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)
--- a/drivers/usb/core/usb.c +++ b/drivers/usb/core/usb.c @@ -748,6 +748,37 @@ void usb_put_intf(struct usb_interface *intf) } EXPORT_SYMBOL_GPL(usb_put_intf); +/**
- usb_get_dma_device - acquire a reference on the usb device's DMA endpoint
- @udev: usb device
- While a USB device cannot perform DMA operations by itself, many USB
- controllers can. A call to usb_get_dma_device() returns the DMA endpoint
- for the given USB device, if any. The returned device structure should be
- released with put_device().
- See also usb_intf_get_dma_device().
- Returns: A reference to the usb device's DMA endpoint; or NULL if none
exists.- */
+struct device *usb_get_dma_device(struct usb_device *udev) +{
- struct device *dmadev;
- if (!udev->bus)
return NULL;- dmadev = get_device(udev->bus->sysdev);
- if (!dmadev || !dmadev->dma_mask) {
put_device(dmadev);return NULL;- }
- return dmadev;
+} +EXPORT_SYMBOL_GPL(usb_get_dma_device);
There's no point making this a separate function, since it has no callers of its own. Just make usb_intf_get_dma_device the only new function.
--- a/include/linux/usb.h +++ b/include/linux/usb.h @@ -711,6 +711,7 @@ struct usb_device { unsigned use_generic_driver:1; }; #define to_usb_device(d) container_of(d, struct usb_device, dev) +#define dev_is_usb(d) ((d)->bus == &usb_bus_type) static inline struct usb_device *interface_to_usbdev(struct usb_interface *intf) { @@ -746,6 +747,29 @@ extern int usb_lock_device_for_reset(struct usb_device *udev, extern int usb_reset_device(struct usb_device *dev); extern void usb_queue_reset_device(struct usb_interface *dev); +extern struct device *usb_get_dma_device(struct usb_device *udev);
+/**
- usb_intf_get_dma_device - acquire a reference on the usb interface's DMA endpoint
- @intf: the usb interface
- While a USB device cannot perform DMA operations by itself, many USB
- controllers can. A call to usb_intf_get_dma_device() returns the DMA endpoint
- for the given USB interface, if any. The returned device structure should be
- released with put_device().
- See also usb_get_dma_device().
- Returns: A reference to the usb interface's DMA endpoint; or NULL if none
exists.- */
+static inline struct device *usb_intf_get_dma_device(struct usb_interface *intf) +{
- if (!intf)
return NULL;
Why would intf ever be NULL?
- return usb_get_dma_device(interface_to_usbdev(intf));
+}
Alan Stern