media: dvb-frontends: avoid stack overflow warnings with clang

A previous patch worked around a KASAN issue in stv0367, now a similar
problem showed up with clang:

drivers/media/dvb-frontends/stv0367.c:1222:12: error: stack frame size (3624) exceeds limit (2048) in 'stv0367ter_set_frontend' [-Werror,-Wframe-larger-than]
 1214 | static int stv0367ter_set_frontend(struct dvb_frontend *fe)

Rework the stv0367_writereg() function to be simpler and mark both
register access functions as noinline_for_stack so the temporary
i2c_msg structures do not get duplicated on the stack when KASAN_STACK
is enabled.

Fixes: 3cd890d ("media: dvb-frontends: fix i2c access helpers for KASAN")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

Author: arndb

drivers/media/dvb-frontends/stv0367.c

@@ -118,50 +118,32 @@ static const s32 stv0367cab_RF_LookUp2[RF_LOOKUP_TABLE2_SIZE][RF_LOOKUP_TABLE2_S
 	}
 };
 
-static
-int stv0367_writeregs(struct stv0367_state *state, u16 reg, u8 *data, int len)
+static noinline_for_stack
+int stv0367_writereg(struct stv0367_state *state, u16 reg, u8 data)
 {
-	u8 buf[MAX_XFER_SIZE];
+	u8 buf[3] = { MSB(reg), LSB(reg), data };
 	struct i2c_msg msg = {
 		.addr = state->config->demod_address,
 		.flags = 0,
 		.buf = buf,
-		.len = len + 2
+		.len = 3,
 	};
 	int ret;
 
-	if (2 + len > sizeof(buf)) {
-		printk(KERN_WARNING
-		       "%s: i2c wr reg=%04x: len=%d is too big!\n",
-		       KBUILD_MODNAME, reg, len);
-		return -EINVAL;
-	}
-
-
-	buf[0] = MSB(reg);
-	buf[1] = LSB(reg);
-	memcpy(buf + 2, data, len);
-
 	if (i2cdebug)
 		printk(KERN_DEBUG "%s: [%02x] %02x: %02x\n", __func__,
-			state->config->demod_address, reg, buf[2]);
+			state->config->demod_address, reg, data);
 
 	ret = i2c_transfer(state->i2c, &msg, 1);
 	if (ret != 1)
 		printk(KERN_ERR "%s: i2c write error! ([%02x] %02x: %02x)\n",
-			__func__, state->config->demod_address, reg, buf[2]);
+			__func__, state->config->demod_address, reg, data);
 
 	return (ret != 1) ? -EREMOTEIO : 0;
 }
 
-static int stv0367_writereg(struct stv0367_state *state, u16 reg, u8 data)
-{
-	u8 tmp = data; /* see gcc.gnu.org/bugzilla/show_bug.cgi?id=81715 */
-
-	return stv0367_writeregs(state, reg, &tmp, 1);
-}
-
-static u8 stv0367_readreg(struct stv0367_state *state, u16 reg)
+static noinline_for_stack
+u8 stv0367_readreg(struct stv0367_state *state, u16 reg)
 {
 	u8 b0[] = { 0, 0 };
 	u8 b1[] = { 0 };