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

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Date:   Fri, 5 Feb 2021 14:53:13 +0100
From:   Andrew Lunn <andrew@...n.ch>
To:     Takashi Iwai <tiwai@...e.de>
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, Feb 05, 2021 at 02:42:23PM +0100, Takashi Iwai wrote:
> 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.

So for the chipidea, it also just 'casually worked'. But now it
probably does not. But that is a separate chipidea issue.  None of my
ARM systems use the chipidea IP core, so i cannot test anything.

	 Andrew