[PATCH 6.6.y 3/4] serial: sc16is7xx: refactor EFR lock

27 Oct 2025

From: Hugo Villeneuve hvilleneuve@dimonoff.com
[ Upstream commit 0c84bea0cabc4e2b98a3de88eeb4ff798931f056 ]
Move common code for EFR lock/unlock of mutex into functions for code reuse
and clarity.
With the addition of old_lcr, move irda_mode within struct sc16is7xx_one to
reduce memory usage:
    Before: /* size: 752, cachelines: 12, members: 10 */
    After:  /* size: 744, cachelines: 12, members: 10 */
Signed-off-by: Hugo Villeneuve hvilleneuve@dimonoff.com
Link: https://lore.kernel.org/r/20231221231823.2327894-17-hugo@hugovil.com
Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
Stable-dep-of: 1c05bf6c0262 ("serial: sc16is7xx: remove useless enable of enhanced features")
Signed-off-by: Sasha Levin sashal@kernel.org
---
 drivers/tty/serial/sc16is7xx.c | 106 ++++++++++++++++++---------------
 1 file changed, 57 insertions(+), 49 deletions(-)

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index 3e4bdc62ae0bb..1fafe3257270a 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -329,8 +329,9 @@ struct sc16is7xx_one {
    struct kthread_work		reg_work;
    struct kthread_delayed_work	ms_work;
    struct sc16is7xx_one_config	config;
-	bool				irda_mode;
    unsigned int			old_mctrl;
+	u8				old_lcr; /* Value before EFR access. */
+	bool				irda_mode;
 };
struct sc16is7xx_port {
@@ -412,6 +413,49 @@ static void sc16is7xx_power(struct uart_port *port, int on)
    		      on ? 0 : SC16IS7XX_IER_SLEEP_BIT);
 }
+/*
+ * In an amazing feat of design, the Enhanced Features Register (EFR)
+ * shares the address of the Interrupt Identification Register (IIR).
+ * Access to EFR is switched on by writing a magic value (0xbf) to the
+ * Line Control Register (LCR). Any interrupt firing during this time will
+ * see the EFR where it expects the IIR to be, leading to
+ * "Unexpected interrupt" messages.
+ *
+ * Prevent this possibility by claiming a mutex while accessing the EFR,
+ * and claiming the same mutex from within the interrupt handler. This is
+ * similar to disabling the interrupt, but that doesn't work because the
+ * bulk of the interrupt processing is run as a workqueue job in thread
+ * context.
+ */
+static void sc16is7xx_efr_lock(struct uart_port *port)
+{
+	struct sc16is7xx_one *one = to_sc16is7xx_one(port, port);
+
+	mutex_lock(&one->efr_lock);
+
+	/* Backup content of LCR. */
+	one->old_lcr = sc16is7xx_port_read(port, SC16IS7XX_LCR_REG);
+
+	/* Enable access to Enhanced register set */
+	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG, SC16IS7XX_LCR_CONF_MODE_B);
+
+	/* Disable cache updates when writing to EFR registers */
+	regcache_cache_bypass(one->regmap, true);
+}
+
+static void sc16is7xx_efr_unlock(struct uart_port *port)
+{
+	struct sc16is7xx_one *one = to_sc16is7xx_one(port, port);
+
+	/* Re-enable cache updates when writing to normal registers */
+	regcache_cache_bypass(one->regmap, false);
+
+	/* Restore original content of LCR */
+	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG, one->old_lcr);
+
+	mutex_unlock(&one->efr_lock);
+}
+
 static void sc16is7xx_ier_clear(struct uart_port *port, u8 bit)
 {
    struct sc16is7xx_port *s = dev_get_drvdata(port->dev);
@@ -538,39 +582,12 @@ static int sc16is7xx_set_baud(struct uart_port *port, int baud)
    	div /= prescaler;
    }
-	/* In an amazing feat of design, the Enhanced Features Register shares
-	 * the address of the Interrupt Identification Register, and is
-	 * switched in by writing a magic value (0xbf) to the Line Control
-	 * Register. Any interrupt firing during this time will see the EFR
