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

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Date:	Tue, 12 Jul 2011 12:03:22 +0200
From:	Linus Walleij <linus.walleij@...aro.org>
To:	Jassi Brar <jassisinghbrar@...il.com>
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 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?

> 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?

We can *maybe* even put the fallback code into dmaengine, so that an
emulated sglist in place for the DMAengine is done automatically of the
DMA controller does not support striding.

> 3) TI may not have just one DMAC IP used in all the SoCs. So if you want
>  vendor specific defines anyway, please atleast also add DMAC version to it.
>  Something like
>>        DMA_SLAVE_CONFIG,
>>        FSLDMA_EXTERNAL_START,
>> +       TI_DMA_v1_STRIDE_CONFIG,

Yep unless we make it generic DMA_STRIDE_CONFIG simply, this makes
a lot of sense.

Linus Walleij
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