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.. SPDX-License-Identifier: GPL-2.0
STM32 DMA-MDMA chaining
This document describes the STM32 DMA-MDMA chaining feature. But before going
further, let's introduce the peripherals involved.
To offload data transfers from the CPU, STM32 microprocessors (MPUs) embed
direct memory access controllers (DMA).
STM32MP1 SoCs embed both STM32 DMA and STM32 MDMA controllers. STM32 DMA
request routing capabilities are enhanced by a DMA request multiplexer
STM32 DMAMUX routes any DMA request from a given peripheral to any STM32 DMA
controller (STM32MP1 counts two STM32 DMA controllers) channels.
**STM32 DMA**
STM32 DMA is mainly used to implement central data buffer storage (usually in
the system SRAM) for different peripheral. It can access external RAMs but
without the ability to generate convenient burst transfer ensuring the best
load of the AXI.
**STM32 MDMA**
STM32 MDMA (Master DMA) is mainly used to manage direct data transfers between
RAM data buffers without CPU intervention. It can also be used in a
hierarchical structure that uses STM32 DMA as first level data buffer
interfaces for AHB peripherals, while the STM32 MDMA acts as a second level
DMA with better performance. As a AXI/AHB master, STM32 MDMA can take control
of the AXI/AHB bus.
STM32 DMA-MDMA chaining feature relies on the strengths of STM32 DMA and
STM32 MDMA controllers.
STM32 DMA has a circular Double Buffer Mode (DBM). At each end of transaction
(when DMA data counter - DMA_SxNDTR - reaches 0), the memory pointers
(configured with DMA_SxSM0AR and DMA_SxM1AR) are swapped and the DMA data
counter is automatically reloaded. This allows the SW or the STM32 MDMA to
process one memory area while the second memory area is being filled/used by
the STM32 DMA transfer.
With STM32 MDMA linked-list mode, a single request initiates the data array
(collection of nodes) to be transferred until the linked-list pointer for the
channel is null. The channel transfer complete of the last node is the end of
transfer, unless first and last nodes are linked to each other, in such a
case, the linked-list loops on to create a circular MDMA transfer.
STM32 MDMA has direct connections with STM32 DMA. This enables autonomous
communication and synchronization between peripherals, thus saving CPU
resources and bus congestion. Transfer Complete signal of STM32 DMA channel
can triggers STM32 MDMA transfer. STM32 MDMA can clear the request generated
by the STM32 DMA by writing to its Interrupt Clear register (whose address is
stored in MDMA_CxMAR, and bit mask in MDMA_CxMDR).
.. table:: STM32 MDMA interconnect table with STM32 DMA
| channels | channels | Transfer | request |
| | | complete | |
| | | signal | |
| Channel *0* | DMA1 channel 0 | dma1_tcf0 | *0x00* |
| Channel *1* | DMA1 channel 1 | dma1_tcf1 | *0x01* |
| Channel *2* | DMA1 channel 2 | dma1_tcf2 | *0x02* |
| Channel *3* | DMA1 channel 3 | dma1_tcf3 | *0x03* |
| Channel *4* | DMA1 channel 4 | dma1_tcf4 | *0x04* |
| Channel *5* | DMA1 channel 5 | dma1_tcf5 | *0x05* |
| Channel *6* | DMA1 channel 6 | dma1_tcf6 | *0x06* |
| Channel *7* | DMA1 channel 7 | dma1_tcf7 | *0x07* |
| Channel *8* | DMA2 channel 0 | dma2_tcf0 | *0x08* |
| Channel *9* | DMA2 channel 1 | dma2_tcf1 | *0x09* |
| Channel *10* | DMA2 channel 2 | dma2_tcf2 | *0x0A* |
| Channel *11* | DMA2 channel 3 | dma2_tcf3 | *0x0B* |
| Channel *12* | DMA2 channel 4 | dma2_tcf4 | *0x0C* |
| Channel *13* | DMA2 channel 5 | dma2_tcf5 | *0x0D* |
| Channel *14* | DMA2 channel 6 | dma2_tcf6 | *0x0E* |
| Channel *15* | DMA2 channel 7 | dma2_tcf7 | *0x0F* |
STM32 DMA-MDMA chaining feature then uses a SRAM buffer. STM32MP1 SoCs embed
three fast access static internal RAMs of various size, used for data storage.
