linux-kernel - Re: [PATCH v3 4/7] dmaengine: stm32-dma: Add DMA/MDMA chaining support

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Message-ID: <20181010040343.GO2372@vkoul-mobl>
Date:   Wed, 10 Oct 2018 09:33:43 +0530
From:   Vinod <vkoul@...nel.org>
To:     Pierre Yves MORDRET <pierre-yves.mordret@...com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        devicetree@...r.kernel.org, dmaengine@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 4/7] dmaengine: stm32-dma: Add DMA/MDMA chaining
 support

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?


> >>  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..

-- 
~Vinod