[PATCH v1] printk: Add CON_NO_SUPPRESS console flag and use it for ramoops

[Pasha Tatashin]

The ramoops console is designed to capture and preserve logs across warm
reboots and liveupdates. It is registered during boot and captures logs
from that point forward. However, if a 'quiet' loglevel is specified on
the kernel command line, ramoops console logs (including important
shutdown/kexec handover logs) can be filtered out and suppressed by
printk.

Specifying 'quiet' on the kernel command line is highly desirable to
avoid severe warm-reboot slowdowns, since physical serial consoles are
typically bottlenecked by slow baud rates. However, we still need to
fully capture all logs in memory via ramoops without suppression.

Introduce a generic console flag CON_NO_SUPPRESS to allow specific
consoles (like the ramoops/pstore console) to opt-out of loglevel
suppression. Check this flag safely in console_emit_next_record() and
nbcon_emit_next_record(). Finally, set this flag on registration of the
pstore console in pstore_register_console().

Signed-off-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
---
 fs/pstore/platform.c    | 2 +-
 include/linux/console.h | 3 +++
 kernel/printk/nbcon.c   | 5 ++++-
 kernel/printk/printk.c  | 4 +++-
 4 files changed, 11 insertions(+), 3 deletions(-)

diff --git a/fs/pstore/platform.c b/fs/pstore/platform.c
index 1d76c9d92056..9efbaa9895da 100644
--- a/fs/pstore/platform.c
+++ b/fs/pstore/platform.c
@@ -420,7 +420,7 @@ static void pstore_register_console(void)
 	 * Always initialize flags here since prior unregister_console()
 	 * calls may have changed settings (specifically CON_ENABLED).
 	 */
-	pstore_console.flags = CON_PRINTBUFFER | CON_ENABLED | CON_ANYTIME;
+	pstore_console.flags = CON_PRINTBUFFER | CON_ENABLED | CON_ANYTIME | CON_NO_SUPPRESS;
 	register_console(&pstore_console);
 }
 
diff --git a/include/linux/console.h b/include/linux/console.h
index 5520e4477ad7..9bd06d3fece3 100644
--- a/include/linux/console.h
+++ b/include/linux/console.h
@@ -180,6 +180,8 @@ static inline void con_debug_leave(void) { }
  *			constraints.
  * @CON_NBCON_ATOMIC_UNSAFE: The write_atomic() callback is not safe and is
  *			therefore only used by nbcon_atomic_flush_unsafe().
+ * @CON_NO_SUPPRESS:	Indicates that the console is exempt from loglevel-based
+ *			message suppression (e.g. from the 'quiet' boot argument).
  */
 enum cons_flags {
 	CON_PRINTBUFFER		= BIT(0),
@@ -192,6 +194,7 @@ enum cons_flags {
 	CON_SUSPENDED		= BIT(7),
 	CON_NBCON		= BIT(8),
 	CON_NBCON_ATOMIC_UNSAFE	= BIT(9),
+	CON_NO_SUPPRESS		= BIT(10),
 };
 
 /**
diff --git a/kernel/printk/nbcon.c b/kernel/printk/nbcon.c
index d7044a7a214b..cd0fc5f42360 100644
--- a/kernel/printk/nbcon.c
+++ b/kernel/printk/nbcon.c
@@ -989,6 +989,7 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_a
 	struct nbcon_state cur;
 	unsigned long dropped;
 	unsigned long ulseq;
+	bool may_suppress;
 
 	/*
 	 * This function should never be called for consoles that have not
@@ -1014,7 +1015,9 @@ static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt, bool use_a
 	if (!nbcon_context_enter_unsafe(ctxt))
 		return false;
 
-	ctxt->backlog = printk_get_next_message(&pmsg, ctxt->seq, is_extended, true);
+	may_suppress = !(console_srcu_read_flags(con) & CON_NO_SUPPRESS);
+	ctxt->backlog = printk_get_next_message(&pmsg, ctxt->seq, is_extended,
+						may_suppress);
 	if (!ctxt->backlog)
 		return nbcon_context_exit_unsafe(ctxt);
 
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 0323149548f6..f6502cb84160 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3131,10 +3131,12 @@ static bool console_emit_next_record(struct console *con, bool *handover, int co
 		.pbufs = &printk_shared_pbufs,
 	};
 	unsigned long flags;
+	bool may_suppress;
 
 	*handover = false;
 
-	if (!printk_get_next_message(&pmsg, con->seq, is_extended, true))
+	may_suppress = !(console_srcu_read_flags(con) & CON_NO_SUPPRESS);
+	if (!printk_get_next_message(&pmsg, con->seq, is_extended, may_suppress))
 		return false;
 
 	con->dropped += pmsg.dropped;
-- 
2.53.0