linux-kernel - Re: [PATCH] dmaengine: qcom_bam

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Date:	Thu, 22 May 2014 16:56:18 +0100
From:	Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:	Andy Gross <agross@...eaurora.org>
CC:	Vinod Koul <vinod.koul@...el.com>, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] dmaengine: qcom_bam_dma: Add descriptor flag APIs



On 22/05/14 16:32, Andy Gross wrote:
> On Thu, May 22, 2014 at 04:27:05PM +0100, Srinivas Kandagatla wrote:
>
> <snip>
>
>>>
>>> The EOT is not used for every transaction.  It is part of a handshaking
>>> protocol with the attached peripheral, much like the NWD (notify when done).  As
>>> near as I can tell today, no peripheral depends on the EOB, so we could drop it
>>> for now until it is needed and cross this bridge when we need to.
>>
>> As EOT behaviour is totally dependent on the attached peripheral(or
>> channel), Can't we make this specific to channel by passing
>> additional flags in the DT dma channel descriptors? This will be
>> better abstraction for drivers as well.
>
> Even for channels where you want to use EOT, you don't use it for every
> transaction.  So a global channel flag isn't going to work.  This is the same
> for NWD.  It is a per descriptor choice.

Thanks Andy for explaining, I got it now.
>
>>
>> I know that EOT flag is part of descriptor but still some channels
>> *must* have EOT to run there state-machine correctly. So making it
>> optional for those channels might be wrong.
>>
>> Are there any use cases for particular *channel* where EOT
>> requirement changes dynamically?
>
> I2C is one example.  You place EOT on the last transaction that makes up a
> write/read transaction.  You may have multiple descriptors to send data, but the
> last one has EOT.  And for read transactions, you place NWD on the last read
> transaction.
>
> <snip>
>
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