linux-kernel - Re: [PATCH v3 1/7] dt-bindings: stm32-dma: Add DMA/MDMA chaining support bindings

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Date:   Tue, 9 Oct 2018 15:46:06 +0200
From:   Pierre Yves MORDRET <pierre-yves.mordret@...com>
To:     Vinod <vkoul@...nel.org>
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 1/7] dt-bindings: stm32-dma: Add DMA/MDMA chaining
 support bindings



On 10/09/2018 10:57 AM, Vinod wrote:
> Hi Pierre,
> 
> On 09-10-18, 09:18, Pierre Yves MORDRET wrote:
> 
>>>>  * DMA client
>>>> @@ -68,7 +84,16 @@ channel: a phandle to the DMA controller plus the following four integer cells:
>>>>  	0x1: 1/2 full FIFO
>>>>  	0x2: 3/4 full FIFO
>>>>  	0x3: full FIFO
>>>> -
>>>> + -bit 2: Intermediate M2M transfer from/to DDR to/from SRAM throughout MDMA
>>>> +	0: MDMA not used to generate an intermediate M2M transfer
>>>> +	1: MDMA used to generate an intermediate M2M transfer.
>>>> + -bit 3-4: indicated SRAM Buffer size in (2^order)*PAGE_SIZE.
>>>> +	PAGE_SIZE is given by Linux at 4KiB: include/asm-generic/page.h.
>>>> +	Order is given by those 2 bits starting at 0.
>>>> +	Valid only whether Intermediate M2M transfer is set.
>>>
>>> why do we need this as a property?
>>
>> In some UC, we need more than 4KiB in case of chaining for better performances.
>> Chaining has to be enabled by client if performance is at sacks.
> 
> Okay if that is the case why is the user not taking care of this?
> Creating DMA txn and chaining them up and starting the chain? Why would
> dmaengine driver need to do that?
> 

User is using standard DMA API (single, sg or cyclic) and is agnostic on what is
behind the scene(almost). As driver I just fulfill the request to transfer what
he wants. My driver scatters transfer into SDRAM chunks defined by user.
Unfortunately all transfer are not % the SDRAM size given in DT. This very last
txn is to flush the last expected bytes.
Whatever user set for chaining(bit 2) the DMA API remains the same at its side.