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

[Greg KH <gregkh@linuxfoundation.org>]

On Sat, May 12, 2012 at 04:47:58PM +0400, Sergei Shtylyov wrote:
> 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.

Ick, no, bit manipulation in sysfs files isn't the nicest thing at all
(especially as Sarah implied that we could have more than 64 ports for a
host, and then what, we have what, 128bit variables here?)

One file per port please, in a generic way.

greg k-h