linux-kernel - [PATCH v1 1/1] serial: 8250_dw: Comment possible corner cases in serial

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Message-ID: <20250317094021.1201512-1-andriy.shevchenko@linux.intel.com>
Date: Mon, 17 Mar 2025 11:40:21 +0200
From: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
	linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Cc: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>,
	Jiri Slaby <jirislaby@...nel.org>
Subject: [PATCH v1 1/1] serial: 8250_dw: Comment possible corner cases in serial_out() implementation

8250 DesignWare driver uses a few custom implementations of the serial_out().
These implementations are carefully made to avoid infinite loops. But this is
not obvious from looking at the code. Comment the possible corner cases in
the respective functions.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
---
 drivers/tty/serial/8250/8250_dw.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index af24ec25d976..49eda7f1fd0e 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -107,11 +107,23 @@ static inline int dw8250_modify_msr(struct uart_port *p, int offset, int value)
 	return value;
 }
 
+/*
+ * This function is being called as part of the uart_port::serial_out()
+ * routine. Hence, it must not call serial_port_out() or serial_out()
+ * against the modified registers here, i.e. LCR.
+ */
 static void dw8250_force_idle(struct uart_port *p)
 {
 	struct uart_8250_port *up = up_to_u8250p(p);
 	unsigned int lsr;
 
+	/*
+	 * The following call currently performs serial_out()
+	 * against the FCR register. Because it differs to LCR
+	 * there will be no infinite loop, but if it ever gets
+	 * modified, we might need a new custom version of it
+	 * that avoids infinite recursion.
+	 */
 	serial8250_clear_and_reinit_fifos(up);
 
 	/*
@@ -128,6 +140,11 @@ static void dw8250_force_idle(struct uart_port *p)
 	(void)p->serial_in(p, UART_RX);
 }
 
+/*
+ * This function is being called as part of the uart_port::serial_out()
+ * routine. Hence, it must not call serial_port_out() or serial_out()
+ * against the modified registers here, i.e. LCR.
+ */
 static void dw8250_check_lcr(struct uart_port *p, int offset, int value)
 {
 	struct dw8250_data *d = to_dw8250_data(p->private_data);
-- 
2.47.2