linux-kernel - Re: Tearing down DMA transfer setup after DMA client has finished

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Date:   Thu, 8 Dec 2016 21:10:18 +0530
From:   Vinod Koul <vinod.koul@...el.com>
To:     Måns Rullgård <mans@...sr.com>
Cc:     Mason <slash.tmp@...e.fr>, Russell King <linux@....linux.org.uk>,
        dmaengine@...r.kernel.org,
        Linus Walleij <linus.walleij@...aro.org>,
        Dan Williams <dan.j.williams@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Jon Mason <jdmason@...zu.us>, Mark Brown <broonie@...nel.org>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Lee Jones <lee.jones@...aro.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Arnd Bergmann <arnd@...db.de>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Peter Ujfalusi <peter.ujfalusi@...com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Sebastian Frias <sf84@...oste.net>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>
Subject: Re: Tearing down DMA transfer setup after DMA client has finished

On Thu, Dec 08, 2016 at 11:44:53AM +0000, Måns Rullgård wrote:
> Vinod Koul <vinod.koul@...el.com> writes:
> 
> > On Wed, Dec 07, 2016 at 04:45:58PM +0000, Måns Rullgård wrote:
> >> Vinod Koul <vinod.koul@...el.com> writes:
> >> 
> >> > On Tue, Dec 06, 2016 at 01:14:20PM +0000, Måns Rullgård wrote:
> >> >> 
> >> >> That's not going to work very well.  Device drivers typically request
> >> >> dma channels in their probe functions or when the device is opened.
> >> >> This means that reserving one of the few channels there will inevitably
> >> >> make some other device fail to operate.
> >> >
> >> > No that doesnt make sense at all, you should get a channel only when you
> >> > want to use it and not in probe!
> >> 
> >> Tell that to just about every single driver ever written.
> >
> > Not really, few do yes which is wrong but not _all_ do that.
> 
> Every driver I ever looked at does.  Name one you consider "correct."

That only tells me you haven't looked enough and want to rant!

FWIW look at ALSA-dmaengine lib, thereby every audio driver that uses it. I
could find other examples and go on and on, but that's besides the point and
looks like you don't want to listen to people telling you something..

-- 
~Vinod