Re: [RFC PATCH 3/4] tty: serial: 8250 core: add runtime pm

From: Tony Lindgren
Date: Wed Jul 09 2014 - 07:48:49 EST

* Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> [140709 04:38]:
> On 07/09/2014 01:17 PM, Tony Lindgren wrote:
> > * One Thousand Gnomes <gnomes@xxxxxxxxxxxxxxxxxxx> [140707 06:22]:
> >> On Fri, 4 Jul 2014 18:34:10 +0200
> >> Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> wrote:
> >>
> >>> While comparing the OMAP-serial and the 8250 part of this I noticed that
> >>> the the latter does not use runtime-pm.
> >>
> >> Yes it does, but 8250 parts (generally - omap presumably is special
> >> here ?) need to be powered on to transmit/receive not just for register
> >> access. The core uart layer implements a "pm" operation for this.
> >
> > At least for omaps the UARTs need to be idled for the SoC to
> > hit off-idle where the SoC is powered off and rebooted during
> > idle.
> >
> > So we can certainly enable this in a generic way, however, this
> > can only be done under the following conditions:

Sorry forgot to mention why I think this can now be done in a
generic way, that's because we have now runtime PM in Linux.

> > 1. We can save and restore the serial register context and detect
> > when context was lost in the serial driver. The context loss
> > can be detected by looking at some registers that are only
> > initialized during init.
> A register check on restore context? DLL+DLH might be a good hint here
> since its value should be >0 in the running case.

OK yeah that would be a generic way to do it potentially ;)

Note that all the context_loss_cnt stuff in omap-serial.c can
then be ignored, that's still only needed for the omap3 legacy
mode booting case and won't be needed much longer.

> > 2. There's a way for the serial RX pin to wake the SoC. On some
> > omaps there's a separate pin specific wake-up interrupt that
> > can be used for that.
> That one would be handled by omap separately.

OK great.

> > 3. The serial PM features must be only enabled if requested by
> > the user via sysfs. Typically extra latency and loss of the
> > first RX character occur which is not acceptable to most
> > applications.
> Unless I'm mistaken the omap driver now initializes the timeout to -1.
> So nothing happens unless this is enabled separately.

Yes that's the only safe approach so the serial port behaves in
the standard Linux way unless specifically requested by the


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