regulator: ti-abb: Fix array out of bound read access on the first transition

At the start of driver initialization, we do not know what bias
setting the bootloader has configured the system for and we only know
for certain the very first time we do a transition.

However, since the initial value of the comparison index is -EINVAL,
this negative value results in an array out of bound access on the
very first transition.

Since we don't know what the setting is, we just set the bias
configuration as there is nothing to compare against. This prevents
the array out of bound access.

NOTE: Even though we could use a more relaxed check of "< 0" the only
valid values(ignoring cosmic ray induced bitflips) are -EINVAL, 0+.

Fixes: 40b1936 ("regulator: Introduce TI Adaptive Body Bias(ABB) on-chip LDO driver")
Link: https://lore.kernel.org/linux-mm/CA+G9fYuk4imvhyCN7D7T6PMDH6oNp6HDCRiTUKMQ6QXXjBa4ag@mail.gmail.com/
Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20201118145009.10492-1-nm@ti.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: nmenon

drivers/regulator/ti-abb-regulator.c

@@ -342,15 +342,25 @@ static int ti_abb_set_voltage_sel(struct regulator_dev *rdev, unsigned sel)
 		return ret;
 	}
 
-	/* If data is exactly the same, then just update index, no change */
 	info = &abb->info[sel];
+	/*
+	 * When Linux kernel is starting up, we are'nt sure of the
+	 * Bias configuration that bootloader has configured.
+	 * So, we get to know the actual setting the first time
+	 * we are asked to transition.
+	 */
+	if (abb->current_info_idx == -EINVAL)
+		goto just_set_abb;
+
+	/* If data is exactly the same, then just update index, no change */
 	oinfo = &abb->info[abb->current_info_idx];
 	if (!memcmp(info, oinfo, sizeof(*info))) {
 		dev_dbg(dev, "%s: Same data new idx=%d, old idx=%d\n", __func__,
 			sel, abb->current_info_idx);
 		goto out;
 	}
 
+just_set_abb:
 	ret = ti_abb_set_opp(rdev, abb, info);
 
 out: