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