Re: PROBLEM: kernel oops with g_serial USB gadget on 2.6.30

[David Brownell <david-b@pacbell.net>]

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