[PATCH 1/2] serial: amba-pl011: unprepare console clock on unregister

[Karl Mehltretter <kmehltretter@gmail.com>]

pl011_console_setup() calls clk_prepare() on the UART clock, but the
console provides no matching teardown, so the clock is never unprepared
when the console is unregistered -- via the sysfs "console" attribute or
a driver unbind. Each re-registration prepares the clock again, leaking
one prepare reference per cycle.

Even where preparing the clock has no hardware effect, the stale
reference leaves the clock framework's prepare count unbalanced. On
providers whose prepare operation enables hardware, the leaked reference
may also keep the UART clock running after unregister.

Add a console .exit() callback that clk_unprepare()s the clock,
balancing the clk_prepare() in pl011_console_setup().

Fixes: 4b4851c65d92 ("clk: amba-pl011: convert to clk_prepare()/clk_unprepare()")
Assisted-by: Claude:claude-fable-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
---
The i.MX console had the same missing teardown, fixed in 9768a37cec37
("serial: imx: disable console clocks on unregister").

Measured on QEMU raspi (bcm2835), whose UART clock is a real gateable
CPRMAN clock: recycling the console via /sys/class/tty/ttyAMA0/console,
the uart clock's prepare_count (/sys/kernel/debug/clk/uart/
clk_prepare_count) climbs by one per cycle -- 2 -> 8 over six cycles --
without this patch, and stays at 2 with it.

 drivers/tty/serial/amba-pl011.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/tty/serial/amba-pl011.c b/drivers/tty/serial/amba-pl011.c
index 8ed91e1da22b..1aa43994a3cd 100644
--- a/drivers/tty/serial/amba-pl011.c
+++ b/drivers/tty/serial/amba-pl011.c
@@ -2552,6 +2552,15 @@ static int pl011_console_setup(struct console *co, char *options)
 	return uart_set_options(&uap->port, co, baud, parity, bits, flow);
 }
 
+static int pl011_console_exit(struct console *co)
+{
+	struct uart_amba_port *uap = amba_ports[co->index];
+
+	clk_unprepare(uap->clk);
+
+	return 0;
+}
+
 /**
  *	pl011_console_match - non-standard console matching
  *	@co:	  registering console
@@ -2705,6 +2714,7 @@ static struct console amba_console = {
 	.name		= "ttyAMA",
 	.device		= uart_console_device,
 	.setup		= pl011_console_setup,
+	.exit		= pl011_console_exit,
 	.match		= pl011_console_match,
 	.write_atomic	= pl011_console_write_atomic,
 	.write_thread	= pl011_console_write_thread,
-- 
2.53.0