linux-kernel - Re: [PATCH 0/7] DMAENGINE: fixes and PrimeCells

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Message-ID: <k2k63386a3d1005070910kcdb516dcga60c24043b1b44de@mail.gmail.com>
Date:	Fri, 7 May 2010 18:10:28 +0200
From:	Linus Walleij <linus.ml.walleij@...il.com>
To:	jassi brar <jassisinghbrar@...il.com>
Cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Ben Dooks <ben-linux@...ff.org>,
	Dan Williams <dan.j.williams@...el.com>,
	linux-mmc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 0/7] DMAENGINE: fixes and PrimeCells

2010/5/7 jassi brar <jassisinghbrar@...il.com>:
> On Fri, May 7, 2010 at 8:43 PM, Linus Walleij
> <linus.ml.walleij@...il.com> wrote:
>> 2010/5/7 Russell King - ARM Linux <linux@....linux.org.uk>:
>>>
>>> Or let me put it another way - if people are happy for Linux to support
>>> new ARM CPU architectures, but with very little attention given to DMA
>>> support on those architectures, then feel free to box the ARM platforms
>>> into a corner on DMA support - but on the understanding that _you_ will
>>> have to deal with the DMA API breakage on those architectures yourself.
>>> Because with ARM platforms not having DMA support, there's absolutely
>>> no way to run any checks what so ever on DMA when the CPU architecture
>>> support is created.
>>
>> I'm doing the best I can to meet exactly this goal. The changes done in
>> the DMA engine were done towards the end of making the DMA
>> engine support *any* DMA controller for the PrimeCells,
>
> with due respect, I think DMA Engine API is very restricting. And it is not
> just the 'async' character of it but also some desirable features like
> Circular-Linked-Buffer are missing. It may be good enough for Mem->Mem
> but is found wanting for Mem<->Dev transfers.
> I would like to see some new API defined that address reasonable
> requirements of extant platform specific implementations.

I understand these concerns, however I believe the DMAdevices/DMAengine
API can surely be refactored towards this end. Dan Williams has proved
*very* cooperative in doing changes and testing for regressions in the
DMAengine, and I see no fundamental problem with it.

Surely circular linked buffers and other goodies can be retrofitted into the
DMAengine without a complete redesign? I only see a new slave call
to support that really, in addition to the existing sglist interface.

I remember we discussed circular buffer device->device transfers with
Dan some while ago, and he was all for including that but wanted some
real-world example to go along with it.

>> So now I guess I have to make it tick on the block known as PL080/PL081
>> as well, and I'll have a try at it.
>
> I always hoped the pl080 core would be segregated out of S3C implementation,
> but for some new common DMA API.

Well, I don't see any fundamental problems with DMAengine, just wanted
extensions really.

I don't know if I'll be able to provide the nice breakout of the PL080 core
from S3C that you'd like to see but I will try to hack something up as
a proof-of-concept that the DMAengine can support the PrimeCells
found on the RealView.

Yours,
Linus Walleij
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