[PATCH v5] drm: Use USB controller's DMA mask when importing dmabufs

[Noralf Trønnes]

Den 26.02.2021 10.26, skrev Thomas Zimmermann:
> 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 at suse.de>
> Fixes: 6eb0233ec2d0 ("usb: don't inherity DMA properties for USB devices")
> Tested-by: Pavel Machek <pavel at ucw.cz>
> Acked-by: Christian König <christian.koenig at amd.com>
> Acked-by: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Christoph Hellwig <hch at lst.de>
> Cc: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
> Cc: <stable at vger.kernel.org> # v5.10+
> ---

> 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

s/Foir/For/

Noralf.

> + *        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);
> +
> +	dmadev = usb_intf_get_dma_device(to_usb_interface(dev->dev));
> +	if (drm_WARN_ONCE(dev, !dmadev, "buffer sharing not supported"))
> +		return ERR_PTR(-ENODEV);
> +
> +	obj = drm_gem_prime_import_dev(dev, dma_buf, dmadev);
> +
> +	put_device(dmadev);
> +
> +	return obj;
> +}
> +EXPORT_SYMBOL(drm_gem_prime_import_usb);
> +#endif