linux-kernel - [PATCH v2 15/15] serial: sc16is7xx: add comments for lock requirements

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Message-Id: <20251002145738.3250272-16-hugo@hugovil.com>
Date: Thu,  2 Oct 2025 10:57:38 -0400
From: Hugo Villeneuve <hugo@...ovil.com>
To: gregkh@...uxfoundation.org,
	jirislaby@...nel.org,
	fvallee@...rea.fr
Cc: linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org,
	hugo@...ovil.com,
	Hugo Villeneuve <hvilleneuve@...onoff.com>
Subject: [PATCH v2 15/15] serial: sc16is7xx: add comments for lock requirements

From: Hugo Villeneuve <hvilleneuve@...onoff.com>

Indicate why lock needs to be asserted when accessing
MSR register, as this is not immediately obvious when looking at this
register in the device datasheet.

Signed-off-by: Hugo Villeneuve <hvilleneuve@...onoff.com>
---
 drivers/tty/serial/sc16is7xx.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index 4898b4235d0da..1fd64a47341d8 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -499,10 +499,10 @@ EXPORT_SYMBOL_GPL(sc16is762_devtype);
 static bool sc16is7xx_regmap_volatile(struct device *dev, unsigned int reg)
 {
 	switch (reg) {
-	case SC16IS7XX_RHR_REG:
-	case SC16IS7XX_IIR_REG:
-	case SC16IS7XX_LSR_REG:
-	case SC16IS7XX_MSR_REG:
+	case SC16IS7XX_RHR_REG: /* Shared address space with THR & DLL */
+	case SC16IS7XX_IIR_REG: /* Shared address space with FCR & EFR */
+	case SC16IS7XX_LSR_REG: /* Shared address space with XON2 */
+	case SC16IS7XX_MSR_REG: /* Shared address space with TCR & XOFF1 */
 	case SC16IS7XX_SPR_REG: /* Shared address space with TLR & XOFF2 */
 	case SC16IS7XX_TXLVL_REG:
 	case SC16IS7XX_RXLVL_REG:
@@ -711,6 +711,7 @@ static void sc16is7xx_update_mlines(struct sc16is7xx_one *one)
 	unsigned long flags;
 	unsigned int status, changed;
 
+	/* Lock required as MSR address is shared with TCR and XOFF1. */
 	lockdep_assert_held_once(&one->lock);
 
 	status = sc16is7xx_get_hwmctrl(port);
-- 
2.39.5