linux-kernel - Re: [PATCH] media: pwc: Fix the URB buffer allocation

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Date:   Fri, 05 Feb 2021 14:42:23 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Hans Verkuil <hverkuil@...all.nl>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Matwey V. Kornilov" <matwey@....msu.ru>,
        Robert Foss <robert.foss@...aro.org>
Subject: Re: [PATCH] media: pwc: Fix the URB buffer allocation

On Fri, 05 Feb 2021 14:13:02 +0100,
Andrew Lunn wrote:
> 
> Hi Takashi
> 
> > Indeed, looks so.  In most cases, this doesn't matter since both point
> > to the same device object.  In some cases like xhci-plat HCD, they
> > differ.  And sysdev  might be a better choice from the consistency
> > POV.
> > 
> > But this brought an interesting question, too.  eg. USB chipidea
> > HCD uses platform devices for both controller and sysdev, and I
> > couldn't find any DMA mask setup.  So, no matter what to use, the uwc
> > driver would be broken on this...  Maybe it's just not covered.
> 
> Did you do a git bisect to see what actually broke it?  "1161db6776bd:
> media: usb: pwc: Don't use coherent DMA buffers for ISO transfer"
> introduced the code, not the regression. If we understand the
> regression, that might give us the answer about chipidea.

It's the recent DMA core change, the commit f959dcd6ddfd ("dma-direct:
Fix potential NULL pointer dereference").  But basically it's a right
fix, and the driver hitting this "regression" has been already broken
but casually worked without setting a proper DMA mask.


Takashi