The MCP251xFD has a dedicated transceiver standby control function on
the INT0/GPIO0/XSTBY pin, controlled by the XSTBYEN bit in IOCON.
When enabled, the hardware automatically manages the transceiver
standby state: the pin is driven low when the controller is active
and high when it enters Sleep mode.

Enable this feature when the 'microchip,xstbyen' device tree property
is present.

Signed-off-by: Viken Dadhaniya <viken.dadhaniya@oss.qualcomm.com>
---
v2 -> v3:

- Configure xstbyen pin before bringing the controller into normal mode.
- Add a check in mcp251xfd_gpio_request() to ensure that GPIO0 cannot be
  used when xstbyen is enabled.

v2 Link: https://lore.kernel.org/all/20260316131950.859748-3-viken.dadhaniya@oss.qualcomm.com/
---
 .../net/can/spi/mcp251xfd/mcp251xfd-core.c    | 37 +++++++++++++++++++
 drivers/net/can/spi/mcp251xfd/mcp251xfd.h     |  1 +
 2 files changed, 38 insertions(+)

diff --git a/drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c b/drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c
index 9c86df08c2c5..92a86083c896 100644
--- a/drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c
+++ b/drivers/net/can/spi/mcp251xfd/mcp251xfd-core.c
@@ -764,6 +764,31 @@ static void mcp251xfd_chip_stop(struct mcp251xfd_priv *priv,
 	mcp251xfd_chip_set_mode(priv, MCP251XFD_REG_CON_MODE_CONFIG);
 }
 
+static int mcp251xfd_chip_xstbyen_enable(const struct mcp251xfd_priv *priv)
+{
+	/* Configure the INT0/GPIO0/XSTBY pin as transceiver standby control:
+	 *
+	 * - XSTBYEN=1: route the pin to the transceiver standby function
+	 * - TRIS0=0:   set output direction; the reset default is 1 (input),
+	 *              which leaves the pin floating HIGH and keeps the
+	 *              transceiver in standby regardless of XSTBYEN
+	 * - LAT0=0:    drive pin LOW => transceiver active (not in standby)
+	 *
+	 * All three bits are included in the mask; only XSTBYEN is set in
+	 * val, so TRIS0 and LAT0 are cleared to 0 atomically.
+	 *
+	 * Pin behaviour by mode:
+	 * - Config mode: controlled by LAT0 (LAT0=0 => LOW => active)
+	 * - Normal mode: hardware drives pin LOW (active)
+	 * - Sleep mode:  hardware drives pin HIGH (standby)
+	 */
+	return regmap_update_bits(priv->map_reg, MCP251XFD_REG_IOCON,
+				  MCP251XFD_REG_IOCON_XSTBYEN |
+				  MCP251XFD_REG_IOCON_TRIS0 |
+				  MCP251XFD_REG_IOCON_LAT0,
+				  MCP251XFD_REG_IOCON_XSTBYEN);
+}
+
 static int mcp251xfd_chip_start(struct mcp251xfd_priv *priv)
 {
 	int err;
@@ -796,6 +821,12 @@ static int mcp251xfd_chip_start(struct mcp251xfd_priv *priv)
 
 	priv->can.state = CAN_STATE_ERROR_ACTIVE;
 
+	if (priv->xstbyen) {
+		err = mcp251xfd_chip_xstbyen_enable(priv);
+		if (err)
+			goto out_chip_stop;
+	}
+
 	err = mcp251xfd_chip_set_normal_mode(priv);
 	if (err)
 		goto out_chip_stop;
@@ -1805,6 +1836,11 @@ static int mcp251xfd_gpio_request(struct gpio_chip *chip, unsigned int offset)
 	u32 pin_mask = MCP251XFD_REG_IOCON_PM(offset);
 	int ret;
 
+	if (priv->xstbyen && offset == 0) {
+		netdev_err(priv->ndev, "Can't use GPIO 0 with XSTBYEN!\n");
+		return -EINVAL;
+	}
+
 	if (priv->rx_int && offset == 1) {
 		netdev_err(priv->ndev, "Can't use GPIO 1 with RX-INT!\n");
 		return -EINVAL;
@@ -2271,6 +2307,7 @@ static int mcp251xfd_probe(struct spi_device *spi)
 	priv->pll_enable = pll_enable;
 	priv->reg_vdd = reg_vdd;
 	priv->reg_xceiver = reg_xceiver;
+	priv->xstbyen = device_property_present(&spi->dev, "microchip,xstbyen");
 	priv->devtype_data = *(struct mcp251xfd_devtype_data *)spi_get_device_match_data(spi);
 
 	/* Errata Reference:
diff --git a/drivers/net/can/spi/mcp251xfd/mcp251xfd.h b/drivers/net/can/spi/mcp251xfd/mcp251xfd.h
index 085d7101e595..d3f4704e2678 100644
--- a/drivers/net/can/spi/mcp251xfd/mcp251xfd.h
+++ b/drivers/net/can/spi/mcp251xfd/mcp251xfd.h
@@ -672,6 +672,7 @@ struct mcp251xfd_priv {
 	struct gpio_desc *rx_int;
 	struct clk *clk;
 	bool pll_enable;
+	bool xstbyen;
 	struct regulator *reg_vdd;
 	struct regulator *reg_xceiver;
 
-- 
2.34.1