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

[Takashi Iwai <tiwai@suse.de>]

On Tue, 23 Feb 2021 11:58:42 +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;

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.


Takashi