[PATCH] serial: 8250_mtk: Simplify clock sequencing and runtime PM

[Chen-Yu Tsai]

The 8250_mtk driver's runtime PM support has some issues:

- The bus clock is enabled (through runtime PM callback) later than a
  register write
- runtime PM resume callback directly called in probe, but no
  pm_runtime_set_active() call is present
- UART PM function calls the callbacks directly, _and_ calls runtime
  PM API
- runtime PM callbacks try to do reference counting, adding yet another
  count between runtime PM and clocks

This fragile setup worked in a way, but broke recently with runtime PM
support added to the serial core. The system would hang when the UART
console was probed and brought up.

Tony provided some potential fixes [1][2], though they were still a bit
complicated. The 8250_dw driver, which the 8250_mtk driver might have
been based on, has a similar structure but simpler runtime PM usage.

Simplify clock sequencing and runtime PM support in the 8250_mtk driver.
Specifically, the clock is acquired enabled and assumed to be active,
unless toggled through runtime PM suspend/resume. Reference counting is
removed and left to the runtime PM core. The serial pm function now
only calls the runtime PM API.

[1] https://lore.kernel.org/linux-serial/20230602092701.GP14287@atomide.com/
[2] https://lore.kernel.org/linux-serial/20230605061511.GW14287@atomide.com/

Fixes: 84a9582fd203 ("serial: core: Start managing serial controllers to enable runtime PM")
Suggested-by: Tony Lindgren <tony at atomide.com>
Signed-off-by: Chen-Yu Tsai <wenst at chromium.org>
---
 drivers/tty/serial/8250/8250_mtk.c | 50 ++++++------------------------
 1 file changed, 10 insertions(+), 40 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_mtk.c b/drivers/tty/serial/8250/8250_mtk.c
index aa8e98164d68..74da5676ce67 100644
--- a/drivers/tty/serial/8250/8250_mtk.c
+++ b/drivers/tty/serial/8250/8250_mtk.c
@@ -431,12 +431,7 @@ static int __maybe_unused mtk8250_runtime_suspend(struct device *dev)
 	while
 		(serial_in(up, MTK_UART_DEBUG0));
 
-	if (data->clk_count == 0U) {
-		dev_dbg(dev, "%s clock count is 0\n", __func__);
-	} else {
-		clk_disable_unprepare(data->bus_clk);
-		data->clk_count--;
-	}
+	clk_disable_unprepare(data->bus_clk);
 
 	return 0;
 }
@@ -444,19 +439,8 @@ static int __maybe_unused mtk8250_runtime_suspend(struct device *dev)
 static int __maybe_unused mtk8250_runtime_resume(struct device *dev)
 {
 	struct mtk8250_data *data = dev_get_drvdata(dev);
-	int err;
 
-	if (data->clk_count > 0U) {
-		dev_dbg(dev, "%s clock count is %d\n", __func__,
-			data->clk_count);
-	} else {
-		err = clk_prepare_enable(data->bus_clk);
-		if (err) {
-			dev_warn(dev, "Can't enable bus clock\n");
-			return err;
-		}
-		data->clk_count++;
-	}
+	clk_prepare_enable(data->bus_clk);
 
 	return 0;
 }
@@ -465,14 +449,12 @@ static void
 mtk8250_do_pm(struct uart_port *port, unsigned int state, unsigned int old)
 {
 	if (!state)
-		if (!mtk8250_runtime_resume(port->dev))
-			pm_runtime_get_sync(port->dev);
+		pm_runtime_get_sync(port->dev);
 
 	serial8250_do_pm(port, state, old);
 
 	if (state)
-		if (!pm_runtime_put_sync_suspend(port->dev))
-			mtk8250_runtime_suspend(port->dev);
+		pm_runtime_put_sync_suspend(port->dev);
 }
 
 #ifdef CONFIG_SERIAL_8250_DMA
@@ -504,7 +486,7 @@ static int mtk8250_probe_of(struct platform_device *pdev, struct uart_port *p,
 		return 0;
 	}
 
-	data->bus_clk = devm_clk_get(&pdev->dev, "bus");
+	data->bus_clk = devm_clk_get_enabled(&pdev->dev, "bus");
 	if (IS_ERR(data->bus_clk))
 		return PTR_ERR(data->bus_clk);
 
@@ -587,25 +569,16 @@ static int mtk8250_probe(struct platform_device *pdev)
 
 	platform_set_drvdata(pdev, data);
 
-	pm_runtime_enable(&pdev->dev);
-	err = mtk8250_runtime_resume(&pdev->dev);
-	if (err)
-		goto err_pm_disable;
-
 	data->line = serial8250_register_8250_port(&uart);
-	if (data->line < 0) {
-		err = data->line;
-		goto err_pm_disable;
-	}
+	if (data->line < 0)
+		return data->line;
 
 	data->rx_wakeup_irq = platform_get_irq_optional(pdev, 1);
 
-	return 0;
-
-err_pm_disable:
-	pm_runtime_disable(&pdev->dev);
+	pm_runtime_set_active(&pdev->dev);
+	pm_runtime_enable(&pdev->dev);
 
-	return err;
+	return 0;
 }
 
 static int mtk8250_remove(struct platform_device *pdev)
@@ -619,9 +592,6 @@ static int mtk8250_remove(struct platform_device *pdev)
 	pm_runtime_disable(&pdev->dev);
 	pm_runtime_put_noidle(&pdev->dev);
 
-	if (!pm_runtime_status_suspended(&pdev->dev))
-		mtk8250_runtime_suspend(&pdev->dev);
-
 	return 0;
 }
 
-- 
2.41.0.rc0.172.g3f132b7071-goog