From: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= <mans@mansr.com>
To: Vinod Koul <vinod.koul@intel.com>
Cc: Mason <slash.tmp@free.fr>, Russell King <linux@arm.linux.org.uk>,
        dmaengine@vger.kernel.org, Linus Walleij <linus.walleij@linaro.org>,
        Dan Williams <dan.j.williams@intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Jon Mason <jdmason@kudzu.us>, Mark Brown <broonie@kernel.org>,
        Lars-Peter Clausen <lars@metafoo.de>, Lee Jones <lee.jones@linaro.org>,
        Laurent Pinchart <laurent.pinchart@ideasonboard.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Maxime Ripard <maxime.ripard@free-electrons.com>,
        Dave Jiang <dave.jiang@intel.com>,
        Peter Ujfalusi <peter.ujfalusi@ti.com>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>,
        Sebastian Frias <sf84@laposte.net>,
        Thibaud Cornic <thibaud_cornic@sigmadesigns.com>
Subject: Re: Tearing down DMA transfer setup after DMA client has finished
References: <20161125045549.GC2698@localhost>
        <092f44ee-4560-be17-25f7-00948dba3cfa@free.fr>
        <20fc9020-7278-bc2f-2a8d-43aff5cabff8@free.fr>
        <20161206051222.GQ6408@localhost> <5846B237.8060409@free.fr>
        <yw1xwpfd2o9f.fsf@unicorn.mansr.com> <20161207164341.GX6408@localhost>
        <yw1xr35j1yd5.fsf@unicorn.mansr.com> <20161208103921.GC6408@localhost>
        <yw1x60mu1w7e.fsf@unicorn.mansr.com> <20161208154018.GD6408@localhost>
Date: Thu, 08 Dec 2016 16:46:15 +0000
In-Reply-To: <20161208154018.GD6408@localhost> (Vinod Koul's message of "Thu,
        8 Dec 2016 21:10:18 +0530")
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Vinod Koul <vinod.koul@intel.com> writes:

> On Thu, Dec 08, 2016 at 11:44:53AM +0000, M?ns Rullg?rd wrote:
>> Vinod Koul <vinod.koul@intel.com> writes:
>> 
>> > On Wed, Dec 07, 2016 at 04:45:58PM +0000, M?ns Rullg?rd wrote:
>> >> Vinod Koul <vinod.koul@intel.com> writes:
>> >> 
>> >> > On Tue, Dec 06, 2016 at 01:14:20PM +0000, M?ns Rullg?rd 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.

-- 
M?ns Rullg?rd