Re: [PATCH 4/4] USB: export autosuspend delay in sysfs

Am Montag, 26. Februar 2007 18:37 schrieb Alan Stern:
> > a) The kernel will (or rather should) export its oppinion whether a device
> > should be suspended (eg. from the blacklist or set by a driver)
> 
> Be careful in your choice of words.  Do you mean "suspended" or
> "autosuspended"?  The kernel basically has no choice about whether a
> device should be allowed to suspend.  It _must_ be allowed; otherwise
> swsusp won't work.

Yes, sorry autosuspend.

> The kernel's opinion about whether a device should be autosuspended _is_
> exported through the power/autosuspend attribute.  Values > 0 mean yes (or
> >= 0 after I change it), other values mean no.

That seems almost deliberately obfuscated.
One file, one value, one meaning.
(sounds vaguely like a song by Queen)
 
> > b) The security implications are different 
> 
> In what way?  Why are you so concerned about security implications?  
> Almost all of the writable files in sysfs are writable only by the 
> superuser; this doesn't have to be any different.

Setting a delay shouldn't be limited to root.

> > > I don't understand your point.  Preventing a device from autosuspending 
> > > isn't a denial-of-service, although it might be a denial-of-power-savings.
> > 
> > _Allowing_ it is, as we have buggy devices which crash.
> 
> Yes.  That's what PAM is for, right?

Operating on files.
 
> > > > Thus a sequence of
> > > > a) read old delay
> > > > b) request 0 delay
> > > > c) restore old delay
> > > > 
> > > > should suspend the device if it is not doing IO.
> > > 
> > > Yes, except that both you and Greg have requested that step c) should 
> > > wake up the device and restart the timer using the reinstated old delay.  
> > > Of course, if you stop after step b) the effect will be to suspend the 
> > > device as soon as it is no longer in use.
> > 
> > I am complicated. I have conditional wishes.
> > I want step (c) to wake up the device only if on/off is included in that
> > value.
> > I can't speak for Greg, though.
> 
> Now you're asking to have two values included in a single sysfs write
> (on/off and delay).  That's a no-no.  :-)

Two files, two values, one in each. 

	Regards
		Oliver

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