[PATCH net v6] net: dsa: yt921x: Fix MIB overflow wraparound routine

[David Yang <mmyangfl@gmail.com>]

Reported by the following Smatch static checker warning:

  drivers/net/dsa/yt921x.c:702 yt921x_read_mib()
  warn: was expecting a 64 bit value instead of '(~0)'

Fixes: 186623f4aa72 ("net: dsa: yt921x: Add support for Motorcomm YT921x")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/netdev/aPsjYKQMzpY0nSXm@stanley.mountain/
Suggested-by: David Laight <david.laight.linux@gmail.com>
Signed-off-by: David Yang <mmyangfl@gmail.com>
---
v6: https://lore.kernel.org/r/20260118013019.1078847-1-mmyangfl@gmail.com
  - send only the first patch targeting net tree
v5: https://lore.kernel.org/r/20260114114745.213252-1-mmyangfl@gmail.com
  - avoid type casting
  - split patches
v4: https://lore.kernel.org/r/20260108004309.4087448-1-mmyangfl@gmail.com
  - add missing u64_stats_init
v3: https://lore.kernel.org/r/20260105020905.3522484-1-mmyangfl@gmail.com
  - use u64_stats_t
  - fix calculations of rx_frames/tx_frames
v2: https://lore.kernel.org/r/20251025171314.1939608-1-mmyangfl@gmail.com
  - run tests and fix MIB parsing in 510026a39849
  - no major changes between versions
v1: https://lore.kernel.org/r/20251024084918.1353031-1-mmyangfl@gmail.com
  - take suggestion from David Laight
  - protect MIB stats with a lock
 drivers/net/dsa/yt921x.c | 15 ++++++++-------
 1 file changed, 8 insertions(+), 7 deletions(-)

diff --git a/drivers/net/dsa/yt921x.c b/drivers/net/dsa/yt921x.c
index 1c511f5dc6ab..7b8c1549a0fb 100644
--- a/drivers/net/dsa/yt921x.c
+++ b/drivers/net/dsa/yt921x.c
@@ -682,21 +682,22 @@ static int yt921x_read_mib(struct yt921x_priv *priv, int port)
 		const struct yt921x_mib_desc *desc = &yt921x_mib_descs[i];
 		u32 reg = YT921X_MIBn_DATA0(port) + desc->offset;
 		u64 *valp = &((u64 *)mib)[i];
-		u64 val = *valp;
 		u32 val0;
-		u32 val1;
+		u64 val;
 
 		res = yt921x_reg_read(priv, reg, &val0);
 		if (res)
 			break;
 
 		if (desc->size <= 1) {
-			if (val < (u32)val)
-				/* overflow */
-				val += (u64)U32_MAX + 1;
-			val &= ~U32_MAX;
-			val |= val0;
+			u64 old_val = *valp;
+
+			val = (old_val & ~(u64)U32_MAX) | val0;
+			if (val < old_val)
+				val += 1ull << 32;
 		} else {
+			u32 val1;
+
 			res = yt921x_reg_read(priv, reg + 4, &val1);
 			if (res)
 				break;
-- 
2.51.0