Re: [PATCH] serial: amba-pl011: touch the watchdogs in write_atomic()

[John Ogness]

On 2026-11-04, Breno Leitao <leitao@xxxxxxxxxx> wrote:
> In emergency context, such as WARN(), nbcon prints the pending records
> through write_atomic() with preemption disabled.
>
> On arm64, WARN()  can run with IRQs masked, so the whole backlog is
> printed with IRQs off as well.
>
> Commit 9bd18e1262c0 ("printk/nbcon: Restore IRQ in atomic flush after
> each emitted record") does not help here, as it only restores the
> caller's IRQ state between records.
>
> pl011_console_write_atomic() busy-waits on the UART for every
> character, and at 115200 baud it prints about 11.5KB per second.
> Flushing a few hundred kilobytes outlasts the hard lockup threshold
> (10 seconds by default), so the printing CPU is reported, or the
> machine panics with hardlockup_panic set. See [1].
>
> The 8250 driver avoids this by calling touch_nmi_watchdog() while it
> polls the transmitter. Do the same in pl011_console_write_atomic(),
> once per record.
>
> Suggested-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/all/87wlqtxb14.fsf@xxxxxxxxxxxxxxxxxxxxx/ [1]
> Signed-off-by: Breno Leitao <leitao@xxxxxxxxxx>

Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx>