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

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Date:   Thu, 8 Dec 2016 11:54:51 +0100
From:   Mason <slash.tmp@...e.fr>
To:     Vinod Koul <vinod.koul@...el.com>, Mans Rullgard <mans@...sr.com>
Cc:     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 08/12/2016 11:39, Vinod Koul wrote:

> 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 doesn't 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.

Vinod,

Could you explain something to me in layman's terms?

I have a NAND Flash Controller driver that depends on the
DMA driver under discussion.

Suppose I move the dma_request_chan() call from the driver's
probe function, to the actual DMA transfer function.

I would want dma_request_chan() to put the calling thread
to sleep until a channel becomes available (possibly with
a timeout value).

But Maxime told me dma_request_chan() will just return
-EBUSY if no channels are available.

Am I supposed to busy wait in my driver's DMA function
until a channel becomes available?

I don't understand how the multiplexing of few memory
channels to many clients is supposed to happen efficiently?

Regards.