linux-kernel - Re: [PATCH 4/5] usb_serial: Kill port mutex

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Date:	Thu, 8 Oct 2009 17:31:07 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	Oliver Neukum <oliver@...kum.org>
cc:	Alan Cox <alan@...rguk.ukuu.org.uk>, Greg KH <greg@...ah.com>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	USB list <linux-usb@...r.kernel.org>
Subject: Re: [PATCH 4/5] usb_serial: Kill port mutex

On Thu, 8 Oct 2009, Oliver Neukum wrote:

> Am Donnerstag, 8. Oktober 2009 22:06:10 schrieb Alan Stern:
> 
> >
> > On further thought, unthrottle should autoresume if the device is
> > open and autosuspended (but it shouldn't do anything if the device is
> > suspended).  After all, the reason for the autosuspend may have been
> > the lack of activity caused by the throttling.
> >
> > In practice this isn't likely to come up.  It would be surprising if
> > throttling lasted long enough to cause an autosuspend or if the core
> > decided to throttle while the device was autosuspended and hence idle.
> 
> So you say that throttle() should do an autopm_put?

The way you've coded the sierra and option drivers, it's not necessary.  
Those drivers do an autopm_get_async during submission and an
autopm_put_async after the completion of every output URB (and they
update the last_busy time in the completion of every input URB).  When
the driver is throttled no URBs will be submitted, so the usage count
will remain at 0 with no effort on the part of throttle().

For other drivers that use the simpler "autoresume on tty install,
autosuspend on tty cleanup" approach provided by usb-serial.c, the
throttle routines obviously don't need to worry about runtime PM.

Alan Stern

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