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

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Message-ID: <s5htuqq38c3.wl-tiwai@suse.de>
Date:   Fri, 05 Feb 2021 15:19:56 +0100
From:   Takashi Iwai <tiwai@...e.de>
To:     Hans Verkuil <hverkuil@...all.nl>
Cc:     Andrew Lunn <andrew@...n.ch>,
        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 15:05:00 +0100,
Hans Verkuil wrote:
> 
> On 05/02/2021 14:53, Andrew Lunn wrote:
> > 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.
> 
> Since few people use the pwc driver anymore, and certainly not on non-intel
> devices, I am happy with Matwey's patch as is merged in our media tree for
> 5.12.
> 
> I'll mark Takashi's patch as Obsolete in patchwork as well.
> 
> I might have an SBC with a chipidea, if so, I'll give it a quick spin.

Fair enough.


Thanks!

Takashi