Due to STM32 DMA legacy (within microcontrollers), STM32 DMA performances are
bad with DDR, while they are optimal with SRAM. Hence the SRAM buffer used
between STM32 DMA and STM32 MDMA. This buffer is split in two equal periods
and STM32 DMA uses one period while STM32 MDMA uses the other period
____________ ' _________ V____________
| STM32 DMA | / __|>_ \ | STM32 MDMA |
|------------| | / \ | |------------|
| DMA_SxM0AR |<=>| | SRAM | |<=>| []-[]...[] |
| DMA_SxM1AR | | \_____/ | | |
|____________| \___<|____/ |____________|
STM32 DMA-MDMA chaining uses (struct dma_slave_config).peripheral_config to
exchange the parameters needed to configure MDMA. These parameters are
gathered into a u32 array with three values:
* the STM32 MDMA request (which is actually the DMAMUX channel ID),
* the address of the STM32 DMA register to clear the Transfer Complete
interrupt flag,
* the mask of the Transfer Complete interrupt flag of the STM32 DMA channel.
Device Tree updates for STM32 DMA-MDMA chaining support
**1. Allocate a SRAM buffer**
SRAM device tree node is defined in SoC device tree. You can refer to it in
your board device tree to define your SRAM pool.
&sram {
my_foo_device_dma_pool: dma-sram@0 {
reg = <0x0 0x1000>;
Be careful of the start index, in case there are other SRAM consumers.
Define your pool size strategically: to optimise chaining, the idea is that
STM32 DMA and STM32 MDMA can work simultaneously, on each buffer of the
If the SRAM period is greater than the expected DMA transfer, then STM32 DMA
and STM32 MDMA will work sequentially instead of simultaneously. It is not a
functional issue but it is not optimal.
Don't forget to refer to your SRAM pool in your device node. You need to
define a new property.
&my_foo_device {
my_dma_pool = &my_foo_device_dma_pool;
Then get this SRAM pool in your foo driver and allocate your SRAM buffer.
**2. Allocate a STM32 DMA channel and a STM32 MDMA channel**
You need to define an extra channel in your device tree node, in addition to
the one you should already have for "classic" DMA operation.
This new channel must be taken from STM32 MDMA channels, so, the phandle of
the DMA controller to use is the MDMA controller's one.
&my_foo_device {
my_dma_pool = &my_foo_device_dma_pool;
dmas = <&dmamux1 ...>, // STM32 DMA channel
<&mdma1 0 0x3 0x1200000a 0 0>; // + STM32 MDMA channel
Concerning STM32 MDMA bindings:
1. The request line number : whatever the value here, it will be overwritten
by MDMA driver with the STM32 DMAMUX channel ID passed through
(struct dma_slave_config).peripheral_config
2. The priority level : choose Very High (0x3) so that your channel will
take priority other the other during request arbitration
3. A 32bit mask specifying the DMA channel configuration : source and
destination address increment, block transfer with 128 bytes per single
4. The 32bit value specifying the register to be used to acknowledge the
request: it will be overwritten by MDMA driver, with the DMA channel
interrupt flag clear register address passed through
(struct dma_slave_config).peripheral_config
5. The 32bit mask specifying the value to be written to acknowledge the
request: it will be overwritten by MDMA driver, with the DMA channel
Transfer Complete flag passed through
(struct dma_slave_config).peripheral_config
Driver updates for STM32 DMA-MDMA chaining support in foo driver
**0. (optional) Refactor the original sg_table if dmaengine_prep_slave_sg()**
In case of dmaengine_prep_slave_sg(), the original sg_table can't be used as
is. Two new sg_tables must be created from the original one. One for
STM32 DMA transfer (where memory address targets now the SRAM buffer instead
of DDR buffer) and one for STM32 MDMA transfer (where memory address targets
the DDR buffer).
