sysrq on serial console

sysrq on serial console

[Dave Hansen]

Since the serial cleanups happened in 2.5, Magic Sysrq doesn't work 
for me on the serial console.

It looks like the UART_LSR_BI bit needs to be set in the status 
variable for the break character to be interpreted as a break in the 
driver.

I doubt that it is actually broken,  but it isn't immediately obvious 
how that bit gets set.  Is there something that I should have set when 
the device was initialized to make sure that UART_LSR_BI is asserted 
in "status" when the interrupt occurs?

-- 
Dave Hansen
haveblue@us.ibm.com

Re: sysrq on serial console

[H. Peter Anvin]

Followup to:  <3D94ED88.5040407@us.ibm.com>
By author:    Dave Hansen <haveblue@us.ibm.com>
In newsgroup: linux.dev.kernel
>
> Since the serial cleanups happened in 2.5, Magic Sysrq doesn't work 
> for me on the serial console.
> 

It's been broken for me in several 2.4 versions as well.  This is
highly troublesome.

> It looks like the UART_LSR_BI bit needs to be set in the status 
> variable for the break character to be interpreted as a break in the 
> driver.
> 
> I doubt that it is actually broken,  but it isn't immediately obvious 
> how that bit gets set.  Is there something that I should have set when 
> the device was initialized to make sure that UART_LSR_BI is asserted 
> in "status" when the interrupt occurs?
-- 
<hpa@transmeta.com> at work, <hpa@zytor.com> in private!
"Unix gives you enough rope to shoot yourself in the foot."
http://www.zytor.com/~hpa/puzzle.txt	<amsp@zytor.com>

Re: sysrq on serial console

[Russell King]

On Fri, Sep 27, 2002 at 04:45:12PM -0700, Dave Hansen wrote:
> It looks like the UART_LSR_BI bit needs to be set in the status 
> variable for the break character to be interpreted as a break in the 
> driver.

That is correct; that is how the UART reports a break character.

> I doubt that it is actually broken,  but it isn't immediately obvious 
> how that bit gets set.  Is there something that I should have set when 
> the device was initialized to make sure that UART_LSR_BI is asserted 
> in "status" when the interrupt occurs?

Now.  Its a status bit from the UART LSR register itself, read from
serial8250_handle_port() and receive_chars().  It will cause an
interrupt any time that the receive interrupt is enabled, ie when
the port is open by user space.

-- 
Russell King (rmk@arm.linux.org.uk)                The developer of ARM Linux
             http://www.arm.linux.org.uk/personal/aboutme.html