linux-kernel - Re: [PATCH v2 0/6] atmel_serial: Cleanups, irq handler splitup & DMA

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Message-ID: <20071219181915.71e9daeb@dhcp-252-066.norway.atmel.com>
Date:	Wed, 19 Dec 2007 18:19:15 +0100
From:	Haavard Skinnemoen <hskinnemoen@...el.com>
To:	"Remy Bohmer" <linux@...mer.net>
Cc:	"Andrew Victor" <linux@...im.org.za>,
	linux-arm-kernel@...ts.arm.linux.org.uk,
	"Russell King" <linux@....linux.org.uk>,
	linux-kernel@...r.kernel.org, kernel@...32linux.org
Subject: Re: [PATCH v2 0/6] atmel_serial: Cleanups, irq handler splitup &
 DMA

On Wed, 19 Dec 2007 17:59:09 +0100
"Remy Bohmer" <linux@...mer.net> wrote:

> > > but I noticed that I sometimes get an input overrun (ttyS0: 1
> > > input overrun(s) ) during stress conditions.
> > > This is something I did not notice before, maybe it was already there,
> > > or has something changed in this area that it is now more sensitive
> > > for this?
> > Hmm...is this with or without DMA?
> 
> DBGU is without DMA.

Right.

> > If it's without DMA, something very strange is going on -- the non-DMA
> > RX code is almost the only thing left in the hardirq handler, so I
> > would really expect overruns to be much less likely to occur now than
> > before...
> 
> As mentioned, maybe it was already there, but I did not run into it
> earlier. I have to figure that out. But, at 115200 and a 1 byte
> receive-'fifo' on DBGU, and still interrupt locks somewhere in the
> tree up to 300us, it is a simple calculation that we can run into
> overrun conditions...

preempt-rt can disable interrupts for 300 us?

If so, then I guess there's really no way to avoid a few overruns.

> Notice that without these interrupt handler splitup, it was much, much
> much worse...

Ok, that's good I guess.

> So, for me it is not a big deal, because it is just a terminal, and
> with my shaky fingers I usually do not type that fast ;-))

If you do, you just need to switch to one of the USARTs instead of the
DBGU so that you can use DMA :-)

We need to fix the break- and error handling though. But my vacation
starts tomorrow, so I probably won't be able to fix it until next year.

Haavard
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