net: ethernet: xscale: Check for PTP support properly

In ixp4xx_get_ts_info() ixp46x_ptp_find() is called
unconditionally despite this feature only existing on
ixp46x, leading to the following splat from tcpdump:

root@OpenWrt:~# tcpdump -vv -X -i eth0
(...)
Unable to handle kernel NULL pointer dereference at virtual address
  00000238 when read
(...)
Call trace:
 ptp_clock_index from ixp46x_ptp_find+0x1c/0x38
 ixp46x_ptp_find from ixp4xx_get_ts_info+0x4c/0x64
 ixp4xx_get_ts_info from __ethtool_get_ts_info+0x90/0x108
 __ethtool_get_ts_info from __dev_ethtool+0xa00/0x2648
 __dev_ethtool from dev_ethtool+0x160/0x234
 dev_ethtool from dev_ioctl+0x2cc/0x460
 dev_ioctl from sock_ioctl+0x1ec/0x524
 sock_ioctl from sys_ioctl+0x51c/0xa94
 sys_ioctl from ret_fast_syscall+0x0/0x44
 (...)
Segmentation fault

Check for ixp46x in ixp46x_ptp_find() before trying to set up
PTP to avoid this.

To avoid altering the returned error code from ixp4xx_hwtstamp_set()
which before this patch was -EOPNOTSUPP, we return -EOPNOTSUPP
from ixp4xx_hwtstamp_set() if ixp46x_ptp_find() fails no matter
the error code. The helper function ixp46x_ptp_find() helper
returns -ENODEV.

Fixes: 9055a2f ("ixp4xx_eth: make ptp support a platform driver")
Signed-off-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260219-ixp4xx-fix-ethernet-v3-1-f235ccc3cd46@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: linusw

drivers/net/ethernet/xscale/ixp4xx_eth.c

@@ -403,15 +403,12 @@ static int ixp4xx_hwtstamp_set(struct net_device *netdev,
 	int ret;
 	int ch;
 
-	if (!cpu_is_ixp46x())
-		return -EOPNOTSUPP;
-
 	if (!netif_running(netdev))
 		return -EINVAL;
 
 	ret = ixp46x_ptp_find(&port->timesync_regs, &port->phc_index);
 	if (ret)
-		return ret;
+		return -EOPNOTSUPP;
 
 	ch = PORT2CHANNEL(port);
 	regs = port->timesync_regs;

drivers/net/ethernet/xscale/ptp_ixp46x.c

@@ -232,6 +232,9 @@ static struct ixp_clock ixp_clock;
 
 int ixp46x_ptp_find(struct ixp46x_ts_regs *__iomem *regs, int *phc_index)
 {
+	if (!cpu_is_ixp46x())
+		return -ENODEV;
+
 	*regs = ixp_clock.regs;
 	*phc_index = ptp_clock_index(ixp_clock.ptp_clock);