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

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Message-ID: <20080919132544.533ab7d0@hskinnemo-gx745.norway.atmel.com>
Date:	Fri, 19 Sep 2008 13:25:44 +0200
From:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
To:	"Dan Williams" <dan.j.williams@...el.com>
Cc:	"Timur Tabi" <timur@...escale.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: dmaengine.c: question about device_alloc_chan_resources

"Dan Williams" <dan.j.williams@...el.com> wrote:
> On Thu, Sep 18, 2008 at 7:45 AM, Haavard Skinnemoen
> <haavard.skinnemoen@...el.com> wrote:
> > Timur Tabi <timur@...escale.com> wrote:  
> >> Haavard Skinnemoen wrote:
> >>  
> >> > Wouldn't it be better if the dmaengine layer made sure it didn't pass
> >> > the same channel several times to a client?  
> >>
> >> Wouldn't that require it to keep track of which clients have already seen which
> >> channels?  It might make more sense, but it's probably easier the current way.  
> >
> > I guess so. What would be even more simple is to simply prevent other
> > clients from taking a channel once someone has acked it, which would be
> > perfect for my purposes, but perhaps not everyone else's...  
> 
> Does not seem too bad to have something like a DMA_EXCL flag in
> dma_chan to tell the core not to show this channel to any other
> clients.

Yes, or maybe provide an interface for simply requesting a channel
without having to register any callbacks. With some drivers, the number
of required channels is fixed, and not using DMA just isn't an option.
A simple "get a channel which satisfies these constraints" interface
would simplify these drivers a lot.

Haavard
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