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

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Message-ID: <20161125135830.GL14217@n2100.armlinux.org.uk>
Date:   Fri, 25 Nov 2016 13:58:30 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Måns Rullgård <mans@...sr.com>
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

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...

-- 
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