The new sg_list items must fit SRAM period length. Here is an example for
* Assuming sgl and nents, respectively the initial scatterlist and its
* length.
* Assuming sram_dma_buf and sram_period, respectively the memory
* allocated from the pool for DMA usage, and the length of the period,
* which is half of the sram_buf size.
struct sg_table new_dma_sgt, new_mdma_sgt;
struct scatterlist *s, *_sgl;
dma_addr_t ddr_dma_buf;
u32 new_nents = 0, len;
int i;
/* Count the number of entries needed */
for_each_sg(sgl, s, nents, i)
if (sg_dma_len(s) > sram_period)
new_nents += DIV_ROUND_UP(sg_dma_len(s), sram_period);
/* Create sg table for STM32 DMA channel */
ret = sg_alloc_table(&new_dma_sgt, new_nents, GFP_ATOMIC);
if (ret)
dev_err(dev, "DMA sg table alloc failed\n");
for_each_sg(new_dma_sgt.sgl, s, new_dma_sgt.nents, i) {
_sgl = sgl;
sg_dma_len(s) = min(sg_dma_len(_sgl), sram_period);
/* Targets the beginning = first half of the sram_buf */
s->dma_address = sram_buf;
* Targets the second half of the sram_buf
* for odd indexes of the item of the sg_list
if (i & 1)
s->dma_address += sram_period;
/* Create sg table for STM32 MDMA channel */
ret = sg_alloc_table(&new_mdma_sgt, new_nents, GFP_ATOMIC);
if (ret)
dev_err(dev, "MDMA sg_table alloc failed\n");
_sgl = sgl;
len = sg_dma_len(sgl);
ddr_dma_buf = sg_dma_address(sgl);
for_each_sg(mdma_sgt.sgl, s, mdma_sgt.nents, i) {
size_t bytes = min_t(size_t, len, sram_period);
sg_dma_len(s) = bytes;
sg_dma_address(s) = ddr_dma_buf;
len -= bytes;
if (!len && sg_next(_sgl)) {
_sgl = sg_next(_sgl);
len = sg_dma_len(_sgl);
ddr_dma_buf = sg_dma_address(_sgl);
} else {
ddr_dma_buf += bytes;
Don't forget to release these new sg_tables after getting the descriptors
with dmaengine_prep_slave_sg().
**1. Set controller specific parameters**
First, use dmaengine_slave_config() with a struct dma_slave_config to
configure STM32 DMA channel. You just have to take care of DMA addresses,
the memory address (depending on the transfer direction) must point on your
SRAM buffer, and set (struct dma_slave_config).peripheral_size != 0.
STM32 DMA driver will check (struct dma_slave_config).peripheral_size to
determine if chaining is being used or not. If it is used, then STM32 DMA
driver fills (struct dma_slave_config).peripheral_config with an array of
three u32 : the first one containing STM32 DMAMUX channel ID, the second one
the channel interrupt flag clear register address, and the third one the
channel Transfer Complete flag mask.
Then, use dmaengine_slave_config with another struct dma_slave_config to
configure STM32 MDMA channel. Take care of DMA addresses, the device address
(depending on the transfer direction) must point on your SRAM buffer, and
the memory address must point to the buffer originally used for "classic"
DMA operation. Use the previous (struct dma_slave_config).peripheral_size
and .peripheral_config that have been updated by STM32 DMA driver, to set
(struct dma_slave_config).peripheral_size and .peripheral_config of the
struct dma_slave_config to configure STM32 MDMA channel.
