netdev - Re: [PATCH net v2] net: ethernet: xscale: Check for PTP support properly

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Message-ID: <CAD++jLmaYU_r+Z+t=6a9GZuVNGOHLGUfp7SDpDH8jnRqD1TLhQ@mail.gmail.com>
Date: Wed, 11 Feb 2026 22:03:50 +0100
From: Linus Walleij <linusw@...nel.org>
To: Andrew Lunn <andrew@...n.ch>
Cc: Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Richard Cochran <richardcochran@...il.com>, Vadim Fedorenko <vadim.fedorenko@...ux.dev>, 
	Vladimir Oltean <vladimir.oltean@....com>, netdev@...r.kernel.org
Subject: Re: [PATCH net v2] net: ethernet: xscale: Check for PTP support properly

On Wed, Feb 11, 2026 at 4:43 PM Andrew Lunn <andrew@...n.ch> wrote:
> On Wed, Feb 11, 2026 at 01:58:21PM +0100, Linus Walleij wrote:
> > 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@...nWrt:~# 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.
>
> >       ret = ixp46x_ptp_find(&port->timesync_regs, &port->phc_index);
> >       if (ret)
> > -             return ret;
> > +             return -EOPNOTSUPP;
>
> Changing the error code is unusual. Maybe add to the commit message
> why this is needed, or somebody might ask :-)

Tricky one, but if you look close changing the error code is what I
try not to do! :D

Before this patch ixp4xx_hwtstamp_set() returned -EOPNOTSUPP:

-       if (!cpu_is_ixp46x())
-               return -EOPNOTSUPP;

After moving the check into ixp46x_ptp_find() as requested in the
review of v1 I have to return some error code from that new check,
but it is now called from more places, so it returns

int ixp46x_ptp_find(struct ixp46x_ts_regs *__iomem *regs, int *phc_index)
 {
+       if (!cpu_is_ixp46x())
+               return -ENODEV;

So this:

     ret = ixp46x_ptp_find(&port->timesync_regs, &port->phc_index);
        if (ret)
-               return ret;
+               return -EOPNOTSUPP;

Is put into ixp4xx_hwtstamp_set() so it will return the same error
code as before if PTP was not found.

(I can copyedit this reasoning into the commit message if you want.)

Yours,
Linus Walleij