linux-kernel - Re: [PATCH] dmaengine: add dma_ctrl_cmd to pass buffer stride configuration

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Message-ID: <CABb+yY1RqHJ6jsMB2RqPNnBLff-bzhwaXG1U-hCD8CpME+a3aA@mail.gmail.com>
Date:	Tue, 12 Jul 2011 16:50:37 +0530
From:	Jassi Brar <jassisinghbrar@...il.com>
To:	Linus Walleij <linus.walleij@...aro.org>
Cc:	Sundaram Raju <sundaram@...com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	davinci-linux-open-source@...ux.davincidsp.com,
	linux@....linux.org.uk, dan.j.williams@...el.com,
	linux-omap@...r.kernel.org
Subject: Re: [PATCH] dmaengine: add dma_ctrl_cmd to pass buffer stride configuration

On Tue, Jul 12, 2011 at 3:33 PM, Linus Walleij <linus.walleij@...aro.org> wrote:
> On Tue, Jul 12, 2011 at 6:17 AM, Jassi Brar <jassisinghbrar@...il.com> wrote:
>
>> 1) Striding, in one form or other, is supported by other DMACs as well.
>>   The number will only increase in future.
>>   Are we to add  <VENDOR>_DMA_STRIDE_CONFIG for each case ?
>
> If we are sure about this and striding will work in a similar way on all
> then let's have the enum named DMA_STRIDE_CONFIG and move the
> passed-in struct to <linux/dmaengine.h) then?
>
> Would that be:
>
> struct dma_stride_config {
>    u32 read_bytes;
>    u32 skip_bytes;
> };
>
> Or something more complex?
Well, I am not sure if striding needs any special treatment at all.
Why not have client drivers prepare and submit sg-list.
Let the DMAC drivers interpret/parse the sg-list and program it
as strides if the h/w supports it.
If anything, we should make preparation and submission of sg-list
as efficient as possible.

>> 2) As Dan noted, client drivers are going to have ifdef hackery in
>> order to be common
>>  to other SoCs.
>
> Don't think so, why? This is a runtime config entirely, and I just illustrated
> in mail to Dan how that can be handled by falling back to a sglist I believe?
Runtime decision isn't neat either.
What if a client driver is common to 'N' SoCs each with different DMACs ?
We would need a switch construct !
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