linux-kernel - Re: [PATCH] dmaengine: at_hdmac: new driver for the Atmel AHB DMA Controller

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Message-ID: <e9c3a7c20812041026v3293786sea3e81e2f777d58d@mail.gmail.com>
Date:	Thu, 4 Dec 2008 11:26:10 -0700
From:	"Dan Williams" <dan.j.williams@...el.com>
To:	"Nicolas Ferre" <nicolas.ferre@...el.com>
Cc:	"Sosnowski, Maciej" <maciej.sosnowski@...el.com>,
	"Haavard Skinnemoen" <hskinnemoen@...el.com>,
	"Linux Kernel list" <linux-kernel@...r.kernel.org>,
	"ARM Linux Mailing List" <linux-arm-kernel@...ts.arm.linux.org.uk>,
	"Andrew Victor" <linux@...im.org.za>
Subject: Re: [PATCH] dmaengine: at_hdmac: new driver for the Atmel AHB DMA Controller

On Tue, Nov 25, 2008 at 8:21 AM, Nicolas Ferre <nicolas.ferre@...el.com> wrote:
>>
>> They will be acknowledged by client code.  Calls like async_memcpy
>> assume that the the ack bit is clear by default so they can specify
>> some actions to run at completion time.  By setting it early, at
>> descriptor allocation time, async_tx will get confused.
>
> This ack bit is annoying me : I cannot figure out how it is used for plain
> memcopy/slave offload calls...
>
> Moreover, at recycle time, if I keep a descriptor chain as a whole, I have
> to introduce another state for my descriptors : consumed but not freed yet
> (with another linked list management).
> If I only take care of the ACK flag for releasing descriptors, I loose the
> dependency in my descriptor chain (in a multi-descriptor memcpy case).

Maybe I misread your driver... in the multi-descriptor memcpy case it
is perfectly fine to set the ack-bit in the driver for all but the
last descriptor in the chain.  The client is only expected to attach
dependencies at the end of the chain.

> Can I only consider this information without taking care of the chaining
> dependency (and loose this information in a multi-descriptor operation) ?

You can lose it for all but the last descriptor.

> Or, may I drop this DMA_CTRL_ACK bit management as I do not have the
> usefulness of redoing an operations on one descriptor (no xor engine) ?
>

If you will never need to run async_memcpy and don't want to handle
the bit then you could set DMA_PRIVATE and the channels would never be
picked up by the async_tx api.  But, I am not convinced we need to
take that step yet.

> Kind regards,
> --
> Nicolas Ferre
>

Thanks,
Dan
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