Switching USB DWC3 controller from host to device mode - usb

Question

I need to use an embedded Linux platform as a USB device in order to stream audio and video from a smartphone. The platform has a USB A receptacle and doesn't support OTG (USB_ID pin is not connected on the host controller).
Now I try to switch from host to device mode using DWC3 controller and the debugfs interface. Therefore I activated DWC3 controller in the kernel configuration and set it to "Dual Role Mode". After mounting the file system I checked the current mode in /sys/kernel/debug/xxxxxxxx.usb3/mode with cat mode and got host as expected. But unfortunately I can't write device to the mode file. After entering the command echo device > mode it remains host and does not change. Does anyone know what could be causing it?

Answer 1

I know that this used to work out of the box, but with the kernel version I'm using in an embedded platform (5.4.0) it doesn't. In order to make it work I had to patch around a bug in the kernel:
diff --git a/drivers/usb/dwc3/core.c b/drivers/usb/dwc3/core.c
index bf1a7a9da..a78990664 100644
--- a/drivers/usb/dwc3/core.c
+++ b/drivers/usb/dwc3/core.c
## -110,9 +110,6 ## static void __dwc3_set_mode(struct work_struct *work)
unsigned long flags;
int ret;
- if (dwc->dr_mode != USB_DR_MODE_OTG)
- return;
-
if (dwc->current_dr_role == DWC3_GCTL_PRTCAP_OTG)
dwc3_otg_update(dwc, 0);
## -192,6 +189,12 ## void dwc3_set_mode(struct dwc3 *dwc, u32 mode)
dwc->desired_dr_role = mode;
spin_unlock_irqrestore(&dwc->lock, flags);
+ if (dwc->dr_mode != USB_DR_MODE_OTG)
+ {
+ __dwc3_set_mode(&dwc->drd_work);
+ return;
+ }
+
queue_work(system_freezable_wq, &dwc->drd_work);
}
After applying the above patch, rebooting to the new kernel, and running:
echo "host" | sudo tee /sys/kernel/debug/*.usb3/mode
I was able to switch modes like I expect (note: I had *.dwc3 instead of *.usb3 in the above command).