linux-kernel - Re: [PATCH] Use tty

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Date:	Wed, 18 Jul 2007 18:19:39 +0100 (BST)
From:	James Simmons <jsimmons@...radead.org>
To:	Paul Fulghum <paulkf@...rogate.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux console project <linuxconsole-dev@...ts.sourceforge.net>
Subject: Re: [PATCH] Use tty_schedule in VT code.


> James Simmons wrote:
> > The low_latency is used by the drivers in the case where its not in a
> > interrupt context. Well we are trusting the drivers.
> > Now if it is true what you said then tty_flip_buffer_push has
> > a bug. Looking at several drivers including serial devices
> > they set the low_latency flag.
> 
> The generic serial driver (8250) is the one that was
> dead locking when that code originally existed.
> It was setting low_latency and calling from interrupt context.

serial8250_interrupt -> serial8250_handle_port ->
receive_chars -> tty_flip_buffer_push

It could still dead lock. The low_latency flag is too weak of a test.
I patched tty_flip_buffer_push to fix this.

> > > And the initial schedule has no reason to add the extra delay.
> > 
> > So do you support a non delay work queue as well?
> 
> No, the delay work must be used for flush_to_ldisc()
> so it makes no sense to define two different work queues
> (one delayed and one not) for the same work.

Sorry I mean move to a just a non delay work queue only.
Which we won't anyways.

> I support your patch.

> The current stuff works and your patch works.
> With your patch, you actually reduce initial
> latency for processing receive data.

Thanks.
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