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Subject: Re: [PATCH v3 4/7] dmaengine: stm32-dma: Add DMA/MDMA chaining
 support
To:     Vinod <vkoul@kernel.org>
CC:     Rob Herring <robh+dt@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexandre Torgue <alexandre.torgue@st.com>,
        Maxime Coquelin <mcoquelin.stm32@gmail.com>,
        Dan Williams <dan.j.williams@intel.com>,
        <devicetree@vger.kernel.org>, <dmaengine@vger.kernel.org>,
        <linux-arm-kernel@lists.infradead.org>,
        <linux-kernel@vger.kernel.org>
References: <1538139715-24406-1-git-send-email-pierre-yves.mordret@st.com>
 <1538139715-24406-5-git-send-email-pierre-yves.mordret@st.com>
 <20181007160030.GB2372@vkoul-mobl>
 <e9cedfdd-ad90-9123-03bb-34a2e84059c5@st.com>
 <20181010040343.GO2372@vkoul-mobl>
From:   Pierre Yves MORDRET <pierre-yves.mordret@st.com>
Message-ID: <d64da587-a680-20c4-7139-f59fd1f299bc@st.com>
Date:   Wed, 10 Oct 2018 09:02:36 +0200
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On 10/10/2018 06:03 AM, Vinod wrote:
> On 09-10-18, 10:40, Pierre Yves MORDRET wrote:
>>
>>
>> On 10/07/2018 06:00 PM, Vinod wrote:
>>> On 28-09-18, 15:01, Pierre-Yves MORDRET wrote:
>>>> This patch adds support of DMA/MDMA chaining support.
>>>> It introduces an intermediate transfer between peripherals and STM32 DMA.
>>>> This intermediate transfer is triggered by SW for single M2D transfer and
>>>> by STM32 DMA IP for all other modes (sg, cyclic) and direction (D2M).
>>>>
>>>> A generic SRAM allocator is used for this intermediate buffer
>>>> Each DMA channel will be able to define its SRAM needs to achieve chaining
>>>> feature : (2 ^ order) * PAGE_SIZE.
>>>> For cyclic, SRAM buffer is derived from period length (rounded on
>>>> PAGE_SIZE).
>>>
>>> So IIUC, you chain two dma txns together and transfer data via an SRAM?
>>
>> Correct. one DMA is DMAv2 (stm32-dma) and the other is MDMA(stm32-mdma).
>> Intermediate transfer is between device and memory.
>> This intermediate transfer is using SDRAM.
> 
> Ah so you use dma calls to setup mdma xtfers? I dont think that is a
> good idea. How do you know you should use mdma for subsequent transfer?
> 

When user bindings told to setup chaining intermediate MDMA transfers are always
triggers.
For instance if a user requests a Dev2Mem transfer with chaining. From client
pov this is still a prep_slave_sg. Internally DMAv2 is setup in cyclic mode (in
double buffer mode indeed => 2 buffer of PAGE_SIZE/2) and destination is SDRAM.
DMAv2 will flip/flop on those 2 buffers.
At the same time DMAv2 driver prepares a MDMA SG that will fetch data from those
2 buffers in SDRAM and fills final destination memory.

> 
>>>>  drivers/dma/stm32-dma.c | 879 ++++++++++++++++++++++++++++++++++++++++++------
>>>
>>> that is a lot of change for a driver, consider splitting it up
>>> logically in smaller changes...
>>>
>>
>> This feature is rather monolithic. Difficult to split up.
>> All the code is required at once.
> 
> It can be enabled at last but split up logically. Intrusive changes to a
> driver make it hard to review..
> 
Ok. I will to think about it how to proceed.