Re: [RFC PATCH] usb/acpi: Add support usb port power off mechanism for device fixed on the motherboard

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Hello.

On 11-05-2012 22:12, Sarah Sharp wrote:

For external ports, this should be associated with sys file control. The users
need to determine when they should be power off.

You don't mean "external", you mean "not described as unconnectable by
ACPI".

So I should work on the external ports without devices firstly and
add the sys file for user to control?

Yes, I think so.  It will be less controversial and probably simpler.
When that mechanism is ready, you should be able to use it
automatically for unconnectable ports.

One tricky thing: In theory, there should be a separate sysfs file for
each port.  That seems like a lot of overhead though; is there any way
to present the information in a single file that won't offend sysfs
purists?

Tianyu proposed having one file per hub, with a bit field that
controlled each port power.  However, I was concerned about different
userspace applications racing with each other to turn or off ports.  For
example, one app could read the bit field, attempt to power off just
port 1, but before it can write to the sysfs file, a second app powers
on port2, and the first app then writes to the sysfs file, leaving port
1 powered off, and port 2 powered off, which is not what the second app
wanted.

But if you can think of a better way to coalesce the port power off
mechanisms into one file, we're all ears. :)

How about two files? You write 1 to the bit that matcheas port you want powered on in the 'poweron' file, and vice versa, you write 1 to the bit that matches port you want powered off in the 'poweroff' file.

Sarah Sharp

WBR, Sergei
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