On Wed, 12 Dec 2012, Timur wrote: > Nexus 7 and USB slave get their power via USB cable. This makes it a > little difficult to connect power, but leave the USB connection > unplugged. I'm not sure how to proceed with this. > > This is the USB Y-adapter I am using: > https://sites.google.com/site/sonicboomworld/_/rsrc/1345753009582/my-projects/otg-diagrams/Y_OTG_CABLE.png > > A photo of all components (USB cables excluded): > http://mehrvarz.github.com/img/n7-otg-power.png Ah, now I understand. > >> Is this not strong evidence the problem is with the host? > > > > Yes, it is. Can you force the host to go into deep sleep while the > > external power remains connected? > > > > Alan Stern > > I investigated this. Unfortunately, the answer seems to be no. I cannot > convince the Nexus 7 to move into deep sleep mode (LP0), when a powered > USB cable is connected to it (despite WiFi, Bt, ADB, etc. being off). Okay. Then how about this: Unplug both the power connector and the slave connector. After the N7 goes into deep sleep, plug the power connector back in but leave the slave unplugged. Then a few seconds later, plug in the slave. Also try doing the same thing, but don't wait for the N7 to go into deep sleep. If this works but the other test doesn't, then clearly the slave is working correctly and the problem lies in the host controller. > A new finding: when the N7 is coming out of deep sleep (in the > problematic scenario), I see that method hub_port_reset() is being > called. It seems to succeed. Because the following shows up in the log: > "usb 2-1: new full speed USB device number 3 using tegra-ehci". However, > this is then followed by hub_set_address() failing and "tegra-ehci > tegra-ehci.0: detected XactErr len 0/8 retry x" and "device not > accepting address 3, error -71" being logged. > > I ran this modified test: while the N7 was in deep sleep, I unplugged > (stole) the USB slave. Then I plugged external power and this time > hub_port_reset() did not succeed and no entry "usb 2-1: new full speed > USB device number 3 using tegra-ehci" was logged. I think this shows > that, after LP0, a connected USB slave device _is_ still detectable. The > problem strikes a moment later, when the host tries to set the address. > > Can I try anything in code, to "reset" the slave, so it will accept an > address? The hub driver already does this, in hub_port_reset(). It wouldn't be surprising if the deep-sleep code in the ehci-tegra driver has some bugs. I don't know how much it has been tested. 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
