linux-kernel - Re: [PATCH 4/5] tty: serial: 8250 core: add runtime pm

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Date:	Wed, 16 Jul 2014 11:46:54 -0500
From:	Felipe Balbi <balbi@...com>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:	<balbi@...com>, <linux-omap@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>,
	Tony Lindgren <tony@...mide.com>,
	<linux-kernel@...r.kernel.org>, <linux-serial@...r.kernel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	<mika.westerberg@...ux.intel.com>
Subject: Re: [PATCH 4/5] tty: serial: 8250 core: add runtime pm

On Wed, Jul 16, 2014 at 06:40:01PM +0200, Sebastian Andrzej Siewior wrote:
> On 07/16/2014 06:06 PM, Felipe Balbi wrote:
> 
> >>> well, other than in probe and other functions which need to
> >>> make sure clocks are on, but it seems unnecessary to
> >>> enable/disable in every function.
> >> 
> >> What do you have in mind? Do you plan to let the uart on while
> >> the minicom is attached but is doing nothing? In that case,
> >> ->startup() and ->shutdown() should be enough.
> > 
> > no the idea was to keep it on for as long as it's transferring 
> > characters and idle it otherwise, if that can't be done easily,
> > then I guess your way is the only way.
> 
> But maybe we have to add some additional logic here to keep it up for
> the transfer. I've been just (maybe over)thinking: If you send 300
> bytes over DMA via 300 baud it should take 10 seconds. The PM-timeout
> could hit before the transfer is complete.

hmm, good point. So we _do_ need a way to get early and only put after
we know transfer has completed and I was assuming stop_tx() to be that
hint, but apparently it isn't.

> Same thing with hw-flowcontrol where you could get stalled for a few
> seconds.
> However it doesn't seem to be a problem in current omap-serial driver.

I don't think current omap-serial driver is a great example.

-- 
balbi

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