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Date:   Tue, 12 Jul 2022 15:37:41 +0200
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Subject: Re: [PATCH 1/4] docs: arm: stm32: introduce STM32 DMA-MDMA chaining
 feature
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To:     Jonathan Corbet <corbet@lwn.net>, Vinod Koul <vkoul@kernel.org>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        Alexandre Torgue <alexandre.torgue@foss.st.com>
CC:     <linux-doc@vger.kernel.org>, <dmaengine@vger.kernel.org>,
        <linux-stm32@st-md-mailman.stormreply.com>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>, Marek Vasut <marex@denx.de>
References: <20220711084703.268481-1-amelie.delaunay@foss.st.com>
 <20220711084703.268481-2-amelie.delaunay@foss.st.com>
 <87a69ffzvk.fsf@meer.lwn.net>
From:   Amelie Delaunay <amelie.delaunay@foss.st.com>
In-Reply-To: <87a69ffzvk.fsf@meer.lwn.net>
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On 7/11/22 17:11, Jonathan Corbet wrote:
> Amelie Delaunay <amelie.delaunay@foss.st.com> 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@foss.st.com>
>> ---
>>   .../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