[PATCH 0/2] Stabilize serial line numbers on MediaTek Genio boards

[Carlo Caione]

On the MediaTek Genio boards only serial0 is aliased in DT, so every
other enabled UART gets a first-free ttyS line number from the 8250
core at probe time. Line numbers are handed out in probe order, and
probe order is not deterministic once a UART defers on its clocks, so
the console UART can lose ttyS0 to another port.

This was observed on the Genio 700 EVK running the Ubuntu 26.04 generic
kernel: uart0 (the debug console) deferred waiting for its clocks while
uart1 probed first and claimed ttyS0. With console=ttyS0 on the kernel
command line the console then pointed at the wrong UART, and the boot
looked like it had hung on the serial console. The failure is
intermittent and depends purely on probe timing.

Pin the line numbers explicitly instead:

 - Patch 1 aliases every enabled UART on the affected Genio boards
   (mt8365-evk, the mt8390 and mt8395 Genio commons, and the Grinn
   Genio SBC), so DT expresses the intended serial<N> -> ttyS<N>
   mapping.

 - Patch 2 makes the 8250_mtk driver read that alias with
   of_alias_get_id() and seed uart.port.line before registering the
   port, so the 8250 core reserves the requested ttyS slot instead of
   falling back to first-free.

Signed-off-by: Carlo Caione <ccaione at baylibre.com>
---
Carlo Caione (2):
      arm64: dts: mediatek: alias all enabled serial ports on Genio boards
      serial: 8250_mtk: honor DT serial aliases

 arch/arm64/boot/dts/mediatek/mt8365-evk.dts              | 2 ++
 arch/arm64/boot/dts/mediatek/mt8390-genio-common.dtsi    | 2 ++
 arch/arm64/boot/dts/mediatek/mt8390-grinn-genio-sbc.dtsi | 2 ++
 arch/arm64/boot/dts/mediatek/mt8395-genio-common.dtsi    | 1 +
 drivers/tty/serial/8250/8250_mtk.c                       | 7 ++++++-
 5 files changed, 13 insertions(+), 1 deletion(-)
---
base-commit: 0e35b9b6ec0ffcc5e23cbdec09f5c622ad532b53
change-id: 20260708-ccaione-genio-serial-aliases-e5f2c28b5ff6

Best regards,
--  
Carlo Caione <ccaione at baylibre.com>