Re: USB not processing APM suspend event properly?

From: Russell King (
Date: Wed Dec 12 2001 - 18:49:16 EST

On Wed, Dec 12, 2001 at 06:40:26PM -0500, Thomas Hood wrote:
> I suppose that the reason why it is presently coded as
> it is is that the drivers can veto the suspend whereas
> the "listeners" can't, and the apm driver wants to tell
> the listeners about the event only if it isn't vetoed.
> With your change, the apm driver really should ignore
> a veto from the drivers since it has already told the
> listeners about the event. Therefore, while your patch
> is an improvement, it still isn't the whole solution.

I think it does perform acceptable behaviour however - if the drivers
(in the unlikely event) refuse the event after we've told user space,
we tell user space we resumed.

> One possible solution is for the apm driver first to _ask_
> the device drivers whether a suspend is allowed; then
> to tell the listeners; then to tell the device drivers
> to suspend.

What if the condition of the suspend gets changed by the act of paging
in memory, or some other change that occured in user space? It's a
chicken and egg problem. 8(

> Actually, it's not true that the original code sends PM_SUSPEND
> each time a listener on /dev/apm_bios replies. The code
> fragment is:
> if (as->suspends_read > 0) {
> as->suspends_read--;
> as->suspends_pending--;
> suspends_pending--;
> } else if (!send_event(APM_USER_SUSPEND))
> return -EAGAIN;
> else
> queue_event(APM_USER_SUSPEND, as);
> This only calls send_event() if the ioctl(suspend) is NOT a
> reply to a notification that was earlier read from the queue.

It is a minor point, but an unwanted one that just obfuscates the operation
of the APM system - only the first PM SUSPEND event causes a change.

Sending 'n' PM_SUSPEND events because you have 'n' listeners on
/dev/apm_bios seems to be a waste of time when it could be handled
more cleanly.

Russell King (                The developer of ARM Linux

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