On Fri, Feb 26, 2021 at 11:29:05AM +0100, Thomas Zimmermann wrote:
> Hi
>
> Am 26.02.21 um 11:19 schrieb Greg KH:
> > On Fri, Feb 26, 2021 at 10:26:47AM +0100, Thomas Zimmermann wrote:
> > > USB devices cannot perform DMA and hence have no dma_mask set in their
> > > device structure. Therefore importing dmabuf into a USB-based driver
> > > fails, which breaks joining and mirroring of display in X11.
> > >
> > > For USB devices, pick the associated USB controller as attachment device.
> > > This allows the DRM import helpers to perform the DMA setup. If the DMA
> > > controller does not support DMA transfers, we're out of luck and cannot
> > > import. Our current USB-based DRM drivers don't use DMA, so the actual
> > > DMA device is not important.
> > >
> > > 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.
> > >
> > > v5:
> > > * provide a helper for USB interfaces (Alan)
> > > * add FIXME item to documentation and TODO list (Daniel)
> > > v4:
> > > * implement workaround with USB helper functions (Greg)
> > > * use struct usb_device->bus->sysdev as DMA device (Takashi)
> > > 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@xxxxxxx>
> > > Fixes: 6eb0233ec2d0 ("usb: don't inherity DMA properties for USB devices")
> > > Tested-by: Pavel Machek <pavel@xxxxxx>
> > > Acked-by: Christian König <christian.koenig@xxxxxxx>
> > > Acked-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > > Cc: Christoph Hellwig <hch@xxxxxx>
> > > Cc: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > Cc: <stable@xxxxxxxxxxxxxxx> # v5.10+
> > > ---
> > > Documentation/gpu/todo.rst | 15 ++++++++++
> > > drivers/gpu/drm/drm_prime.c | 45 ++++++++++++++++++++++++++++++
> > > drivers/gpu/drm/tiny/gm12u320.c | 2 +-
> > > drivers/gpu/drm/udl/udl_drv.c | 2 +-
> > > drivers/usb/core/usb.c | 31 ++++++++++++++++++++
> > > include/drm/drm_gem_shmem_helper.h | 16 +++++++++++
> > > include/drm/drm_prime.h | 5 ++++
> > > include/linux/usb.h | 24 ++++++++++++++++
> > > 8 files changed, 138 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
> > > index f872d3d33218..c185e0a2951e 100644
> > > --- a/Documentation/gpu/todo.rst
> > > +++ b/Documentation/gpu/todo.rst
> > > @@ -617,6 +617,21 @@ Contact: Daniel Vetter
> > > Level: Intermediate
> > > +Remove automatic page mapping from dma-buf importing
> > > +----------------------------------------------------
> > > +
> > > +When importing dma-bufs, the dma-buf and PRIME frameworks automatically map
> > > +imported pages into the importer's DMA area. This is a problem for USB devices,
> > > +which do not support DMA operations. By default, importing fails for USB
> > > +devices. USB-based drivers work around this problem by employing
> > > +drm_gem_prime_import_usb(). To fix the issue, automatic page mappings should
> > > +be removed from the buffer-sharing code.
> > > +
> > > +Contact: Thomas Zimmermann <tzimmermann@xxxxxxx>, Daniel Vetter
> > > +
> > > +Level: Advanced
> > > +
> > > +
> > > Better Testing
> > > ==============
> > > diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
> > > index 2a54f86856af..59013bb1cd4b 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 +1056,47 @@ 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().
> > > + *
> > > + * FIXME: The dma-buf framework expects to map the exported pages into
> > > + * the importer's DMA area. USB devices don't support DMA, and
> > > + * importing would fail. Foir the time being, this function provides
> > > + * a workaround by using the USB controller's DMA area. The real
> > > + * solution is to remove page-mapping operations from the dma-buf
> > > + * framework.
> > > + *
> > > + * Returns: A GEM object on success, or a pointer-encoder errno value otherwise.
> > > + */
> > > +#ifdef CONFIG_USB
> > > +struct drm_gem_object *drm_gem_prime_import_usb(struct drm_device *dev,
> > > + struct dma_buf *dma_buf)
> > > +{
> > > + struct device *dmadev;
> > > + struct drm_gem_object *obj;
> > > +
> > > + if (!dev_is_usb(dev->dev))
> > > + return ERR_PTR(-ENODEV);
> >
> > I have resisted the "dev_is_*()" type of function for USB for a long
> > time now, and I really don't want to add it now.
> >
> > The driver core explicitly was not created with RTI (run type
> > identification), but over time it has been slowly added on a per-bus
> > basis for various reasons, some good and others not good.
> >
> > In this function, why would a drm device that was NOT a usb device ever
> > call it? Because of that, I don't think dev_is_usb() is needed at all,
> > just don't call this function unless it really is a USB device.
>
> It was simply a safety measure. There's really no reason a non-USB device
> would ever call this function. So not using dev_is_usb() is fine. It'll be
> gone in v6.
Great, thanks!
Other than that, and the documentation comment I made, looks semi-sane
to me.
greg k-h
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