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

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Hi

Am 23.02.21 um 18:30 schrieb Greg KH:
On Tue, Feb 23, 2021 at 11:00:54AM -0500, Alan Stern wrote:
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.

Yes, sorry for not being clear, that is what I think would make the most
sense, then all USB drivers could use it easily and it can be changed in
one location if the DMA logic ever changes.

We can certainly do that. Thanks for clarifying. I'll send out an updated patch soon.

Best regards
Thomas


thanks,

greg k-h
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--
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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