linux-kernel - Re: UV serial regression since 2.6.36

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Date:	Wed, 04 Jan 2012 19:41:47 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Jiri Slaby <jirislaby@...il.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Arnd Bergmann <arnd@...db.de>, Greg KH <gregkh@...e.de>
Subject: Re: UV serial regression since 2.6.36

On 12/06/2011 11:34 PM, Jiri Slaby wrote:
> Hi,
> 
> In short, the issue causes serial port traffic to stop after some time
> on UV machines. Only when serial is used as a console. It worked
> perfectly with 2.6.32. It's a standard 16550 on 0x3f8 (well, it's
> emulated to be at that port). Also in the PNP subsys.
> 
> To reproduce that 'debug' kernel parameter must not be used.
> 
> The root cause seems to be that the serial "chip" there has problems
> with interrupts. It generates some but doesn't indicate it's the source
> or somebody clears it (NOINT is set). And when it is supposed to
> generate one (on THRE), it does not. So there are bytes in the TX buffer
> which are never sent. Until the port is kicked e.g. by "echo h >
> /proc/sysrq-trigger".
> 
> The unhandled interrupts were always an issue, but somehow hidden. It
> became a real problem with patches post 2.6.32.
> 
> I bisected it to these two commits _together_:
> 
> commit 3f582b8c11014e4ce310d9839fb335164195333f
> Author: Arnd Bergmann <arnd@...db.de>
> Date:   Tue Jun 29 22:31:40 2010 +0200
> 
>     serial: fix termios settings in open
> 
> AND
> 
> commit 74c2107759dc6efaa1b9127014be58a742a1e7ac
> Author: Alan Cox <alan@...ux.intel.com>
> Date:   Tue Jun 1 22:53:00 2010 +0200
> 
>     serial: Use block_til_ready helper
> 
> Those reverted on the top of 2.6.37 makes it work again. Those reverted
> on the top of 3.0 plus mine 3 reverted which removed update_set_termios
> completely makes it work again.
> 
> I didn't look closely to why those patches are causing that. Like
> c_cflag copy in uart_update_termios and "tty->termios->c_cflag & CBAUD"
> test in tty_port_block_til_ready don't look good to me...

My vacation is over. I suppose you haven't had a chance to take a look
into the regression? (Just in case any of you is looking into that right
now to not duplicate the effort.)

> thanks,
-- 
js
suse labs
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