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

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Message-ID: <20140522153214.GB22327@qualcomm.com>
Date:	Thu, 22 May 2014 10:32:14 -0500
From:	Andy Gross <agross@...eaurora.org>
To:	Srinivas Kandagatla <srinivas.kandagatla@...aro.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 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.

> 
> 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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