Re: [PATCH 1/4] docs: arm: stm32: introduce STM32 DMA-MDMA chaining feature
From: Jonathan Corbet
Date: Mon Jul 11 2022 - 11:11:33 EST
Amelie Delaunay <amelie.delaunay@xxxxxxxxxxx> writes:
> STM32 DMA-MDMA chaining feature is available on STM32 SoCs which embed
> STM32 DMAMUX, DMA and MDMA controllers. It is the case on STM32MP1 SoCs but
> also on STM32H7 SoCs. But focus is on STM32MP1 SoCs, using DDR.
> This documentation aims to explain how to use STM32 DMA-MDMA chaining
> feature in drivers of STM32 peripheral having request lines on STM32 DMA.
> Signed-off-by: Amelie Delaunay <amelie.delaunay@xxxxxxxxxxx>
> .../arm/stm32/stm32-dma-mdma-chaining.rst | 365 ++++++++++++++++++
> 1 file changed, 365 insertions(+)
> create mode 100644 Documentation/arm/stm32/stm32-dma-mdma-chaining.rst
When you add a new RST file you also need to add it to index.rst
somewhere so that it becomes part of the docs build.
> diff --git a/Documentation/arm/stm32/stm32-dma-mdma-chaining.rst b/Documentation/arm/stm32/stm32-dma-mdma-chaining.rst
> new file mode 100644
> index 000000000000..bfbbadc45aa7
> --- /dev/null
> +++ b/Documentation/arm/stm32/stm32-dma-mdma-chaining.rst
> @@ -0,0 +1,365 @@
> +.. 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.
Please keep to the 80-column limit for documentation, it makes it easier
> + 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.
> + 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).
> + .. csv-table:: STM32 MDMA interconnect table with STM32 DMA
> + :header: "STM32 DMAMUX channels", "STM32 DMA controllers channels",
> + "STM32 DMA Transfer Complete signal", "STM32 MDMA request"
If at all possible, please use simple tables; that makes the plain text
documentation much easier to read.