Hi Alan, Thanks for the confirmative answer, since I suspect it's the bug of the hub first, but all the three hubs at my hand report the same status. I can see why the hubs are reported as self powered, e.g. if there's a device configured with max power equals to 120 mA, if the hub is self powered, the configuration will be rejected by kernel. But if it's reported as self powered, per port power limit is 500 mA, so the device may be configured and maybe work normal. But will it cause a runtime error? Say I have a embeded usb host which can supply 500 mA at most, when the hub reported as self powered and attached with two devices both with max power equals to 250 mA. Both device can be configured, but if they are both working at the most power requirement at the same time, there should be some power problem, right? Regards Rong On Tue, Mar 31, 2015 at 9:48 PM, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote: > On Tue, 31 Mar 2015, Rong Wang wrote: > >> Hi all, >> >> I'm on USB certification of a vendor SoC, and there's a case called >> "Bus powered hub power exceeded test" in the compliance plan defined >> by the usb.org. It requires an error message when a device (Max power >> descriptor >100mA) is attached to an external bus powered hub. >> >> But when I attached the bus powered hub to Linux (Ubuntu 14.04), I >> found that it always reported as self powered > > ... > >> The hub->mA_per_port is always set as full_load (500mA for usb 2.0 >> hub) since reported as self powered hub. >> So when I attach a usb device with ( 100mA < Max power descriptor <= >> 500mA), linux won't show error messages. >> >> I'm wondering why the bus powered hub reported as self powered? And is >> any switch to make it bus powered? > > The reason is simple: The hub is buggy. It tells the computer that it > is self-powered even though it isn't. > > No, there is no switch to fix it. > > Alan Stern > -- 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
