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

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Date:   Fri, 9 Dec 2016 23:26:06 +0530
From:   Vinod Koul <vinod.koul@...el.com>
To:     Mason <slash.tmp@...e.fr>
Cc:     Sebastian Frias <sf84@...oste.net>, 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 Fri, Dec 09, 2016 at 06:34:15PM +0100, Mason wrote:
> 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.

Right, but in that case the fallback would be PIO mode, and if that is
not availble (IIRC some f your devices don't) then reject the usage with
EAGAIN.


-- 
~Vinod