linux-kernel - Re: [PATCH v4] dma-debug: New interfaces to debug dma mapping errors

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Date:	Wed, 10 Oct 2012 15:50:25 -0600
From:	Shuah Khan <shuah.khan@...com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	konrad.wilk@...cle.com, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, rob@...dley.net, stern@...land.harvard.edu,
	joerg.roedel@....com, bhelgaas@...gle.com,
	LKML <linux-kernel@...r.kernel.org>, linux-doc@...r.kernel.org,
	devel@...uxdriverproject.org, x86@...nel.org, shuahkhan@...il.com
Subject: Re: [PATCH v4] dma-debug: New interfaces to debug dma mapping errors

On Tue, 2012-10-09 at 14:02 -0700, Andrew Morton wrote:
> On Mon, 08 Oct 2012 11:07:20 -0600
> Shuah Khan <shuah.khan@...com> wrote:
> 
> > > 
> > > Still seems overly complicated to me, but whatev.
> > > 
> > > I think the way to handle this is pretty simple: set a flag in the dma
> > > entry when someone runs dma_mapping_error() and, if that flag wasn't
> > > set at unmap time, emit a loud warning.
> > > 
> > > From my reading of the code, this patch indeed does that, along with a
> > > bunch of other (unnecessary?) stuff.  But boy, the changelog conceals
> > > this information well!
> > 
> > Are you referring to the system wide error counters when you say
> > unnecessary stuff. The reason I added those was to catch errors when
> > drivers don't do unmap. Several drivers fail to do unmap. Checking flag
> > from unmap debug interfaces, doesn't cover these cases. However, I think
> > the value of system wide counters is limited in the sense that they
> > don't really identify the driver that needs fixing. In that sense it can
> > be deemed unnecessary. I dropped them in v5 patch, which I am sending
> > out.
> 
> hm.  Could we keep a counter of the number of map/unmap calls within
> the dma object and then emit a warning if that is non-zero at teardown
> time?  That should identify the offending driver.
> 

Thanks. That would work. Will take a look and see what it takes to
implement.

-- Shuah



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