-	 * where it expects the IIR to be, leading to "Unexpected interrupt"
-	 * messages.
-	 *
-	 * Prevent this possibility by claiming a mutex while accessing the
-	 * EFR, and claiming the same mutex from within the interrupt handler.
-	 * This is similar to disabling the interrupt, but that doesn't work
-	 * because the bulk of the interrupt processing is run as a workqueue
-	 * job in thread context.
-	 */
-	mutex_lock(&one->efr_lock);
-
-	lcr = sc16is7xx_port_read(port, SC16IS7XX_LCR_REG);
-
-	/* Open the LCR divisors for configuration */
-	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG,
-			     SC16IS7XX_LCR_CONF_MODE_B);
-
    /* Enable enhanced features */
-	regcache_cache_bypass(one->regmap, true);
+	sc16is7xx_efr_lock(port);
    sc16is7xx_port_update(port, SC16IS7XX_EFR_REG,
    		      SC16IS7XX_EFR_ENABLE_BIT,
    		      SC16IS7XX_EFR_ENABLE_BIT);
-
-	regcache_cache_bypass(one->regmap, false);
-
-	/* Put LCR back to the normal mode */
-	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG, lcr);
-
-	mutex_unlock(&one->efr_lock);
+	sc16is7xx_efr_unlock(port);
/* If bit MCR_CLKSEL is set, the divide by 4 prescaler is activated. */
    sc16is7xx_port_update(port, SC16IS7XX_MCR_REG,
@@ -579,7 +596,8 @@ static int sc16is7xx_set_baud(struct uart_port *port, int baud)
mutex_lock(&one->efr_lock);
-	/* Open the LCR divisors for configuration */
+	/* Backup LCR and access special register set (DLL/DLH) */
+	lcr = sc16is7xx_port_read(port, SC16IS7XX_LCR_REG);
    sc16is7xx_port_write(port, SC16IS7XX_LCR_REG,
    		     SC16IS7XX_LCR_CONF_MODE_A);
@@ -589,7 +607,7 @@ static int sc16is7xx_set_baud(struct uart_port *port, int baud)
    sc16is7xx_port_write(port, SC16IS7XX_DLL_REG, div % 256);
    regcache_cache_bypass(one->regmap, false);
-	/* Put LCR back to the normal mode */
+	/* Restore LCR and access to general register set */
    sc16is7xx_port_write(port, SC16IS7XX_LCR_REG, lcr);
mutex_unlock(&one->efr_lock);
@@ -1070,17 +1088,7 @@ static void sc16is7xx_set_termios(struct uart_port *port,
    if (!(termios->c_cflag & CREAD))
    	port->ignore_status_mask |= SC16IS7XX_LSR_BRK_ERROR_MASK;
-	/* As above, claim the mutex while accessing the EFR. */
-	mutex_lock(&one->efr_lock);
-
-	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG,
-			     SC16IS7XX_LCR_CONF_MODE_B);
-
    /* Configure flow control */
-	regcache_cache_bypass(one->regmap, true);
-	sc16is7xx_port_write(port, SC16IS7XX_XON1_REG, termios->c_cc[VSTART]);
-	sc16is7xx_port_write(port, SC16IS7XX_XOFF1_REG, termios->c_cc[VSTOP]);
-
    port->status &= ~(UPSTAT_AUTOCTS | UPSTAT_AUTORTS);
    if (termios->c_cflag & CRTSCTS) {
    	flow |= SC16IS7XX_EFR_AUTOCTS_BIT |
@@ -1092,16 +1100,16 @@ static void sc16is7xx_set_termios(struct uart_port *port,
    if (termios->c_iflag & IXOFF)
    	flow |= SC16IS7XX_EFR_SWFLOW1_BIT;
-	sc16is7xx_port_update(port,
-			      SC16IS7XX_EFR_REG,
-			      SC16IS7XX_EFR_FLOWCTRL_BITS,
-			      flow);
-	regcache_cache_bypass(one->regmap, false);
-
    /* Update LCR register */
    sc16is7xx_port_write(port, SC16IS7XX_LCR_REG, lcr);
-	mutex_unlock(&one->efr_lock);
+	/* Update EFR registers */
+	sc16is7xx_efr_lock(port);
+	sc16is7xx_port_write(port, SC16IS7XX_XON1_REG, termios->c_cc[VSTART]);
+	sc16is7xx_port_write(port, SC16IS7XX_XOFF1_REG, termios->c_cc[VSTOP]);
+	sc16is7xx_port_update(port, SC16IS7XX_EFR_REG,
+			      SC16IS7XX_EFR_FLOWCTRL_BITS, flow);
+	sc16is7xx_efr_unlock(port);
/* Get baud rate generator configuration */
    baud = uart_get_baud_rate(port, termios, old,
-- 
2.51.0



    

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[PATCH 6.6.y 3/4] serial: sc16is7xx: refactor EFR lock