[PATCH 1/2] power: supply: sbs-battery: Use a per-device serial number buffer

sbs_get_battery_serial_number() formats the battery serial number into
sbs_serial[], a single file-scope buffer shared by every sbs-battery
instance, and points val->strval at it.

Nothing restricts this driver to one instance. It binds per I2C client,
and sbs-manager registers one muxed I2C channel per supported battery
specifically so that the smart battery driver can be bound to each of
them, so several sbs-battery instances on one system is a supported
configuration.

The power supply core reads strval after the driver's get_property()
callback has returned: power_supply_show_property() fills a local
union power_supply_propval, then formats it with sysfs_emit(). Two
concurrent POWER_SUPPLY_PROP_SERIAL_NUMBER reads on different batteries
therefore race for the shared buffer - battery B's sprintf() can land
between battery A filling the buffer and the core reading it, and
battery A then reports battery B's serial number.

Move the buffer into struct sbs_info so that each battery formats into
its own storage. It is deliberately not added to the chip->strings[]
array: those entries hold the cached constant strings that
sbs_invalidate_cached_props() clears on presence changes, whereas the
serial number is re-read from its word register on every access.

Fixes: d3ab61ecbab2 ("bq20z75: Add support for more power supply properties")
Assisted-by: Claude:claude-opus-5
Signed-off-by: Babanpreet Singh <[email protected]>
---
 drivers/power/supply/sbs-battery.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/power/supply/sbs-battery.c b/drivers/power/supply/sbs-battery.c
index 017ec06be766..9bdb6c599c5f 100644
--- a/drivers/power/supply/sbs-battery.c
+++ b/drivers/power/supply/sbs-battery.c
@@ -217,6 +217,7 @@ struct sbs_info {
 	u32				flags;
 	int				technology;
 	char				strings[NR_STRING_BUFFERS][I2C_SMBUS_BLOCK_MAX + 1];
+	char				serial[5];
 };
 
 static char *sbs_get_string_buf(struct sbs_info *chip,
@@ -821,18 +822,18 @@ static int sbs_get_battery_capacity(struct i2c_client *client,
 	return 0;
 }
 
-static char sbs_serial[5];
 static int sbs_get_battery_serial_number(struct i2c_client *client,
 	union power_supply_propval *val)
 {
+	struct sbs_info *chip = i2c_get_clientdata(client);
 	int ret;
 
 	ret = sbs_read_word_data(client, sbs_data[REG_SERIAL_NUMBER].addr);
 	if (ret < 0)
 		return ret;
 
-	sprintf(sbs_serial, "%04x", ret);
-	val->strval = sbs_serial;
+	sprintf(chip->serial, "%04x", ret);
+	val->strval = chip->serial;
 
 	return 0;
 }
-- 
2.43.0