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

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Date:   Fri, 9 Dec 2016 18:34:15 +0100
From:   Mason <slash.tmp@...e.fr>
To:     Vinod Koul <vinod.koul@...el.com>,
        Sebastian Frias <sf84@...oste.net>
Cc:     Mans Rullgard <mans@...sr.com>,
        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>,
        Thibaud Cornic <thibaud_cornic@...madesigns.com>
Subject: Re: Tearing down DMA transfer setup after DMA client has finished

On 09/12/2016 18:17, Vinod Koul wrote:

> On Fri, Dec 09, 2016 at 11:25:57AM +0100, Sebastian Frias wrote:
>>
>> What concrete solution do you propose?
> 
> I have already proposed two solutions.
> 
> A) Request a channel only when you need it. Obviously we can't do virtual
> channels with this (though we should still use virt-channels framework).
> The sbox setup and teardown can be done as part of channel request and
> freeup. PL08x already does this.
> 
> Downside is that we can only have as many consumers at a time as channels.
> 
> I have not heard any technical reason for not doing this apart from drivers
> grab the channel at probe, which is incorrect and needs to be fixed
> irrespective of the problem at hand.
> 
> This is my preferred option.

There is one important drawback with this solution. If a driver calls
dma_request_chan() when no channels are currently available, it will
get -EBUSY. If there were a flag in dma_request_chan to be put to
sleep (with timeout) until a channel is available, then it would
work. But busy waiting in the client driver is a waste of power.

Regards.