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

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Date:	Tue, 9 Oct 2012 14:02:15 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	shuah.khan@...com
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 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.

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