linux-kernel - Re: [PATCH] atmel_serial: update the powersave handler to match serial core

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Date:	Sat, 20 Sep 2008 15:01:56 +0200
From:	Haavard Skinnemoen <haavard.skinnemoen@...el.com>
To:	Anti Sullin <anti.sullin@...ecdesign.ee>
Cc:	Michael Trimarchi <trimarchimichael@...oo.it>,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	Andrew Victor <linux@...im.org.za>
Subject: Re: [PATCH] atmel_serial: update the powersave handler to match
 serial core

Anti Sullin <anti.sullin@...ecdesign.ee> wrote:
> Haavard Skinnemoen wrote:
> > I don't think the gpio layer is supposed to touch the portmux. David
> > has always been very clear about that. But if we somehow manage to get
> > the pin configured as a GPIO, we can use the GPIO layer to request a
> > pin change interrupt.
> >
> > It _might_ work even if you don't reconfigure the pin as a GPIO...but
> > then I think we'd be relying on undocumented behaviour.
> >   
> I believe that you don't need to reconfigure the pin. The gpio hw does
> not require the pin to be multiplexed to any given state, so does not request_irq (correct?).
> I did this trick in my [2/3] MCI driver patch I sent to arm-linux-kernel at 18.03.2008 (the
> e-mail subject line was wrong, [1/3], though) and I'm using that patch still on my
> production devices to avoid some rare but critical SD data corruption (mainline kernel still 
> screws up the filesystem on SD sometimes). The same should be easily usable on serial port, too.

You're right. It works, and it's documented. I don't think it's
guaranteed by the GPIO API, but as long as it's guaranteed on AT91 and
AVR32, we should be fine.

> So in many applications we could not use this. But this might still come handy
> in a lot of cases we can poll and find out what caused the data on the serial
> port etc. Or on applications, where this loss of data does not matter (like debug
> console where the resume is usable even if it does not wake up on the first byte).

Yes, as long as you have some sort of timeout/retry mechanism in place,
it might be useful.

Haavard
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