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

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Message-ID: <yw1xh96v8xnb.fsf@unicorn.mansr.com>
Date:   Fri, 25 Nov 2016 14:03:20 +0000
From:   Måns Rullgård <mans@...sr.com>
To:     Russell King - ARM Linux <linux@...linux.org.uk>
Cc:     Vinod Koul <vinod.koul@...el.com>, Mason <slash.tmp@...e.fr>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Dave Jiang <dave.jiang@...el.com>,
        Arnd Bergmann <arnd@...db.de>, Mark Brown <broonie@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        dmaengine@...r.kernel.org, Dan Williams <dan.j.williams@...el.com>,
        Jon Mason <jon.mason@...el.com>,
        Lee Jones <lee.jones@...aro.org>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>
Subject: Re: Tearing down DMA transfer setup after DMA client has finished

Russell King - ARM Linux <linux@...linux.org.uk> writes:

> On Fri, Nov 25, 2016 at 01:50:35PM +0000, Måns Rullgård wrote:
>> Russell King - ARM Linux <linux@...linux.org.uk> writes:
>> > It would be unfair to augment the API and add the burden on everyone
>> > for the new API when 99.999% of the world doesn't require it.
>> 
>> I don't think making this particular dma driver wait for the descriptor
>> callback to return before reusing a channel quite amounts to a horrid
>> hack.  It certainly wouldn't burden anyone other than the poor drivers
>> for devices connected to it, all of which are specific to Sigma AFAIK.
>
> Except when you stop to think that delaying in a tasklet is exactly
> the same as randomly delaying in an interrupt handler - the tasklet
> runs on the return path back to the parent context of an interrupt
> handler.  Even if you sleep in the tasklet, you're sleeping on behalf
> of the currently executing thread - if it's a RT thread, you effectively
> destroy the RT-ness of the thread.  Let's hope no one cares about RT
> performance on that hardware...

That's why I suggested to do this only if the needed delay is known to
be no more than a few bus cycles.  The completion callback is currently
the only post-transfer interaction we have between the dma and device
drivers.  To handle an arbitrarily long delay, some new interface will
be required.

-- 
Måns Rullgård