kernel: serial: 8250_mtk: track busclk state to avoid bus error

UARTs not used as boot console are currently broken on some MediaTek
targets due to register access depending on the bus clock being enabled.
Add patch to make sure this dependency is always met.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
(cherry picked from commit 9f62abbb60b849c710cc7e40498d661827e8f852)

1 files changed, 61 insertions, 0 deletions

diff --git a/target/linux/generic/pending-5.15/860-serial-8250_mtk-track-busclk-state-to-avoid-bus-error.patch b/target/linux/generic/pending-5.15/860-serial-8250_mtk-track-busclk-state-to-avoid-bus-error.patch
new file mode 100644
index 0000000000..c5db5d9491
--- /dev/null
+++ b/target/linux/generic/pending-5.15/860-serial-8250_mtk-track-busclk-state-to-avoid-bus-error.patch
@@ -0,0 +1,61 @@
+From 629c701fc39f1ada9416e0766a86729e83bde86c Mon Sep 17 00:00:00 2001
+Message-ID: <629c701fc39f1ada9416e0766a86729e83bde86c.1694465766.git.daniel@makrotopia.org>
+From: Daniel Golle <daniel@makrotopia.org>
+Date: Mon, 11 Sep 2023 21:27:44 +0100
+Subject: [PATCH] serial: 8250_mtk: track busclk state to avoid bus error
+To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
+    Jiri Slaby <jirislaby@kernel.org>,
+    Matthias Brugger <matthias.bgg@gmail.com>,
+    AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>,
+    Daniel Golle <daniel@makrotopia.org>,
+    John Ogness <john.ogness@linutronix.de>,
+    Chen-Yu Tsai <wenst@chromium.org>,
+    Changqi Hu <changqi.hu@mediatek.com>,
+    linux-kernel@vger.kernel.org,
+    linux-serial@vger.kernel.org,
+    linux-arm-kernel@lists.infradead.org,
+    linux-mediatek@lists.infradead.org
+
+Commit e32a83c70cf9 ("serial: 8250-mtk: modify mtk uart power and
+clock management") introduced polling a debug register to make sure
+the UART is idle before disabling the bus clock. However, at least on
+some MediaTek SoCs access to that very debug register requires the bus
+clock being enabled. Hence calling the suspend function while already
+in suspended state results in that register access triggering a bus
+error. In order to avoid that, track the state of the bus clock and
+only poll the debug register if not already in suspended state.
+
+Fixes: e32a83c70cf9 ("serial: 8250-mtk: modify mtk uart power and clock management")
+Signed-off-by: Daniel Golle <daniel@makrotopia.org>
+---
+ drivers/tty/serial/8250/8250_mtk.c | 11 ++++++++++-
+ 1 file changed, 10 insertions(+), 1 deletion(-)
+
+--- a/drivers/tty/serial/8250/8250_mtk.c
++++ b/drivers/tty/serial/8250/8250_mtk.c
+@@ -32,7 +32,7 @@
+ #define MTK_UART_RXTRI_AD	0x14	/* RX Trigger address */
+ #define MTK_UART_FRACDIV_L	0x15	/* Fractional divider LSB address */
+ #define MTK_UART_FRACDIV_M	0x16	/* Fractional divider MSB address */
+-#define MTK_UART_DEBUG0	0x18
++#define MTK_UART_DEBUG0		0x18
+ #define MTK_UART_IER_XOFFI	0x20	/* Enable XOFF character interrupt */
+ #define MTK_UART_IER_RTSI	0x40	/* Enable RTS Modem status interrupt */
+ #define MTK_UART_IER_CTSI	0x80	/* Enable CTS Modem status interrupt */
+@@ -418,13 +418,12 @@ static int __maybe_unused mtk8250_runtim
+ 	struct mtk8250_data *data = dev_get_drvdata(dev);
+ 	struct uart_8250_port *up = serial8250_get_port(data->line);
+ 
+-	/* wait until UART in idle status */
+-	while
+-		(serial_in(up, MTK_UART_DEBUG0));
+-
+ 	if (data->clk_count == 0U) {
+ 		dev_dbg(dev, "%s clock count is 0\n", __func__);
+ 	} else {
++		/* wait until UART in idle status */
++		while
++			(serial_in(up, MTK_UART_DEBUG0));
+ 		clk_disable_unprepare(data->bus_clk);
+ 		data->clk_count--;
+ 	}