RE: [PATCH v2 2/2] serial: amba-pl011: keep console clock enabled for atomic writes

[John Ogness <john.ogness@linutronix.de>]

Hi Toshiyuki,

On 2026-07-27, "Toshiyuki Sato (Fujitsu)" <fj6611ie@fujitsu.com> wrote:
> Regarding Petr's comment [1] as well, I'm concerned about the
> potential impact when the clk remains enabled during periods without
> console output.

Can you elaborate on your concerns? Do you actually need such low-power
_and_ kernel logging directly on serial?

> When creating nbcon patch, I saw a similar patch [2] from the past.
> Have you considered coordinating with the clk subsystem implementation
> for this?

AFAICT there was no real justification for enabling clocks per write
other than because we can. A lot has changed since 2013 and neither
spin_locks nor raw_spin_locks are appropriate because atomic printing
can occur in _any_ context (including NMI's).

If the amba-pl011 insists on enabling clocks per write, I would
recommend not implementing the write_atomic() callback. Since I assume a
significant amount of users _will_ want atomic printing support, perhaps
you can add a Kconfig to toggle building with clock-disabling and no
atomic, or clock-always-on and atomic.

Note that there is also CON_NBCON_ATOMIC_UNSAFE available, if the driver
wants to somehow blindly enable clocks on panic in order to unsafely
dump panic logs.

John Ogness

> [1] https://lore.kernel.org/all/al4KdsU9YLOmwDoV@pathway.suse.cz/
> [2] https://lore.kernel.org/all/1359475526-17523-1-git-send-email-walimisdev@gmail.com/