linux-kernel - Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic transfers

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Message-ID: <20150807163303.GS7576@n2100.arm.linux.org.uk>
Date:	Fri, 7 Aug 2015 17:33:04 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Peter Ujfalusi <peter.ujfalusi@...com>,
	Vinod Koul <vinod.koul@...el.com>,
	Dan Williams <dan.j.williams@...el.com>,
	dmaengine@...r.kernel.org, linux-kernel@...r.kernel.org,
	nsekhar@...com, linux-omap@...r.kernel.org,
	linux-serial@...r.kernel.org, john.ogness@...utronix.de,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] dma: omap-dma: add support for pause of non-cyclic
 transfers

On Fri, Aug 07, 2015 at 12:07:11PM -0400, Peter Hurley wrote:
> On 08/07/2015 11:29 AM, Russell King - ARM Linux wrote:
> > On Fri, Aug 07, 2015 at 11:08:48AM -0400, Peter Hurley wrote:
> >> [ + Greg KH ]
> >>
> >> On 08/07/2015 09:57 AM, Russell King - ARM Linux wrote:
> >>> As it is something that the driver has _not_ supported, you are clearly
> >>> adding a feature to an existing driver.  It's not a bug fix.
> >>>
> >>>>> If something else has been converted to pause channels and that is causing
> >>>>> a problem, then _that_ conversion is where the bug lies, not the lack of
> >>>>> support in the omap-dma.
> >>
> >> FWIW, the actual bug is the api that silently does nothing.
> > 
> > Incorrect.
> > 
> > static int omap_dma_pause(struct dma_chan *chan)
> > {
> >         struct omap_chan *c = to_omap_dma_chan(chan);
> > 
> >         /* Pause/Resume only allowed with cyclic mode */
> >         if (!c->cyclic)
> >                 return -EINVAL;
> > 
> > Asking for the channel to be paused will return an error code indicating
> > that the request failed.  That will be propagated back through to the
> > return code of dmaengine_pause().
> > 
> > If we look at what 8250-dma.c is doing:
> > 
> >                 if (dma->rx_running) {
> >                         dmaengine_pause(dma->rxchan);
> > 
> > It's 8250-dma.c which is silently _ignoring_ the return code, failing
> > to check that the operation it requested worked.  Maybe this should be
> > WARN_ON(dmaengine_pause(dma->rxchan)) or at least it should print a
> > message?
> 
> Thanks for the suggestion; I'll hold on to that and push it after we add
> the 8250 omap dma pause in mainline.

Why wait?  You're hiding a data loss bug which is clearly the result of
code you allegedly maintain.

> Well, instead of me saying something snide about the lack of upstream serial
> driver unit tests, I'll say I've been working on cleaning up and organizing
> my own tty/serial subsystem and driver units tests which I hope to upstream
> in the fall.
> 
> Those include i/o validators that ran this driver for days at time without
> error at max line rate. Unfortunately, that hardware does not exhibit the
> same problem as the DRA7 noted in the changelog.

What you have is a race condition in the code you a responsible for
maintaining, caused by poorly implemented code.  Fix it, rather than
whinging about drivers outside of your subsystem having never implemented
_optional_ things that you choose to merge broken code which relied upon
it _without_ checking that the operation succeeded.

It is _entirely_ your code which is wrong here.

I will wait for that to be fixed before acking the omap-dma change since
you obviously need something to test with.

-- 
FTTC broadband for 0.8mile line: currently at 10.5Mbps down 400kbps up
according to speedtest.net.
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