[PATCH printk v2 2/2] printk/nbcon: Change nbcon_irq_work to IRQ_WORK_LAZY

[John Ogness]

For some platforms it is a problem to queue irq_work when entering
cpuidle states. Since nbcon uses irq_work for waking the printing
kthreads, any printk() calls when entering cpuidle states can lead
to the affected hardware hanging. Tegra20 and Tegra30 are examples
of such platforms. Avoiding raising the irq_work IRQ has shown to
circumvent the problem.

Change the nbcon_irq_work to be IRQ_WORK_LAZY, thus not raising
an IRQ upon irq_work queuing. The irq_work is then handled on the
next interrupt (worst case, kernel tick). This additional delay is
acceptable because nbcon_irq_work is only responsible for
non-emergency deferred printing, which is delayed anyway. This also
has the benefit of not needing to raise an IRQ for each printk()
call.

Link: https://lore.kernel.org/lkml/f3757a75-0ba1-4558-bf57-f19ab7e59a4c@xxxxxxxxxx
Fixes: 76f258bf3f2a ("printk: nbcon: Introduce printer kthreads")
Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
---
 kernel/printk/nbcon.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index 218b1922a58de..e5aede99304ed 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -1782,7 +1782,7 @@ bool nbcon_alloc(struct console *con)
 	}
 
 	rcuwait_init(&con->rcuwait);
-	init_irq_work(&con->irq_work, nbcon_irq_work);
+	con->irq_work = IRQ_WORK_INIT_LAZY(nbcon_irq_work);
 	atomic_long_set(&ACCESS_PRIVATE(con, nbcon_prev_seq), -1UL);
 	nbcon_state_set(con, &state);
 
-- 
2.47.3