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

Tue Feb 23 12:14:30 UTC 2021

On Tue, Feb 23, 2021 at 1:02 PM Greg KH <gregkh at linuxfoundation.org> wrote:
>
> On Tue, Feb 23, 2021 at 12:46:20PM +0100, Daniel Vetter wrote:
> > 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 at suse.de>
> > > > Fixes: 6eb0233ec2d0 ("usb: don't inherity DMA properties for USB devices")
> > > > Cc: Christoph Hellwig <hch at lst.de>
> > > > Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> > > > Cc: Johan Hovold <johan at kernel.org>
> > > > Cc: Alan Stern <stern at rowland.harvard.edu>
> > > > Cc: Andy Shevchenko <andriy.shevchenko at linux.intel.com>
> > > > Cc: Sebastian Andrzej Siewior <bigeasy at linutronix.de>
> > > > Cc: Mathias Nyman <mathias.nyman at linux.intel.com>
> > > > Cc: Oliver Neukum <oneukum at suse.com>
> > > > Cc: Thomas Gleixner <tglx at linutronix.de>
> > > > Cc: <stable at 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?
> >
> > The trouble is that this is a regression fix for 5.9, because the dma-api
> > got more opinionated about what it allows. The proper fix is a lot more
> > invasive (we essentially need to rework the drm_prime.c to allow dma-buf
> > importing for just cpu access), and that's a ton more invasive than just a
> > small patch with can stuff into stable kernels.
> >
> > This here is ugly, but it should at least get rid of black screens again.
> >
> > I think solid FIXME comment explaining the situation would be good.
>
> Why can't I take a USB patch for a regression fix?  Is drm somehow
> stand-alone that you make changes here that should belong in other
> subsystems?
>
> {hint, it shouldn't be}
>
> When you start poking in the internals of usb controller structures,
> that logic belongs in the USB core for all drivers to use, not in a
> random tiny subsystem where no USB developer will ever notice it?  :)

Because the usb fix isn't the right fix here, it's just the duct-tape.
We don't want to dig around in these internals, it's just a convenient
way to shut up the dma-api until drm has sorted out its confusion.

We can polish the turd if you want, but the thing is, it's still a turd ...

The right fix is to change drm_prime.c code to not call dma_map_sg
when we don't need it. The problem is that roughly 3 layers of code
(drm_prime, dma-buf, gem shmem helpers) are involved. Plus, since
drm_prime is shared by all drm drivers, all other drm drivers are
impacted too. We're not going to be able to cc: stable that kind of
stuff. Thomas actually started with that series, until I pointed out
how bad things really are.

And before you ask: The dma-api change makes sense, and dma-api vs drm
relations are strained since years, so we're not going ask for some
hack there for our abuse to paper over the regression. I've been in
way too many of those threads, only result is shouting and failed
anger management.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch