usbnet_link_change() fails to call netif_carrier_on()

Hi,

ASIX AX88772B based USB 10/100 Ethernet adapter doesn't come
up ("carrier off"), despite the built-in 100BASE-FX PHY positive link
indication.

The problem appears to be in usbnet.c framework:

void usbnet_link_change(struct usbnet *dev, bool link, bool need_reset)
{
	/* update link after link is reseted */
	if (link && !need_reset)
		netif_carrier_on(dev->net);
	else
		netif_carrier_off(dev->net);

	if (need_reset && link)
		usbnet_defer_kevent(dev, EVENT_LINK_RESET);
	else
		usbnet_defer_kevent(dev, EVENT_LINK_CHANGE);
}

I think the author's idea was a bit different than what the code really
ended doing. Especially when called with link = 1 and need_reset = 1.

It seems it may have already caused problems - possible workarounds:
- commit 7be4cb7189f7 ("net: usb: ax88179_178a: improve reset check")
- commit ecf848eb934b ("net: usb: ax88179_178a: fix link status when
  link is set to down/up")
Can't check those due to -ENOHW but ax88179_link_reset() adds
a netif_carrier_on() call on link ups.

Not sure about the "reset" name, though (and the comment in
usbnet_link_change()). It seems it's when the link goes up.

Signed-off-by: Krzysztof Hałasa <khalasa@piap.pl>
Fixes: ac64995da872 ("usbnet: introduce usbnet_link_change API")
Fixes: 4b49f58fff00 ("usbnet: handle link change")


The code has been introduced in 2013, by 4b49f58fff00 and a bunch of
related commits. Perhaps it visibly affects only AXIS and dm9601
adapters, though.

On Tue, Nov 19, 2024 at 02:46:59PM +0100, Krzysztof Hałasa wrote:
> Hi,
> 
> ASIX AX88772B based USB 10/100 Ethernet adapter doesn't come
> up ("carrier off"), despite the built-in 100BASE-FX PHY positive link
> indication.
> 
> The problem appears to be in usbnet.c framework:
> 
> void usbnet_link_change(struct usbnet *dev, bool link, bool need_reset)
> {
> 	/* update link after link is reseted */
> 	if (link && !need_reset)
> 		netif_carrier_on(dev->net);
> 	else
> 		netif_carrier_off(dev->net);
> 
> 	if (need_reset && link)
> 		usbnet_defer_kevent(dev, EVENT_LINK_RESET);
> 	else
> 		usbnet_defer_kevent(dev, EVENT_LINK_CHANGE);
> }

static int ax88772_phylink_setup(struct usbnet *dev)
{
        struct asix_common_private *priv = dev->driver_priv;
        phy_interface_t phy_if_mode;
        struct phylink *phylink;

        priv->phylink_config.dev = &dev->net->dev;
        priv->phylink_config.type = PHYLINK_NETDEV;
        priv->phylink_config.mac_capabilities = MAC_SYM_PAUSE | MAC_ASYM_PAUSE |
                MAC_10 | MAC_100;

etc.

This device is using phylink to manage the PHY. phylink will than
manage the carrier. It assumes it is solely responsible for the
carrier. So i think your fix is wrong. You probably should be removing
all code in this driver which touches the carrier.

	Andrew