[PATCH v2] iio: adc: ti-ads7138: replace kmalloc() with stack allocation in i2c_write_block

[Giorgi Tchankvetadze]

The ads7138_i2c_write_block() function currently utilizes kmalloc()
to allocate a buffer for I2C transfers. However, the length
parameter passed to this function is strictly 2 bytes across all
driver invocations, making the total payload buffer size exactly 4 bytes.
Invoking the heap allocator for a 4-byte buffer introduces
unnecessary SLUB overhead. 

Replace the kmalloc() call with a statically sized 4-byte stack array.
Add a boundary check returning -EINVAL to ensure future driver
modifications do not overflow the stack buffer if a length
greater than 2 is requested.
Furthermore, dropping the dynamic allocation entirely removes the need to
include <linux/slab.h> (which was the original motivation for this patch)
and streamlines the PIO transfer path.

Suggested-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
Suggested-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
Signed-off-by: Giorgi Tchankvetadze <giorgitchankvetadze1997@xxxxxxxxx>
---
 drivers/iio/adc/ti-ads7138.c | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/drivers/iio/adc/ti-ads7138.c b/drivers/iio/adc/ti-ads7138.c
index ee5c1b8e3a8e..172893cbc17d 100644
--- a/drivers/iio/adc/ti-ads7138.c
+++ b/drivers/iio/adc/ti-ads7138.c
@@ -102,21 +102,20 @@ static const int ads7138_oversampling_ratios[] = {
 static int ads7138_i2c_write_block(const struct i2c_client *client, u8 reg,
 				   u8 *values, u8 length)
 {
+	u8 buf[4];
 	int ret;
-	int len = length + 2; /* "+ 2" for OPCODE and reg */
 
-	u8 *buf __free(kfree) = kmalloc(len, GFP_KERNEL);
-	if (!buf)
-		return -ENOMEM;
+	if (length != 2)
+		return -EINVAL;
 
 	buf[0] = ADS7138_OPCODE_BLOCK_WRITE;
 	buf[1] = reg;
 	memcpy(&buf[2], values, length);
 
-	ret = i2c_master_send(client, buf, len);
+	ret = i2c_master_send(client, buf, ARRAY_SIZE(buf));
 	if (ret < 0)
 		return ret;
-	if (ret != len)
+	if (ret != ARRAY_SIZE(buf))
 		return -EIO;
 
 	return 0;
-- 
2.52.0