On 2013/3/29 6:43, Sarah Sharp wrote:
On Thu, Mar 28, 2013 at 05:00:23PM -0400, Alan Stern wrote:
On Thu, 28 Mar 2013, Sarah Sharp wrote:
On Thu, Mar 28, 2013 at 01:11:02AM +0800, Lan Tianyu wrote:
Some devices' firmware will be broken at some points. Power down
and power on device can help device to rework in this case.
This patch is to add ioctl cmd USBDEVFS_POWER_RESET for usbfs node
to repower usb device. First, call hub_port_logical_disconnect() to
disconnect device. Second, Power down and up usb port.
I don't think this is the right approach. We want to be able to power
off a port, even if no devices are attached. One of the use case
scenarios we discussed was allowing distros to turn off empty USB ports
according to some policy (e.g. screen blank, lost bluetooth connection
to their phone that indicates the user walked away from the computer,
server admin wants to save power, etc).
With the current patches in 3.9, I don't see a way we can do that. The
ports have power/control, which can only be set to 'on' or 'auto'. You
should add an 'off' option to that file.
Won't empty ports naturally be powered off by runtime PM, so long as
there isn't a PM_QOS constraint to prevent it? The user (or system
software) may need to write "auto" to the port's power/control file,
which may require us to put the ports on the usb_bus_type or to make
them class devices. But however we do it, this should work
automatically.
Now I'm a bit confused and I'll have to look at the code again. I can't
remember if Tianyu made the kernel add a PM_QOS constraint to set the no
power off flag for hot pluggable ports, or if we expect userspace to use
the connect type sysfs file to figure out it shouldn't write auto to the
file.
Actually, I exposed pm qos flags for usb port via dev_pm_qos_expose_flags().
It creates power/pm_qos_no_power_off under usb port sysfs directory.
User can echo 0 > pm_qos_no_power_off to power off the empty port.
If the kernel isn't adding that constraint, then Tianyu *really* needs
to document that.
"power/pm_qos_no_power_off" is device's power attribute and it has been
described in the sysfs-devices-power. I think I should mention in the
usb/power-management.text.
Sarah Sharp
--
Best Regards
Tianyu Lan
linux kernel enabling team
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