On Wed, 27 Mar 2013, David Linares wrote: > Please correct me if I am wrong. > On my iMX25 board, I have got a 1-port root_hub which will provide > 500mA max to its unique child. You should check that against the iMX25's specs. It's possible that the root hub has a current limit lower than 500 mA. > Now that's where I am getting confused. > Its child, a 2-port hub, will then be bus-powered. But if it is, it > won't provide these 500mA shared between the 2 ports. It will only > provide 100mA per port, right? Actually, I don't know. What I said earlier was under the assumption that the 2-port hub receives all its power from the USB bus. But in theory, it's possible for the hub to receive power directly from the iMX25 board's power supply. So the answer depends on how the hardware is wired up. If the 2-port hub doesn't have an additional power source then you are right -- it is bus powered and should provide only 100 mA to each port. But if it does have an external (to the USB bus) supply then it could very well try to provide 500 mA to each port. Even if the external supply isn't strong enough to permit this. > But for me to be able to plug my wireless dongle, in the code, I guess > I will need to have "hub->mA_per_port = 500;" and therefore have the > USB_DEVICE_SELF_POWERED bit set in the hubstatus. Is that correct? > Because otherwise, my dongle will always pass the test "if > (c->desc.bMaxPower * 2 > udev->bus_mA)" if this udev->bus_mA is set to > 100mA and always end up in insufficient power. Sort of. Your emphasis is backward; this is a hardware issue, and whether or not a software test is passed won't affect the underlying problem. That test is there so that the system will avoid trying to use a device when there isn't enough power for it. Unfortunately the test doesn't always work, because many devices lie about their power sources. Also, hubs don't always limit the amount of current to their downstream ports the way they should. A bus-powered hub is supposed to get an over-current condition and turn off port power if the device attached to the port tries to draw more than 100 mA. But not all of them do this. In any case, you said there were occasional problems even without the dongle attached. In that situation, power should not be an issue. > > You never posted any usbmon results. If you could see the actual data > > sent by the hub when it's not working, you might get a better idea of > > where the problem is. > > True. Didn't get a chance to do this experiment yet. As you can read, > I am still having some difficulties understanding how it should work > in practice and who gives which power :-) > Will do that tomorrow too. Okay. 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
