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

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Feb 23, 2021 at 01:14:30PM +0100, Daniel Vetter wrote:
> On Tue, Feb 23, 2021 at 1:02 PM Greg KH <gregkh@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@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?
> > >
> > > 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.

Let's do it right.  If this is a regression from 5.9, it isn't a huge
one as that kernel was released last October.  I don't like to see this
messing around with USB internals in non-USB-core code please.

thanks,

greg k-h