serial: max310x: fix IO data corruption in batched operations

After upgrading from 5.16 to 6.1, our board with a MAX14830 started
producing lots of garbage data over UART. Bisection pointed out commit
285e76fc04 as the culprit. That patch tried to replace hand-written
code which I added in 2b4bac48c1 ("serial: max310x: Use batched reads
when reasonably safe") with the generic regmap infrastructure for
batched operations.

Unfortunately, the `regmap_raw_read` and `regmap_raw_write` which were
used are actually functions which perform IO over *multiple* registers.
That's not what is needed for accessing these Tx/Rx FIFOs; the
appropriate functions are the `_noinc_` versions, not the `_raw_` ones.

Fix this regression by using `regmap_noinc_read()` and
`regmap_noinc_write()` along with the necessary `regmap_config` setup;
with this patch in place, our board communicates happily again. Since
our board uses SPI for talking to this chip, the I2C part is completely
untested.

Fixes: 285e76fc04 ("serial: max310x: use regmap methods for SPI batch operations")
Cc: stable@vger.kernel.org
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jan Kundrát <jan.kundrat@cesnet.cz>
Link: https://lore.kernel.org/r/79db8e82aadb0e174bc82b9996423c3503c8fb37.1680732084.git.jan.kundrat@cesnet.cz
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/max310x.c

@@ -525,6 +525,11 @@ static bool max310x_reg_precious(struct device *dev, unsigned int reg)
 	return false;
 }
 
+static bool max310x_reg_noinc(struct device *dev, unsigned int reg)
+{
+	return reg == MAX310X_RHR_REG;
+}
+
 static int max310x_set_baud(struct uart_port *port, int baud)
 {
 	unsigned int mode = 0, div = 0, frac = 0, c = 0, F = 0;
@@ -651,14 +656,14 @@ static void max310x_batch_write(struct uart_port *port, u8 *txbuf, unsigned int
 {
 	struct max310x_one *one = to_max310x_port(port);
 
-	regmap_raw_write(one->regmap, MAX310X_THR_REG, txbuf, len);
+	regmap_noinc_write(one->regmap, MAX310X_THR_REG, txbuf, len);
 }
 
 static void max310x_batch_read(struct uart_port *port, u8 *rxbuf, unsigned int len)
 {
 	struct max310x_one *one = to_max310x_port(port);
 
-	regmap_raw_read(one->regmap, MAX310X_RHR_REG, rxbuf, len);
+	regmap_noinc_read(one->regmap, MAX310X_RHR_REG, rxbuf, len);
 }
 
 static void max310x_handle_rx(struct uart_port *port, unsigned int rxlen)
@@ -1468,6 +1473,10 @@ static struct regmap_config regcfg = {
 	.writeable_reg = max310x_reg_writeable,
 	.volatile_reg = max310x_reg_volatile,
 	.precious_reg = max310x_reg_precious,
+	.writeable_noinc_reg = max310x_reg_noinc,
+	.readable_noinc_reg = max310x_reg_noinc,
+	.max_raw_read = MAX310X_FIFO_SIZE,
+	.max_raw_write = MAX310X_FIFO_SIZE,
 };
 
 #ifdef CONFIG_SPI_MASTER
@@ -1553,6 +1562,10 @@ static struct regmap_config regcfg_i2c = {
 	.volatile_reg = max310x_reg_volatile,
 	.precious_reg = max310x_reg_precious,
 	.max_register = MAX310X_I2C_REVID_EXTREG,
+	.writeable_noinc_reg = max310x_reg_noinc,
+	.readable_noinc_reg = max310x_reg_noinc,
+	.max_raw_read = MAX310X_FIFO_SIZE,
+	.max_raw_write = MAX310X_FIFO_SIZE,
 };
 
 static const struct max310x_if_cfg max310x_i2c_if_cfg = {