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SubjectRe: [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

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