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

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Date:   Thu, 08 Dec 2016 16:46:15 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Vinod Koul <vinod.koul@...el.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

Vinod Koul <vinod.koul@...el.com> writes:

> 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..

ALSA is a particularly poor example.  ALSA drivers request a dma channel
at some point between the device being opened and playback (or capture)
starting.  They then set up a cyclic transfer that runs continuously
until playback is stopped.  It's a completely different model of
operation than we're talking about here.

-- 
Måns Rullgård