linux-kernel - Re: [linux-usb-devel] error to be returned while suspended

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Date:	Fri, 6 Oct 2006 09:21:51 +0200
From:	Oliver Neukum <oliver@...kum.org>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org,
	linux-usb-devel@...ts.sourceforge.net
Subject: Re: [linux-usb-devel] error to be returned while suspended

Am Donnerstag, 5. Oktober 2006 23:45 schrieb Alan Stern:
> On Thu, 5 Oct 2006, Oliver Neukum wrote:
> 
> > I have a few observations, but no solution either:
> > - if root tells a device to suspend, it shall do so
> 
> Probably everyone will agree on that.

But should it stay suspended until explictely resumed? Do we have
consensus on that?

> > - the issues of manual & automatic suspend and remote wakeup are orthogonal
> 
> Except for the fact that remote wakeup kicks in only when a device is 
> suspended.

Yes.
 
> > - there should be a common API for all devices
> 
> It would be nice, wouldn't it?  But we _already_ have several vastly
> different power-management APIs.  Consider for example DPMI and IDE 
> spindown.

No reason to make matters worse.
 
> > - there's no direct connection between power save and open()
> 
> Why shouldn't a device always be put into a power-saving mode whenever it 
> isn't open?  Agreed, you might want to reduce its power usage at times 
> even when it is open...

That and you are putting the latency/power choice into kernel space.
I've seen GPS recievers that need 30 seconds to get a fix. Autosuspend
needs to be in kernel space. But that doesn't mean that it is sufficient
as a mechanism nor that it doesn't need parameters supplied from
user space.

> > The question when a device is in use is far from trivial.
> 
> Yes.  It has to be decided by each individual driver.  For simple 
> character-oriented devices, "open" is a good first start.

Yes. However, simple character devices are the first candidates for
libusb so kernel space is left with the hard cases.

	Regards
		Oliver
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