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Subject: RE: dmaengine: DMA_CTRL_ACK flag signification
Date: Tue, 7 Oct 2008 10:09:31 +0100
Message-ID: <7F38996F7185A24AB9071ED4950AD8C102121A73@swsmsx413.ger.corp.intel.com>
In-Reply-To: <48DA2B94.3020804@atmel.com>
Thread-Topic: dmaengine: DMA_CTRL_ACK flag signification
Thread-Index: AckePQS/9x3ZGzOSRvW2pU5zSPgSPgKFTI9Q
References: <48DA2B94.3020804@atmel.com>
From: "Sosnowski, Maciej" <maciej.sosnowski@intel.com>
To: "Nicolas Ferre" <nicolas.ferre@atmel.com>,
       "Williams, Dan J" <dan.j.williams@intel.com>,
       "Linux Kernel list" <linux-kernel@vger.kernel.org>
Cc: "Haavard Skinnemoen" <hskinnemoen@atmel.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1308
Lines: 37

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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