linux-kernel - Re: [PATCH 15/15] tty: serial: 8250: omap: add dma support

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Date:	Tue, 2 Sep 2014 05:05:46 +0200
From:	Sebastian Reichel <sre@...nel.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Tony Lindgren <tony@...mide.com>, balbi@...com,
	linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	Vinod Koul <vinod.koul@...el.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Paul Walmsley <paul@...an.com>
Subject: Re: [PATCH 15/15] tty: serial: 8250: omap: add dma support

Hi,

On Mon, Sep 01, 2014 at 07:47:53PM +0200, Sebastian Andrzej Siewior wrote:
> On 08/29/2014 06:12 PM, Tony Lindgren wrote:
> > Looks like the paste bug is there for sure, doing off idle and pasting
> > 240 characters to the console can hang the UART RX after few attempts,
> > and pasting 16 charactes won't show up at all if the system is idling.
> > So you may want to play with that too a bit :)
> 
> One character wakes it up. After that you can send 16, 64 and you see
> them. Right away. No delay.
> 
> If you send "a lot" data in one-go it takes approx 152 characters until
> the first one is displayed properly at 115200,8N1. That is approx 13ms.
> Could it take that long to get up and be ready?

I noticed the same behaviour when I tested the runtime PM stuff on
my N900 with the existing serial-omap driver and I also assumed,
that the chip needs that long to get up.

> Comparing it with serial-omap I see the same thing: I takes approx the
> same amount of data until the first one is displayed. After a lot of
> "long" writes which wake the chip up from idle I manage to freeze both,
> the serial-omap driver and mine driver.
> 
> One thing that is probably a dumb idea is that printk in
> omap_8250_mdr1_errataset().
> Would it be possible that when I hit a printk in the resume path that I
> may deadlock and box will freeze?

-- Sebastian

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