Re: Linux serial console patch

[Russell King <rmk+lkml@arm.linux.org.uk>]

On Mon, Sep 06, 2004 at 04:45:22PM +0100, James Courtier-Dutton wrote:
> Russell King wrote:
> > On Mon, Sep 06, 2004 at 10:32:27AM +0000, Danny ter Haar wrote:
> > 
> >>James Courtier-Dutton  <James@superbug.demon.co.uk> wrote:
> >>
> >>>>I have read your posts to lkml containing your serial console flow control
> >>>>patches firstly for 2.4.x and then for 2.6.x kernels.
> >>>
> >>>Does this fix junk being output from the serial console?
> >>>If one is using Pentium 4 HT, it seems that both CPU cores try to send 
> >>>characters to the serial port at the same time, resulting in lost 
> >>>characters as one CPU over writes the output from the other.
> >>
> >>We have multiple P4-HT enabled servers with debian installed & serial
> >>console enabled (RPB++ ;-) and _i_ have never seen this behaviour.
> > 
> > 
> > I don't think this is a serial problem as such, but a problem with the
> > kernel console subsystem (printk) itself.  Maybe James can provide an
> > example output to confirm exactly what he's seeing.
> > 
> 
> http://www.superbug.demon.co.uk/latency/
> 
> There are 2 oops traces there. At about line 176, the corruption starts.

They both look like a two printk's overlapping each other, which isn't
unreasonable since one is an oops.  We try real hard to get oopses
out, which means "busting" the printk spinlocks.  The side effect of
busting those spinlocks is of course no console locking.

It may be annoying, but unless some SMP person wants to fix the spinlock
busting to be a little more inteligent, you can expect this situation
to continue.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:  2.6 PCMCIA      - http://pcmcia.arm.linux.org.uk/
                 2.6 Serial core