On Thu, 2015-10-22 at 11:16 +0100, David Howells wrote: > printk() currently discards earlier messages to make space for new messages > arriving. This has the distinct downside that if the kernel starts > churning out messages because of some initial incident, the report of the > initial incident is likely to be lost under a blizzard of: > > ** NNN printk messages dropped ** > > messages from console_unlock(). > > The first message generated (typically an oops) is usually the most > important - the one you want to solve first - so we really want to see > that.
But wait... didn't I watch you muttering on IRC about the actual bug
you were trying to catch here... and didn't you have a *serial* console
hooked up?
What broke such that serial console stopped giving you *every* message?
Serial console was always *synchronous*.
We could do stuff like...
printk("Going to do foo...\n");
outb(foo, baz);
printk("Did foo and the machine didn't catch fire! Now bar\n");
outb(bar, baz);
printk("Done\n");
And with a serial console I could know *precisely* the point at which
the machine locked up.
And I could enable the silly debugging levels on things like JFFS2 and
be sure that with a serial console I could catch *every* printk
reliably — which led to a number of cases where people would reproduce
a bug with a serial console and debugging, mail me a huge log file, and
get a patch back in reply.
We *need* to have a mode where serial console is actually *reliable*,
and we can know that the message has been sent out the port before the
printk() call returns.
What happened to it? And how do we fix it?
--
dwmw2
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