usb: typec: fsa4480: Check if the chip is really there

Currently, the driver will happily register the switch/mux devices, and
so long as the i2c master doesn't complain, the user would never know
there's something wrong.

Add a device id check (based on [1]) and return -ENODEV if the read
fails or returns nonsense.

Checking the value on a Qualcomm SM6115P-based Lenovo Tab P11 tablet,
the ID mentioned in the datasheet does indeed show up:
 fsa4480 1-0042: Found FSA4480 v1.1 (Vendor ID = 0)

[1] https://www.onsemi.com/pdf/datasheet/fsa4480-d.pdf

Fixes: 1dc2463 ("usb: typec: mux: Add On Semi fsa4480 driver")
Cc: stable <stable@kernel.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Konrad Dybcio <konrad.dybcio@linaro.org>
Link: https://lore.kernel.org/r/20240729-topic-fs4480_check-v3-1-f5bf732d3424@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/typec/mux/fsa4480.c

@@ -13,6 +13,10 @@
 #include <linux/usb/typec_dp.h>
 #include <linux/usb/typec_mux.h>
 
+#define FSA4480_DEVICE_ID	0x00
+ #define FSA4480_DEVICE_ID_VENDOR_ID	GENMASK(7, 6)
+ #define FSA4480_DEVICE_ID_VERSION_ID	GENMASK(5, 3)
+ #define FSA4480_DEVICE_ID_REV_ID	GENMASK(2, 0)
 #define FSA4480_SWITCH_ENABLE	0x04
 #define FSA4480_SWITCH_SELECT	0x05
 #define FSA4480_SWITCH_STATUS1	0x07
@@ -251,6 +255,7 @@ static int fsa4480_probe(struct i2c_client *client)
 	struct typec_switch_desc sw_desc = { };
 	struct typec_mux_desc mux_desc = { };
 	struct fsa4480 *fsa;
+	int val = 0;
 	int ret;
 
 	fsa = devm_kzalloc(dev, sizeof(*fsa), GFP_KERNEL);
@@ -268,6 +273,15 @@ static int fsa4480_probe(struct i2c_client *client)
 	if (IS_ERR(fsa->regmap))
 		return dev_err_probe(dev, PTR_ERR(fsa->regmap), "failed to initialize regmap\n");
 
+	ret = regmap_read(fsa->regmap, FSA4480_DEVICE_ID, &val);
+	if (ret || !val)
+		return dev_err_probe(dev, -ENODEV, "FSA4480 not found\n");
+
+	dev_dbg(dev, "Found FSA4480 v%lu.%lu (Vendor ID = %lu)\n",
+		FIELD_GET(FSA4480_DEVICE_ID_VERSION_ID, val),
+		FIELD_GET(FSA4480_DEVICE_ID_REV_ID, val),
+		FIELD_GET(FSA4480_DEVICE_ID_VENDOR_ID, val));
+
 	/* Safe mode */
 	fsa->cur_enable = FSA4480_ENABLE_DEVICE | FSA4480_ENABLE_USB;
 	fsa->mode = TYPEC_STATE_SAFE;