[PATCH] serial: sh-sci: Fix off-by-one error in FIFO threshold register setting

[Geert Uytterhoeven <geert+renesas@glider.be>]

The Receive FIFO Data Count Trigger field (RTRG[6:0]) in the Receive
FIFO Data Count Trigger Register (HSRTRGR) of HSCIF can only hold values
ranging from 0-127.  As the FIFO size is equal to 128 on HSCIF, the user
can write an out-of-range value, touching reserved bits.

Fix this by limiting the trigger value to the FIFO size minus one.
Reverse the order of the checks, to avoid rx_trig becoming zero if the
FIFO size is one.

Note that this change has no impact on other SCIF variants, as their
maximum supported trigger value is lower than the FIFO size anyway, and
the code below takes care of enforcing these limits.

Reported-by: Linh Phung <linh.phung.jy@renesas.com>
Fixes: a380ed461f66d1b8 ("serial: sh-sci: implement FIFO threshold register setting")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
---
Compile-tested only.

The BSP contains a different patch[1], which masks the value to write by
0x7f.  This is IMHO incorrect, as it would set the trigger value to zero
when 128 is requested.

[1] "serial: sh-sci: Using mask when writing to HSRTRGR"
    https://github.com/renesas-rcar/linux-bsp/commit/9915223f41c7d680aaaed12971601dc038ce76a3
---
 drivers/tty/serial/sh-sci.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/sh-sci.c b/drivers/tty/serial/sh-sci.c
index ef37fdf37612f82f..4baf1316ea729931 100644
--- a/drivers/tty/serial/sh-sci.c
+++ b/drivers/tty/serial/sh-sci.c
@@ -1023,10 +1023,10 @@ static int scif_set_rtrg(struct uart_port *port, int rx_trig)
 {
 	unsigned int bits;
 
+	if (rx_trig >= port->fifosize)
+		rx_trig = port->fifosize - 1;
 	if (rx_trig < 1)
 		rx_trig = 1;
-	if (rx_trig >= port->fifosize)
-		rx_trig = port->fifosize;
 
 	/* HSCIF can be set to an arbitrary level. */
 	if (sci_getreg(port, HSRTRGR)->size) {
-- 
2.25.1