[PATCH 6.1.y 2/3] serial: sc16is7xx: add comments for lock requirements

[Sasha Levin <sashal@kernel.org>]

From: Hugo Villeneuve <hvilleneuve@dimonoff.com>

[ Upstream commit 42405cb77fd35b5acc0e5a11d74eef68c2afa484 ]

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@dimonoff.com>
Link: https://patch.msgid.link/20251027142957.1032073-15-hugo@hugovil.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Stable-dep-of: 1c41d09dcd07 ("serial: sc16is7xx: declare MCR and IER as volatile registers")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 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 ccac831bc69e2..c55cad61ff0bd 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -530,10 +530,10 @@ static const struct sc16is7xx_devtype 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:
@@ -768,6 +768,7 @@ 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);
-- 
2.53.0