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From: David Brownell <david-b@pacbell.net>
To: Marek Szyprowski <m.szyprowski@samsung.com>
Subject: Re: PROBLEM: kernel oops with g_serial USB gadget on 2.6.30
Date: Tue, 23 Jun 2009 09:52:37 -0700
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Cc: "'Alan Stern'" <stern@rowland.harvard.edu>,
       "'Peter Korsgaard'" <jacmet@sunsite.dk>,
       "'USB list'" <linux-usb@vger.kernel.org>,
       "'Kernel development list'" <linux-kernel@vger.kernel.org>,
       kyungmin.park@samsung.com
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On Tuesday 23 June 2009, Marek Szyprowski wrote:
> /* 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.

... and that has, at some point since 2.6.26 or so, become a
problem that caused oopsing.


> 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?

Change the u_serial.c code so that this newish tty behavior
stops causing problems:  don't set low_latency.

But also try and sort through any consequences of that, and
don't forget to update the comments which talk about how the
low_latency setting is affecting code flow.

- Dave
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