linux-kernel - Re: serial ports now asserting DTR and RTS during boot; breaks connected hardware

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Date:	Tue, 03 Jan 2012 17:00:45 +0100
From:	Jiri Slaby <jslaby@...e.cz>
To:	Dave Jones <davej@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: serial ports now asserting DTR and RTS during boot; breaks connected
 hardware

On 01/03/2012 04:37 PM, Dave Jones wrote:
> Jiri,
> We got this report from a user who notes a change in behaviour for
> his serial hardware over the last few kernel versions.

Hi!

I'm busy right now, however just after a quick look, it may be related to:
https://lkml.org/lkml/2011/12/6/573

I'll take a look later.

> https://bugzilla.redhat.com/show_bug.cgi?id=771010
> 
> This sounds like it might be related to your DTR/RTS changes back in March 2011 maybe ? 

What changes do you mean? In serial-core.c? That one is not used by USB
serials. Hence this wouldn't occur with FTDI.

>  > Description of problem:
>  > 
>  > After upgrade from FC14 to FC16, it was discovered that during kernel boot, the
>  > DTR and RTS signals of serial devices (both real UART and FTDI USB devices) are
>  > being asserted from driver load to approx 20 seconds afterward.  This causes
>  > some types of hardware connected to these ports to fail as they expect a
>  > different sequence or controlled activation of these signals.  In particular,
>  > ham radio equipment that uses these signals to key transmitters is now going
>  > into uncontrolled transmit for nearly the duration of kernel boot.
>  > 
>  > 
>  > Steps to Reproduce:
>  > 1. connect RS232 breakout box to serial port
>  > 2. boot system
>  > 3. watch DTR and RTS LEDs light as kernel is booting
>  > 
>  > Actual results:
>  > 
>  > LEDs light and stay light for almost entire boot duration indicating that DTR
>  > and RTS are asserting as the kernel boots.
>  > 
>  > Expected results:
>  > 
>  > DTR and RTS should not activate while kernel is booting.  They should only
>  > activate when the port is opened by an application and it has performed a
>  > termios function to enable these signals.
>  > 
>  > 
>  > Additional info:
>  > 
>  > The following code in the ftdi_sio driver is one suspect but this behavior
>  > happens on 8250 UARTs also so it is systemic now,
>  > 
>  > ftdi_sio.c, line 2161,
>  > 
>  >                 /* Ensure RTS and DTR are raised when baudrate changed from 0
>  > */
>  >                 if (!old_termios || (old_termios->c_cflag & CBAUD) == B0)
>  >                         set_mctrl(port, TIOCM_DTR | TIOCM_RTS);
>  > 
> 
> 

regards,
-- 
js
suse labs
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