On Thu, 8 Jul 2010, Amit Nagal wrote: > Hi , > > i am facing forced disconnect of my usb composite device under the > situation described below : > > 1 ) i plugged a 4 port bus powered external usb hub into linux pc's root hub . > > 2) i plugged the usb device into one of the 4 ports of external usb hub . > > 3) i plugged another 4 port external usb hub (same vendor as for 1st > external hub ) into one of the available port > of first external usb hub . topology looks like this > > root hub <- 1st 4 port external usb hub <- usb device > > <- 2nd 4 port external usb hub > > > but most of the time when 2nd usb hub is plugged into the 1st hub , > usb device gets disconnected automatically from 1st hub , > though physically it is plugged in to the 1st external usb hub . > > > 4) under what conditions a device can automatically get disconnected > from a hub ? Basically, whenever something goes wrong. It's entirely up to the hub to decide when this should happen. > 5) is it something related to power issue ? It could very well be. Did you know that USB does not allow one bus-powered hub to be plugged into another? The combined voltage drop is too great. > 6) if a hub cannot meet the power requirements of a device attached > to its port , > will it do a disconnect to the device ? It might. In theory this shouldn't happen because both the hub and the device are supposed to tell the kernel how much current they need. But a lot of devices don't do this correctly. > 7) my dmesg logs are attached for the following situations : > > a) when usb device is plugged into first external usb hub > (dmesg_device_connect_No_2nd_hub.txt) > b) when 2nd usb hub is plugged into 1st usb hub and the device > disconnects . This is a good example. The kernel prints a warning message in the log when it sees an unpowered hub plugged into another unpowered hub. The fact that the warning isn't present in your log means that the hubs lie to the kernel: They don't tell it they are unpowered. Another way to see this is that the log doesn't include any messages about "hub controller current requirement"; these messages are skipped when the hub claims to be self-powered. You should be able to see this in the "lsusb -v" output for those hubs. 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