struct dma_slave_config dma_conf;
struct dma_slave_config mdma_conf;
memset(&dma_conf, 0, sizeof(dma_conf));
config.direction = DMA_DEV_TO_MEM;
config.dst_addr = sram_dma_buf; // SRAM buffer
config.peripheral_size = 1; // peripheral_size != 0 => chaining
dmaengine_slave_config(dma_chan, &dma_config);
memset(&mdma_conf, 0, sizeof(mdma_conf));
config.direction = DMA_DEV_TO_MEM;
mdma_conf.src_addr = sram_dma_buf; // SRAM buffer
mdma_conf.dst_addr = rx_dma_buf; // original memory buffer
mdma_conf.peripheral_size = dma_conf.peripheral_size; // <- dma_conf
mdma_conf.peripheral_config = dma_config.peripheral_config; // <- dma_conf
dmaengine_slave_config(mdma_chan, &mdma_conf);
**2. Get a descriptor for STM32 DMA channel transaction**
In the same way you get your descriptor for your "classic" DMA operation,
you just have to replace the original sg_list (in case of
dmaengine_prep_slave_sg()) with the new sg_list using SRAM buffer, or to
replace the original buffer address, length and period (in case of
dmaengine_prep_dma_cyclic()) with the new SRAM buffer.
**3. Get a descriptor for STM32 MDMA channel transaction**
If you previously get descriptor (for STM32 DMA) with
* dmaengine_prep_slave_sg(), then use dmaengine_prep_slave_sg() for
* dmaengine_prep_dma_cyclic(), then use dmaengine_prep_dma_cyclic() for
Use the new sg_list using SRAM buffer (in case of dmaengine_prep_slave_sg())
or, depending on the transfer direction, either the original DDR buffer (in
case of DMA_DEV_TO_MEM) or the SRAM buffer (in case of DMA_MEM_TO_DEV), the
source address being previously set with dmaengine_slave_config().
**4. Submit both transactions**
Before submitting your transactions, you may need to define on which
descriptor you want a callback to be called at the end of the transfer
(dmaengine_prep_slave_sg()) or the period (dmaengine_prep_dma_cyclic()).
Depending on the direction, set the callback on the descriptor that finishes
the overal transfer:
* DMA_DEV_TO_MEM: set the callback on the "MDMA" descriptor
* DMA_MEM_TO_DEV: set the callback on the "DMA" descriptor
Then, submit the descriptors whatever the order, with dmaengine_tx_submit().
**5. Issue pending requests (and wait for callback notification)**
As STM32 MDMA channel transfer is triggered by STM32 DMA, you must issue
STM32 MDMA channel before STM32 DMA channel.
If any, your callback will be called to warn you about the end of the overal
transfer or the period completion.
Don't forget to terminate both channels. STM32 DMA channel is configured in
cyclic Double-Buffer mode so it won't be disabled by HW, you need to terminate
it. STM32 MDMA channel will be stopped by HW in case of sg transfer, but not
in case of cyclic transfer. You can terminate it whatever the kind of transfer.
**STM32 DMA-MDMA chaining DMA_MEM_TO_DEV special case**
STM32 DMA-MDMA chaining in DMA_MEM_TO_DEV is a special case. Indeed, the
STM32 MDMA feeds the SRAM buffer with the DDR data, and the STM32 DMA reads
data from SRAM buffer. So some data (the first period) have to be copied in
SRAM buffer when the STM32 DMA starts to read.
A trick could be pausing the STM32 DMA channel (that will raise a Transfer
Complete signal, triggering the STM32 MDMA channel), but the first data read
by the STM32 DMA could be "wrong". The proper way is to prepare the first SRAM
period with dmaengine_prep_dma_memcpy(). Then this first period should be
"removed" from the sg or the cyclic transfer.
Due to this complexity, rather use the STM32 DMA-MDMA chaining for
DMA_DEV_TO_MEM and keep the "classic" DMA usage for DMA_MEM_TO_DEV, unless
you're not afraid.
Application note, datasheet and reference manual are available on ST website
Dedicated focus on three application notes (AN5224_, AN4031_ & AN5001_)
dealing with STM32 DMAMUX, STM32 DMA and STM32 MDMA.
.. _STM32MP1:
.. _AN5224:
.. _AN4031:
.. _AN5001:
- Amelie Delaunay <>