[PATCH 2/3] serial: 8250: dispatch SysRq character in serial8250_handle_irq()

[Jacques Nilo]

serial8250_handle_irq() captures a SysRq character into port->sysrq_ch
inside serial8250_handle_irq_locked() via uart_prepare_sysrq_char()
(reached from serial8250_read_char()). Dispatch of that captured
character to handle_sysrq() is expected to happen at port-unlock time,
through uart_unlock_and_check_sysrq[_irqrestore]().

After commit 8324a54f604d ("serial: 8250: Add
serial8250_handle_irq_locked()") the function was reduced to a wrapper
that takes the port lock via guard(uart_port_lock_irqsave) whose
destructor is plain uart_port_unlock_irqrestore(). The sysrq-aware
unlock helper is no longer called, so port->sysrq_ch is captured but
never dispatched: BREAK + SysRq key is consumed silently.

This was the very condition Johan Hovold's 853a9ae29e978 ("serial:
8250: fix handle_irq locking", 2021) introduced
uart_unlock_and_check_sysrq_irqrestore() to address.

Switch to the new guard(uart_port_lock_sysrq_irqsave), whose destructor
is the sysrq-aware unlock helper, restoring the pre-split behaviour.
Update the Context: comment on serial8250_handle_irq_locked() so future
HW-specific 8250 wrappers know to use the same guard or the explicit
sysrq-aware unlock.

Verified on RTL8196E with CONFIG_MAGIC_SYSRQ_SERIAL=y: BREAK + 'h' on
the console UART produces the SysRq help dump in dmesg and the brk
counter in /proc/tty/driver/serial increments correctly.

Fixes: 8324a54f604d ("serial: 8250: Add serial8250_handle_irq_locked()")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jacques Nilo <jnilo@xxxxxxx>
---
 drivers/tty/serial/8250/8250_port.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index e4e6a53eb..64f3487e8 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -1786,7 +1786,10 @@ static bool handle_rx_dma(struct uart_8250_port *up, unsigned int iir)
 }
 
 /*
- * Context: port's lock must be held by the caller.
+ * Context: port's lock must be held by the caller. The caller must
+ * release it via guard(uart_port_lock_sysrq_irqsave) or
+ * uart_unlock_and_check_sysrq_irqrestore(), which captures SysRq
+ * character on unlock.
  */
 void serial8250_handle_irq_locked(struct uart_port *port, unsigned int iir)
 {
@@ -1839,7 +1842,7 @@ int serial8250_handle_irq(struct uart_port *port, unsigned int iir)
 	if (iir & UART_IIR_NO_INT)
 		return 0;
 
-	guard(uart_port_lock_irqsave)(port);
+	guard(uart_port_lock_sysrq_irqsave)(port);
 	serial8250_handle_irq_locked(port, iir);
 
 	return 1;
-- 
2.43.0