linux-kernel - Re: dmaengine.c: question about device_alloc_chan

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Message-ID: <e9c3a7c20809201600s3862da7qaab4a917220778e9@mail.gmail.com>
Date:	Sat, 20 Sep 2008 16:00:40 -0700
From:	"Dan Williams" <dan.j.williams@...el.com>
To:	"Timur Tabi" <timur@...escale.com>
Cc:	"Haavard Skinnemoen" <haavard.skinnemoen@...el.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: dmaengine.c: question about device_alloc_chan_resources

On Fri, Sep 19, 2008 at 7:34 AM, Timur Tabi <timur@...escale.com> wrote:
> Haavard Skinnemoen wrote:
>
>> Yes, or maybe provide an interface for simply requesting a channel
>> without having to register any callbacks.
>
> I could use this feature.  The sound drivers for our MPC8610 processor use DMA,
> but the drivers need to control the DMA hardware directly, so I can't use
> dmaengine.  I would like to be able to just reserve the channels and program
> them as I see fit.
>

I think its a good idea especially since it would be best not to
needlessly proliferate client implementations with competing channel
allocation schemes.  However it would need to be more descriptive
than:

struct dma_chan *dma_request_channel(dma_cap_mask_t request_mask);

Why:
1/  What if the requester initializes before a dmaengine device has
been registered?  What if a device is never registered?
2/ What about platform specific concerns where dma_cap_mask_t is not
descriptive enough e.g. only one memcpy channel can address a certain
bus?  Currently a client implementation can have some intelligence to
return DMA_DUP for channels that do not have the platform capability.

At the very least clients should be allowed to set an 'exclusive' bit
to prevent the channel from leaking elsewhere.

--
Dan
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