On Wednesday 18 July 2012 12:40:38 Alan Stern wrote:
> Oliver, you seem to be arguing both sides of this discussion. You
But there are more than two sides in this discussion.
> point out the the power-off operation is too dangerous in general for
> the kernel to do it, and now you say that it's too racy for userspace
> to do it.
It is too dangerous in general. Therefore it may be safe in particular.
> Are we to infer that you don't want it to be done at all?
No, now that I think about it an attribute for the drivers is necessary.
Like drivers have "supports_autosuspend" they also should have
"supports_power_off". In addition it is necessary for ports to have
an attribute in sysfs which allows user space to block power off.
And it is a bit complicated. Power may be cut, if
a) a port is internal and unpluggable, or
b) a port is internal and it's interfaces' drivers set "supports_power_off",
unless:
1) remote wakeup is requested
2) user space has blocked it via the new sysfs attribute
3) USB_QUIRK_RESET_MORPHS is set
> What about things like the ubiquitous USB webcams one finds in laptops
> these days? I practically never use the one in my laptop, so turning
> off its bus power seems like a good idea. Even if that means I have to
> turn the power on explicitly before using the webcam.
That would work. Btusb wouldn't work.
Regards
Oliver
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