[PATCH v4 1/2] hwmon: (spd5118) Select page 0 unconditionally during probe

[Armin Wolf <W_Armin@gmx.de>]

Some Intel i2c controllers can be configured by the BIOS to reject
writes to the SPD device. This often causes problems when the register
page needs to be changed, usually during resume.

Avoid probing on affected devices by unconditionally selecting page 0
by writing the SPD5118_REG_I2C_LEGACY_MODE register during probe.
This will fail on affected controllers and thus prevent the driver
from probing.

Signed-off-by: Armin Wolf <W_Armin@gmx.de>
---
Changes since v3:
- return early on obviously unsupported devices

Changes since v2:
- restore original register content if i2c init fails

Changes since v1:
- Avoid zeroing reserved bits inside SPD5118_REG_I2C_LEGACY_MODE
---
 drivers/hwmon/spd5118.c | 65 +++++++++++++++++++++--------------------
 1 file changed, 34 insertions(+), 31 deletions(-)

diff --git a/drivers/hwmon/spd5118.c b/drivers/hwmon/spd5118.c
index 9724cf70b61d..f79b46085ecc 100644
--- a/drivers/hwmon/spd5118.c
+++ b/drivers/hwmon/spd5118.c
@@ -637,44 +637,47 @@ static int spd5118_i2c_init(struct i2c_client *client)
 				     I2C_FUNC_SMBUS_WORD_DATA))
 		return -ENODEV;
 
+	/* Early check to avoid obviously unsupported I2C devices */
 	regval = i2c_smbus_read_word_swapped(client, SPD5118_REG_TYPE);
-	if (regval < 0 || (regval && regval != 0x5118))
+	if (regval < 0)
+		return regval;
+
+	/*
+	 * Some SPD5118 devices report 0x0 when page 0 is not selected,
+	 * so we only fail here if the register value is not 0x0 or 0x5118.
+	 */
+	if (regval && regval != 0x5118)
 		return -ENODEV;
 
 	/*
-	 * If the device type registers return 0, it is possible that the chip
-	 * has a non-zero page selected and takes the specification literally,
+	 * We must select page 0 to ensure that we can reliably read the volatile
+	 * registers on chips that take the specification literally,
 	 * i.e. disables access to volatile registers besides the page register
 	 * if the page is not 0. The Renesas/ITD SPD5118 Hub Controller is known
-	 * to show this behavior. Try to identify such chips.
+	 * to show this behavior.
+	 *
+	 * We must also perform an unconditional register write to detect if
+	 * the i2c controller blocks write accesses to the SPD device. Some Intel
+	 * controllers might be configured by the BIOS to do this.
 	 */
-	if (!regval) {
-		/* Vendor ID registers must also be 0 */
-		regval = i2c_smbus_read_word_data(client, SPD5118_REG_VENDOR);
-		if (regval)
-			return -ENODEV;
-
-		/* The selected page in MR11 must not be 0 */
-		mode = i2c_smbus_read_byte_data(client, SPD5118_REG_I2C_LEGACY_MODE);
-		if (mode < 0 || (mode & ~SPD5118_LEGACY_MODE_MASK) ||
-		    !(mode & SPD5118_LEGACY_PAGE_MASK))
-			return -ENODEV;
-
-		err = i2c_smbus_write_byte_data(client, SPD5118_REG_I2C_LEGACY_MODE,
-						mode & SPD5118_LEGACY_MODE_ADDR);
-		if (err)
-			return -ENODEV;
-
-		/*
-		 * If the device type registers are still bad after selecting
-		 * page 0, this is not a SPD5118 device. Restore original
-		 * legacy mode register value and abort.
-		 */
-		regval = i2c_smbus_read_word_swapped(client, SPD5118_REG_TYPE);
-		if (regval != 0x5118) {
-			i2c_smbus_write_byte_data(client, SPD5118_REG_I2C_LEGACY_MODE, mode);
-			return -ENODEV;
-		}
+	mode = i2c_smbus_read_byte_data(client, SPD5118_REG_I2C_LEGACY_MODE);
+	if (mode < 0)
+		return mode;
+
+	err = i2c_smbus_write_byte_data(client, SPD5118_REG_I2C_LEGACY_MODE,
+					mode & ~SPD5118_LEGACY_PAGE_MASK);
+	if (err < 0)
+		return err;
+
+	/* We only need to access SPD5118_REG_TYPE again if regval was 0x0 */
+	if (regval == 0x5118)
+		return 0;
+
+	regval = i2c_smbus_read_word_swapped(client, SPD5118_REG_TYPE);
+	if (regval != 0x5118) {
+		/* Restore original register content */
+		i2c_smbus_write_byte_data(client, SPD5118_REG_I2C_LEGACY_MODE, mode);
+		return -ENODEV;
 	}
 
 	/* We are reasonably sure that this is really a SPD5118 hub controller */
-- 
2.39.5