[RFT PATCH] regmap-i2c: Only use I2C_FUNC_I2C if val_bits == 8

[Guenter Roeck <linux@roeck-us.net>]

On chips with a register value size of 16 bits, I2C block reads will
typically not return the expected values. Instead, returned values
are most likely undefined if an operation crosses a register boundary.

This can be observed, for example, with a TMP102 connected to an omap
i2c controller. With this chip, the initial regmap read operation to fill
the register cache looks as follows (debug log from modified kernel).

omap_i2c 48070000.i2c: MSG[0]: add=0x0048, len: 1, flags: 0x0
omap_i2c 48070000.i2c: [0] 0x00
omap_i2c 48070000.i2c: MSG[1]: add=0x0048, len: 8, flags: 0x1
omap_i2c 48070000.i2c: [0] 0x23
omap_i2c 48070000.i2c: [1] 0xa0
omap_i2c 48070000.i2c: [2] 0xff
omap_i2c 48070000.i2c: [3] 0xff
omap_i2c 48070000.i2c: [4] 0xff
omap_i2c 48070000.i2c: [5] 0xff
omap_i2c 48070000.i2c: [6] 0xff
omap_i2c 48070000.i2c: [7] 0xff

Reported-by: Nishanth Menon <nm@ti.com>
Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
---
Nishanth,

it would be great if you can test this patch, to see if it solves the problem
you had observed. Please note that https://patchwork.kernel.org/patch/9191185/
needs to be applied first (it is available in next-20160624).

 drivers/base/regmap/regmap-i2c.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/base/regmap/regmap-i2c.c b/drivers/base/regmap/regmap-i2c.c
index 4735318f4268..f319e6829816 100644
--- a/drivers/base/regmap/regmap-i2c.c
+++ b/drivers/base/regmap/regmap-i2c.c
@@ -257,7 +257,8 @@ static struct regmap_bus regmap_i2c_smbus_i2c_block = {
 static const struct regmap_bus *regmap_get_i2c_bus(struct i2c_client *i2c,
 					const struct regmap_config *config)
 {
-	if (i2c_check_functionality(i2c->adapter, I2C_FUNC_I2C))
+	if (config->val_bits == 8 &&
+	    i2c_check_functionality(i2c->adapter, I2C_FUNC_I2C))
 		return &regmap_i2c;
 	else if (config->val_bits == 8 && config->reg_bits == 8 &&
 		 i2c_check_functionality(i2c->adapter,
-- 
2.5.0