serial: sc16is7xx: fix invalid FIFO access with special register set

[ Upstream commit 7d3b793faaab1305994ce568b59d61927235f57b ]

When enabling access to the special register set, Receiver time-out and
RHR interrupts can happen. In this case, the IRQ handler will try to read
from the FIFO thru the RHR register at address 0x00, but address 0x00 is
mapped to DLL register, resulting in erroneous FIFO reading.

Call graph example:
    sc16is7xx_startup(): entry
    sc16is7xx_ms_proc(): entry
    sc16is7xx_set_termios(): entry
    sc16is7xx_set_baud(): DLH/DLL = $009C --> access special register set
    sc16is7xx_port_irq() entry            --> IIR is 0x0C
    sc16is7xx_handle_rx() entry
    sc16is7xx_fifo_read(): --> unable to access FIFO (RHR) because it is
                               mapped to DLL (LCR=LCR_CONF_MODE_A)
    sc16is7xx_set_baud(): exit --> Restore access to general register set

Fix the problem by claiming the efr_lock mutex when accessing the Special
register set.

Fixes: dfeae61 ("serial: sc16is7xx")
Cc: [email protected]
Signed-off-by: Hugo Villeneuve <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Greg Kroah-Hartman <[email protected]>
[ Resolve minor conflicts ]
Signed-off-by: Bin Lan <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
(cherry picked from commit dc5ead0e8fc5ef53b8553394d4aab60c277976b3)

drivers/tty/serial/sc16is7xx.c

@@ -545,6 +545,8 @@ static int sc16is7xx_set_baud(struct uart_port *port, int baud)
 			      SC16IS7XX_MCR_CLKSEL_BIT,
 			      prescaler == 1 ? 0 : SC16IS7XX_MCR_CLKSEL_BIT);
 
+	mutex_lock(&one->efr_lock);
+
 	/* Open the LCR divisors for configuration */
 	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG,
 			     SC16IS7XX_LCR_CONF_MODE_A);
@@ -558,6 +560,8 @@ static int sc16is7xx_set_baud(struct uart_port *port, int baud)
 	/* Put LCR back to the normal mode */
 	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG, lcr);
 
+	mutex_unlock(&one->efr_lock);
+
 	return DIV_ROUND_CLOSEST((clk / prescaler) / 16, div);
 }