Re: [PATCH printk v1 00/10] printk: introduce atomic consoles and sync mode

On Tue, Aug 03, 2021 at 03:18:51PM +0206, John Ogness wrote:

Hi,

This is the next part of our printk-rework effort (points 3 and
4 of the LPC 2019 summary [0]).

Here the concept of "atomic consoles" is introduced through  a
new (optional) write_atomic() callback for console drivers. This
callback must be implemented as an NMI-safe variant of the
write() callback, meaning that it can function from any context
without relying on questionable tactics such as ignoring locking
and also without relying on the synchronization of console
semaphore.

As an example of how such an atomic console can look like, this
series implements write_atomic() for the 8250 UART driver.

This series also introduces a new console printing mode called
"sync mode" that is only activated when the kernel is about to
end (such as panic, oops, shutdown, reboot). Sync mode can only
be activated if atomic consoles are available. A system without
registered atomic consoles will be unaffected by this series.

When in sync mode, the console printing behavior becomes:

- only consoles implementing write_atomic() will be called

- printing occurs within vprintk_store() instead of
  console_unlock(), since the console semaphore is irrelevant
  for atomic consoles

For systems that have registered atomic consoles, this series
improves the reliability of seeing crash messages by using new
locking techniques rather than "ignoring locks and hoping for
the best". In particular, atomic consoles rely on the
CPU-reentrant spinlock (i.e. the printk cpulock) for
synchronizing console output.

If console is runtime suspended, who will bring it up?
Does it mean that this callback can't be implemented on the consoles that
do runtime suspend (some of 8250 currently, for example)?

-- 
With Best Regards,
Andy Shevchenko



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