net: usb: lan78xx: skip LTM configuration for LAN7850

commit d9cc0e4 upstream.

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: stable@vger.kernel.org
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://patch.msgid.link/20260305143429.530909-4-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: olerem

drivers/net/usb/lan78xx.c

@@ -2672,6 +2672,10 @@ static void lan78xx_init_ltm(struct lan78xx_net *dev)
 	u32 buf;
 	u32 regs[6] = { 0 };
 
+	/* LAN7850 is USB 2.0 and does not support LTM */
+	if (dev->chipid == ID_REV_CHIP_ID_7850_)
+		return;
+
 	ret = lan78xx_read_reg(dev, USB_CFG1, &buf);
 	if (buf & USB_CFG1_LTM_ENABLE_) {
 		u8 temp[2];