linux-kernel - RE: PROBLEM: kernel oops with g

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Date:	Tue, 23 Jun 2009 10:37:23 +0200
From:	Marek Szyprowski <m.szyprowski@...sung.com>
To:	'David Brownell' <david-b@...bell.net>
Cc:	'Alan Stern' <stern@...land.harvard.edu>,
	'Peter Korsgaard' <jacmet@...site.dk>,
	'USB list' <linux-usb@...r.kernel.org>,
	'Kernel development list' <linux-kernel@...r.kernel.org>,
	kyungmin.park@...sung.com,
	'Marek Szyprowski' <m.szyprowski@...sung.com>
Subject: RE: PROBLEM: kernel oops with g_serial USB gadget on 2.6.30

Hello, 

On Tuesday, June 23, 2009 9:23 AM, David Brownell wrote:

> > > Like, oh, changing a spinlock to a mutex.   You might change the
> > > low_latency setting and review how that's now supposed to behave.
> >
> > In in gs_open() function in drivers/usb/gadget/u_serial.c, after the
> > line 780 I've added:
> >
> > tty->low_latency = 1;
> 
> That's already how it's set.  Try instead *REMOVING* the line
> which sets that flag.

My fault. I didn't notice that a few lines lower the low_latency flag is
alredy set and I thought I should set it. After removing that, g_serial
started working. :) Thanks a lot.

I noticed that there is also a comment:
/* low_latency means ldiscs work in tasklet context, without
 * needing a workqueue schedule ... easier to keep up.
 */

So in low latency mode calls are made from a tasklet. This is not true in
my case, as the S3C OTG UDC driver does all its job in interrupts. This
way also a (usb) packet_done callback is done from an interrupt. I expect
that g_serial driver interacts with tty layer in that packet_done callback,
so this is the source of the problems. I noticed that some other UDC
drivers also does all its job from an interrupt, so they also might be
affected. How this bug should be properly resolved?

Best regards
--
Marek Szyprowski
Samsung Poland R&D Center

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