On Tue, Sep 25, 2012 at 12:09:06AM +0200, Rafael J. Wysocki wrote: > On Monday, September 24, 2012, Alan Stern wrote: > > On Mon, 24 Sep 2012, Rafael J. Wysocki wrote: > > > > > Second, I'm not sure if there's any way for user space to figure out what > > > > > ports are connected to what sockets visible to user space. If there is such > > > > > a way, I wonder what it is, if not, the proposed interface is just plain > > > > > dangerous. > > > > ACPI _PLD method provides a lot of information to describe where the > > > > port located in. But currently it is not exposed to user space. > > > > > > Well, precisely. Which means that the user would have to apply trial-and-error > > > to figure out which sysfs file corresponds to which physical port on his/her > > > machine. > > > > > > That doesn't sound really user friendly. > > > > It doesn't have to be trial-and-error. We should add symlinks between > > the sysfs directory for a USB device and the directory for the port it > > is plugged into. > > > > In fact, Tianyu, that would be a good patch to do next. > > Well, in my opinion it's rather requisite for adding the direct port power > control interface. > > What about hubs connected to such ports? I don't think they're going to > work when power is removed from it, are they? USB hubs would have remote wakeup enabled, so we would never power off their port with the "auto" policy. > > > > > Finally, it follows from my experience that interfaces of this kind often > > > > > are sources of pain and grief for distro support folks who need to handle > > > > > problems reported by users. I used to do that and I know what kind of pain > > > > > that is. So, in my opinion it's better not to expose low-level functionality > > > > > to user space directly, like in this case. > > > > > > > > You mean force power on and power off? There is a demand that if a usb > > > > device was abnormal, user space driver or app could make it rework via > > > > power off. > > > > > > Well, then implement it as a "hard reset" attribute on the device. Namely, > > > if the device is attached to a power-manageable port, writing 1 (for example) > > > to its "hard reset" attribute will cause the port to be power-cycled (as > > > long as the port has its own power resource, that is). > > > > A few people want to use their USB ports as digital output signals. > > For this purpose they want to be able to control the port power > > directly. However, this is relatively rare. > > Out of curiosity, does it apply to empty ports or ports with devices connected > to them? There are things like USB lights or fans that pull port power, but don't actually enumerate as a USB device. So to the OS, those would be "empty" ports. But they would be external ports, so the default "auto" policy would never power off external ports. The user could write a small script to PWM the fan on and off to control the speed. It would just be a novelty though, as Alan said. Sarah Sharp -- 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
