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

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Feb 23, 2021 at 12:19:56PM +0100, Greg KH wrote:
> On Tue, Feb 23, 2021 at 11:58:42AM +0100, Thomas Zimmermann wrote:
> > USB devices cannot perform DMA and hence have no dma_mask set in their
> > device structure. Importing dmabuf into a USB-based driver fails, which
> > break joining and mirroring of display in X11.
> > 
> > For USB devices, pick the associated USB controller as attachment device,
> > so that it can perform DMA. If the DMa controller does not support DMA
> > transfers, we're aout of luck and cannot import.
> > 
> > Drivers should use DRM_GEM_SHMEM_DROVER_OPS_USB to initialize their
> > instance of struct drm_driver.
> > 
> > Tested by joining/mirroring displays of udl and radeon un der Gnome/X11.
> > 
> > v3:
> > 	* drop gem_create_object
> > 	* use DMA mask of USB controller, if any (Daniel, Christian, Noralf)
> > v2:
> > 	* move fix to importer side (Christian, Daniel)
> > 	* update SHMEM and CMA helpers for new PRIME callbacks
> > 
> > Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
> > Fixes: 6eb0233ec2d0 ("usb: don't inherity DMA properties for USB devices")
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > Cc: Johan Hovold <johan@kernel.org>
> > Cc: Alan Stern <stern@rowland.harvard.edu>
> > Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> > Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> > Cc: Mathias Nyman <mathias.nyman@linux.intel.com>
> > Cc: Oliver Neukum <oneukum@suse.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: <stable@vger.kernel.org> # v5.10+
> > ---
> >  drivers/gpu/drm/drm_prime.c        | 36 ++++++++++++++++++++++++++++++
> >  drivers/gpu/drm/tiny/gm12u320.c    |  2 +-
> >  drivers/gpu/drm/udl/udl_drv.c      |  2 +-
> >  include/drm/drm_gem_shmem_helper.h | 13 +++++++++++
> >  include/drm/drm_prime.h            |  5 +++++
> >  5 files changed, 56 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> > index 2a54f86856af..9015850f2160 100644
> > --- a/drivers/gpu/drm/drm_prime.c
> > +++ b/drivers/gpu/drm/drm_prime.c
> > @@ -29,6 +29,7 @@
> >  #include <linux/export.h>
> >  #include <linux/dma-buf.h>
> >  #include <linux/rbtree.h>
> > +#include <linux/usb.h>
> > 
> >  #include <drm/drm.h>
> >  #include <drm/drm_drv.h>
> > @@ -1055,3 +1056,38 @@ void drm_prime_gem_destroy(struct drm_gem_object *obj, struct sg_table *sg)
> >  	dma_buf_put(dma_buf);
> >  }
> >  EXPORT_SYMBOL(drm_prime_gem_destroy);
> > +
> > +/**
> > + * drm_gem_prime_import_usb - helper library implementation of the import callback for USB devices
> > + * @dev: drm_device to import into
> > + * @dma_buf: dma-buf object to import
> > + *
> > + * This is an implementation of drm_gem_prime_import() for USB-based devices.
> > + * USB devices cannot perform DMA directly. This function selects the USB host
> > + * controller as DMA device instead. Drivers can use this as their
> > + * &drm_driver.gem_prime_import implementation.
> > + *
> > + * See also drm_gem_prime_import().
> > + */
> > +#ifdef CONFIG_USB
> > +struct drm_gem_object *drm_gem_prime_import_usb(struct drm_device *dev,
> > +						struct dma_buf *dma_buf)
> > +{
> > +	struct usb_device *udev;
> > +	struct device *usbhost;
> > +
> > +	if (dev->dev->bus != &usb_bus_type)
> > +		return ERR_PTR(-ENODEV);
> > +
> > +	udev = interface_to_usbdev(to_usb_interface(dev->dev));
> > +	if (!udev->bus)
> > +		return ERR_PTR(-ENODEV);
> > +
> > +	usbhost = udev->bus->controller;
> > +	if (!usbhost || !usbhost->dma_mask)
> > +		return ERR_PTR(-ENODEV);
> 
> If individual USB drivers need access to this type of thing, shouldn't
> that be done in the USB core itself?
> 
> {hint, yes}
> 
> There shouldn't be anything "special" about a DRM driver that needs this
> vs. any other driver that might want to know about DMA things related to
> a specific USB device.  Why isn't this an issue with the existing
> storage or v4l USB devices?

Also, where is the locking for any of the above?  What guarantees that
the device will not go away during those pointer walks?

thanks,

greg k-h