linux-kernel - RE: dmaengine: DMA_CTRL

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Date:	Tue, 7 Oct 2008 10:09:31 +0100
From:	"Sosnowski, Maciej" <maciej.sosnowski@...el.com>
To:	"Nicolas Ferre" <nicolas.ferre@...el.com>,
	"Williams, Dan J" <dan.j.williams@...el.com>,
	"Linux Kernel list" <linux-kernel@...r.kernel.org>
Cc:	"Haavard Skinnemoen" <hskinnemoen@...el.com>
Subject: RE: dmaengine: DMA_CTRL_ACK flag signification

Nicolas Ferre wrote:
> Hi all,
> 
> I am in the process of writing a driver for an on-chip Atmel DMA
> engine. 
> 
> I am a little confused about the use of the flag DMA_CTRL_ACK : It
> seems that it is set in most of the descriptors in use except the
> first or last of a descriptor chain. So, I cannot find where this
> flag is cleared. In short, I do not see what it is used for : how
> must I take it into account in my driver (in device_prep_dma_memcpy()
> for instance) ? 
> 
> Can you enlighten me ?
> 
> Regards,

Hi Nicolas,

Sorry for the delay in response.
Generally the idea behind DMA_CTRL_ACK is to let an application 
safely set a chain of dependent operations.
What a DMA driver needs to do is to check 
if a given descriptor has been already acked (using async_tx_test_ack())
before it recycles or releases it.
You are right that there is no place where DMA_CTRL_ACK 
is cleared at the moment. I would say it is the offload engine driver 
responsibility to clear the flag when it recycles the descriptor.
Dan, could you confirm?

Regards,
Maciej
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