On Fri, Aug 19, 2016 at 04:25:40PM -0500, Bin Liu wrote: > Hi, > > On Mon, Aug 15, 2016 at 09:21:25PM +0200, Hans de Goede wrote: > > Hi All, > > > > Here is a patch series which implements run-time changing the dr-mode > > of sunxi musb controllers through the (already existing) musb "mode" > > sysfs attribute. > > > > This is useful on boards where there is no id pin, e.g. some tv-boxes > > use the musb controller to get an extra usb A port without needing > > a hub chip. Except for the missing id pin when using a usb A<->A cable > > these ports can do peripheral mode just fine. This series makes it > > possible to do e.g. this by doing echo "peripheral" > mode before > > plugging in the usb A<->A cable. > > Well, this is an illegal usecase. A-A cable is invalid by USB Spec. TV boxes with the Sunchip CX-A99 board inside are an example of such devices and they ship with a cable that has a type-A plug at both ends, precisely to allow you to connect the board to a USB host. It is standard practice to do so with that board [1][2] and I've had mine connected that way for the last few months while working on mainline kernel support. > With type-A receptacle the controller should be in host-only mode, > switching to peripheral mode should not be allowed. [...] > This is not a problem. With a type-A connector, the dual-role controller > should work in host-only mode. > > Role switching should only be allowed if an AB connector is used. The Openmoko Neo 1973 and Neo Freerunner have just a Mini-B connector but the s3c24xx USB port is switchable between host mode and peripheral mode.[3] Should that also be disallowed then? > Using the sysfs entry to switch roles for generic purpose is really a > bad idea, it opens up ton of problems. Openmoko kernels have since November 2008 had the funtionality to switch roles of the USB port using a sysfs attribute. Can you be more specific about the "ton of problems"? I have one of those devices as well as a CX-A99, so I'd like to know what I've overlooked. [1] http://forum.tronsmart.com/forum/tronsmart-draco-aw80/211-instructions-for-firmware-updating-via-pc#642 [2] https://linux-sunxi.org/FEL [3] http://wiki.openmoko.org/wiki/Specialized_USB_cables -- Rask Ingemann Lambertsen -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
