linux-kernel - Re: [linux-pm][PATCH] base: Change power/wakeup output from "" to "unsupported" if wakeup feature isn't supported by a device

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Date:	Mon, 7 Jan 2008 02:57:58 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	yi.y.yang@...el.com
Cc:	David Brownell <david-b@...bell.net>, pavel@....cz,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [linux-pm][PATCH] base: Change power/wakeup output from "" to  "unsupported" if wakeup feature isn't supported by a device

On Monday, 7 of January 2008, Yi Yang wrote:
> On Fri, 2008-01-04 at 08:09 -0800, David Brownell wrote:
> > > > This patch changes empty output to "unsupported" in order that a user knows
> > > > wakeup feature isn't supported by this device when he/she
> > > > 'cat /sys/devices/.../power/wakeup', please consider to apply,
> > > > thanks.
> > >
> > > What about simply removing "wakuep" file if wakeup is not supported?
> > 
> > It may not *stay* unsupported, so I think changing it in either
> > of those permanent ways would be confusing/misleading.
> > 
> > For example, USB devices have multiple states ... minimally, an
> > unconfigured state, and a configured state.  Some have multiple
> > configurations.  Only configured states can be wakeup-capable.
> > So a given device might have three states, but support wakeup
> > except in one of them...
> If so, we can change "unsupported" to "unconfigurable" or "inoperable"
> which can cover the states "unconfigured", "unsupported" and other
> unknown states. :-).

The main problem with your patch is that is changes a documented behavior
visible by the user space.  That should be done very cautiously.

Thanks,
Rafael
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