Re: [PATCH] dmaengine: qcom_bam_dma: Add descriptor flag APIs

From: Andy Gross
Date: Thu May 22 2014 - 11:32:21 EST


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>

--
sent by an employee of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
hosted by The Linux Foundation
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/