On 2012/9/24 21:18, Rafael J. Wysocki wrote:
On Saturday, September 22, 2012, Rafael J. Wysocki wrote:
On Friday, September 21, 2012, Sarah Sharp wrote:
Two weeks ago at Linux Plumbers Conference, I presented about the Intel
Lynx Point USB port power off mechanism. This email is a report out of
what was discussed, and a kick off point for further discussion.
[...]
ca9c9d0 usb : Add sysfs files to control port power.
http://git.kernel.org/?p=linux/kernel/git/gregkh/usb.git;a=commit;h=ca9c9d0
I have reviewed the whole series (I suppose there's too late for adding
Reviewed-by tags to it, but oh well ;-)) and the only patch I have some serious
doubts about is the one above. In fact, I'd prefer it to be reverted for now.
First off, if the power resources manipulated through those sysfs files
are shared between ports (i.e. there are power domains), the interface isn't
really well defined, because the advertised functionality (powering off/on a
USB port) is not there in general.
USB port power off is defined in the usb spec that when the port's
PORT_POWER feature is removed, port power should be turn off.
But most hubs don't implement it. So the feature should be general and
not common.
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.
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.
Of course, I understand the desire to make the feature actually useful to
someone, but what about making system suspend/hibernation use it to start
with? The logic there should be quite straightforward.
Thanks,
Rafael
--
Best Regards
Tianyu Lan
linux kernel enabling team
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