transaction
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struct dma_async_tx_descriptor
*dmaengine_prep_slave_single(parameters);
Parameters:
struct dma_chan *chan
dma_addr_t buff
size_t len
enum dma_transfer_direction dir
unsigned long flags
DMA_PREPARE_INTERRUPT, DMA_CTRL_ACK

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Prepare a Descriptor & Setting the Callback Example
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Allocate a 1K buffer of cached contiguous memory
char *buf = kmalloc(1024, GFP_KERNEL | GFP_DMA);
Get the physical address for the buffer
dma_buf = dma_map_single(device, buf, 1024,
DMA_TO_DEVICE);
Prepare the descriptor for the DMA transaction
Enum dma_ctrl_flags flags = DMA_CTRL_ACK |
DMA_PREP_INTERRUPT;
chan_des = dma_engine_prep_slave_single(chan, dma_buf,
1024, DMA_MEM_TO_DEV, flags);
Set up the callback function for the descriptor
chan_des->callback = <callback when the transfer completes>;
chan_des->callback_param = cmp;

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Submit & Start the Transaction
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The dmaengine_submit() function submits the
descriptor to the DMA engine to be put into the
pending queue
The returned cookie can be used to check the progress
The dma_sync_issue_pending() function is used to
start the DMA transaction that was previously put in
pending queue
If the channel is idle, then the first transaction in queue is
started and the subsequent transactions are queued up
On completion of each in queue, the next in queue is
started and a tasklet is triggered. The tasklet will then call
the client driver completion callback routine for notification,
if set

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References
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<kernel src tree>/Documentation/dmaengine/
client.txt
External Video Reference
https://www.xilinx.com/video/soc/linux-dma-in-device-driver
s.html

Linux dma engine

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