Re: [PATCH 1/4] docs: arm: stm32: introduce STM32 DMA-MDMA chaining feature

[Amelie Delaunay]

On 7/11/22 17:11, Jonathan Corbet wrote:
> 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.

Thanks for you review.

I'll add it to index.rst, with other stm32 documentations.

> > 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
> > +=======================
> > +
> > +
> > +Introduction
> > +------------
> > +
> > +  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 read.

OK, I prepare a v2 with a 80-column limit for the documentation patch.

> > +  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).
> > +
> > +  .. 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.

It is possible, with some extra lines. I'll update it in v2 coming soon.

> [...]
>
> Thanks,
>
> jon

Regards,
Amelie