[PATCH v3] drm: Use USB controller's DMA mask when importing dmabufs

[Alan Stern]

On Tue, Feb 23, 2021 at 03:06:07PM +0100, Thomas Zimmermann wrote:
> Hi
> 
> Am 23.02.21 um 14:44 schrieb Takashi Iwai:

> > Aside from the discussion whether this "workaround" is needed, the use
> > of udev->bus->controller here looks a bit suspicious.  As the old USB
> > code (before the commit 6eb0233ec2d0) indicated, it was rather
> > usb->bus->sysdev that was used for the DMA mask, and it's also the one
> > most of USB core code refers to.  A similar question came up while
> > fixing the same kind of bug in the media subsystem, and we concluded
> > that bus->sysdev is a better choice.
> 
> Good to hear that we're not the only ones affected by this. Wrt the original
> code, using sysdev makes even more sense.

Hmmm, I had forgotten about this.  So DMA masks aren't inherited after 
all, thanks to commit 6eb0233ec2d0.  That leas me to wonder how well 
usb-storage is really working these days...

The impression I get is that Greg would like the USB core to export a 
function which takes struct usb_interface * as argument and returns the 
appropriate DMA mask value.  Then instead of messing around with USB 
internals, drm_gem_prime_import_usb could just call this new function.

Adding such a utility function would be a sufficiently small change that 
it could go into the -stable kernels with no trouble.

Alan Stern