Re: [microblaze-uclinux] early printk support for microblaze

[David McCullough <davidm@xxxxxxxxxxxx>]

Jivin John Williams lays it down ...
> Yasushi SHOJI wrote:
> >
> >if you apply the patch it produce boot log like the above. note that
> >the top part of boot log is duplicated. The reason I said was that I
> >misunderstood what CON_PRINTBUFFER is for.
> >
> >it might be better to redirect early printk's out put to debug uart if
> >the platform has one.  what do you think?
> 
> OK, i'm slowly catching up!  Sorry, I'd never heard of early printk 
> before - but a google search suggests it's a capability to use printk() 
> before the kernel calls console_init(), is that right?
> 
> What I don't understand, is why it is necessary?  Any printk() output 
> that is sent before calling console_init() gets buffered, and is output 
> eventually.
> 
> Or should pre-console_init() printk's be output immediately, in case the 
> kernel hangs/crashes before console_init()?

Early printk is (from what I have seen) a hack for debugging new
platforms that do not make it that far through the boot for
CON_PRINTBUFFER to kick in.

It's always a bit messy IMO.

What you probably want is a config option.  If EARLY_PRINTK is set,
don't set CON_PRINTBUFFER and have the two work it out so that you do
not get the output twice.

The "sh" platform has this arrangement,  even though I use "sh" every
day I still haven't used the EARLY_PRINTK support :-)  I usually call
a debug specific putc routine if I need early print outs,  because when
I need early I usually need it in head.S.  This also saves messing up
the serial driver ;-)

Cheers,
Davidm

-- 
David McCullough, davidm@snapgear.com  Ph:+61 7 34352815 http://www.SnapGear.com
Custom Embedded Solutions + Security   Fx:+61 7 38913630 http://www.uCdot.org
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