net: usb: lan78xx: skip LTM configuration for LAN7850

Do not configure Latency Tolerance Messaging (LTM) on USB 2.0 hardware.

The LAN7850 is a High-Speed (USB 2.0) only device and does not support
SuperSpeed features like LTM. Currently, the driver unconditionally
attempts to configure LTM registers during initialization. On the
LAN7850, these registers do not exist, resulting in writes to invalid
or undocumented memory space.

This issue was identified during a port to the regmap API with strict
register validation enabled. While no functional issues or crashes have
been observed from these invalid writes, bypassing LTM initialization
on the LAN7850 ensures the driver strictly adheres to the hardware's
valid register map.

Fixes: 55d7de9 ("Microchip's LAN7800 family USB 2/3 to 10/100/1000 Ethernet device driver")
Cc: [email protected]
Signed-off-by: Oleksij Rempel <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: olerem

drivers/net/usb/lan78xx.c

@@ -3119,6 +3119,10 @@ static int lan78xx_init_ltm(struct lan78xx_net *dev)
 	int ret;
 	u32 buf;
 
+	/* LAN7850 is USB 2.0 and does not support LTM */
+	if (dev->chipid == ID_REV_CHIP_ID_7850_)
+		return 0;
+
 	ret = lan78xx_read_reg(dev, USB_CFG1, &buf);
 	if (ret < 0)
 		goto init_ltm_failed;