[v2 PATCH 1/1] gpio: exar: access MPIO registers on slave chips

[Qingtao Cao <qingtao.cao.au@xxxxxxxxx>]

When EXAR xr17v35x chips are cascaded in order to access the MPIO registers
(part of the Device Configuration Registers) of the slave chips, an offset
needs to be applied based on the number of master chip's UART channels.

Signed-off-by: Qingtao Cao <qingtao.cao@xxxxxxxx>
---
 drivers/gpio/gpio-exar.c | 40 +++++++++++++++++++++++++++++++++++-----
 1 file changed, 35 insertions(+), 5 deletions(-)

diff --git a/drivers/gpio/gpio-exar.c b/drivers/gpio/gpio-exar.c
index d37de78247a6..d2ed2897fabb 100644
--- a/drivers/gpio/gpio-exar.c
+++ b/drivers/gpio/gpio-exar.c
@@ -21,6 +21,12 @@
 #define EXAR_OFFSET_MPIOLVL_HI 0x96
 #define EXAR_OFFSET_MPIOSEL_HI 0x99
 
+/*
+ * The Device Configuration and UART Configuration Registers
+ * for each UART channel take 1KB of memory address space.
+ */
+#define EXAR_UART_CHANNEL_SIZE 0x400
+
 #define DRIVER_NAME "gpio_exar"
 
 static DEFINE_IDA(ida_index);
@@ -31,26 +37,39 @@ struct exar_gpio_chip {
 	int index;
 	char name[20];
 	unsigned int first_pin;
+	/*
+	 * The offset to the slave device's (if existing)
+	 * Device Configuration Registers.
+	 */
+	unsigned int slave_offset;
 };
 
 static unsigned int
 exar_offset_to_sel_addr(struct exar_gpio_chip *exar_gpio, unsigned int offset)
 {
-	return (offset + exar_gpio->first_pin) / 8 ? EXAR_OFFSET_MPIOSEL_HI
-						   : EXAR_OFFSET_MPIOSEL_LO;
+	unsigned int pin = exar_gpio->first_pin + (offset % 16);
+	unsigned int slave = offset / 16;
+	unsigned int addr = pin / 8 ? EXAR_OFFSET_MPIOSEL_HI : EXAR_OFFSET_MPIOSEL_LO;
+
+	return addr + (slave ? exar_gpio->slave_offset : 0);
 }
 
 static unsigned int
 exar_offset_to_lvl_addr(struct exar_gpio_chip *exar_gpio, unsigned int offset)
 {
-	return (offset + exar_gpio->first_pin) / 8 ? EXAR_OFFSET_MPIOLVL_HI
-						   : EXAR_OFFSET_MPIOLVL_LO;
+	unsigned int pin = exar_gpio->first_pin + (offset % 16);
+	unsigned int slave = offset / 16;
+	unsigned int addr = pin / 8 ? EXAR_OFFSET_MPIOLVL_HI : EXAR_OFFSET_MPIOLVL_LO;
+
+	return addr + (slave ? exar_gpio->slave_offset : 0);
 }
 
 static unsigned int
 exar_offset_to_bit(struct exar_gpio_chip *exar_gpio, unsigned int offset)
 {
-	return (offset + exar_gpio->first_pin) % 8;
+	unsigned int pin = exar_gpio->first_pin + (offset % 16);
+
+	return pin % 8;
 }
 
 static int exar_get_direction(struct gpio_chip *chip, unsigned int offset)
@@ -153,6 +172,17 @@ static int gpio_exar_probe(struct platform_device *pdev)
 	if (!exar_gpio)
 		return -ENOMEM;
 
+	/*
+	 * If cascaded, xr17v354 or xr17v358 slaves have the same amount
+	 * of MPIOs as their masters and the last 4 bits of the master's
+	 * PCI Device ID is the number of its UART channels.
+	 */
+	if (pcidev->device & GENMASK(15, 12)) {
+		ngpios += ngpios;
+		exar_gpio->slave_offset = (pcidev->device & GENMASK(3, 0)) *
+				EXAR_UART_CHANNEL_SIZE;
+	}
+
 	/*
 	 * We don't need to check the return values of mmio regmap operations (unless
 	 * the regmap has a clock attached which is not the case here).
-- 
2.34.1