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+.. SPDX-License-Identifier: GPL-2.0
+
+=======================
+STM32 DMA-MDMA chaining
+=======================
+
+
+Introduction
+------------
+
+ 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).
+
+ **STM32 DMAMUX**
+
+ 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.
+
+
+Principles
+----------
+
+ 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
+
+ +--------------+----------------+-----------+------------+
+ | STM32 DMAMUX | STM32 DMA | STM32 DMA | STM32 MDMA |
+ | 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
+ simultaneously.
+ ::
+
+ dma[1:2]-tcf[0:7]
+ .----------------.
+ ____________ ' _________ 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
+ SRAM.
+ 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
+ transfer
+
+ 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
+ DMA_DEV_TO_MEM:
+ ::
+
+ /*
+ * 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);
+ else
+ new_nents++;
+
+ /* 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
+ STM32 MDMA;
+ * dmaengine_prep_dma_cyclic(), then use dmaengine_prep_dma_cyclic() for
+ STM32 MDMA.
+
+ 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.
+
+Resources
+---------
+
+ Application note, datasheet and reference manual are available on ST website
+ (STM32MP1_).
+
+ Dedicated focus on three application notes (AN5224_, AN4031_ & AN5001_)
+ dealing with STM32 DMAMUX, STM32 DMA and STM32 MDMA.
+
+.. _STM32MP1: https://www.st.com/en/microcontrollers-microprocessors/stm32mp1-series.html
+.. _AN5224: https://www.st.com/resource/en/application_note/an5224-stm32-dmamux-the-dma-request-router-stmicroelectronics.pdf
+.. _AN4031: https://www.st.com/resource/en/application_note/dm00046011-using-the-stm32f2-stm32f4-and-stm32f7-series-dma-controller-stmicroelectronics.pdf
+.. _AN5001: https://www.st.com/resource/en/application_note/an5001-stm32cube-expansion-package-for-stm32h7-series-mdma-stmicroelectronics.pdf
+
+:Authors:
+
+- Amelie Delaunay <amelie.delaunay@foss.st.com> \ No newline at end of file