From: Greg KH <gregkh@linuxfoundation•org>
To: Nguyen Minh Tien <zizuzacker@gmail•com>
Cc: stable@vger•kernel.org, Woojung Huh <woojung.huh@microchip•com>,
UNGLinuxDriver@microchip•com, netdev@vger•kernel.org
Subject: Re: [PATCH 6.6.y] net: usb: lan78xx: program MAC_CR for LAN7801 fixed-PHY link
Date: Fri, 5 Jun 2026 16:57:35 +0200 [thread overview]
Message-ID: <2026060535-subpanel-proven-595a@gregkh> (raw)
In-Reply-To: <20260605121535.51414-1-zizuzacker@gmail.com>
On Fri, Jun 05, 2026 at 07:15:35PM +0700, Nguyen Minh Tien wrote:
> While bringing up a LAN7801 wired over RGMII to an MDIO-less switch, I
> hit a link that the fixed PHY reported as "up" but that could not pass a
> single packet: every frame came out corrupted at the switch and no ARP
> reply ever made it back.
>
> It turned out the MAC_CR speed/duplex bits are never set on this path.
> lan7801_phy_init() registers a fixed PHY at SPEED_1000/DUPLEX_FULL, but
> nothing programs the MAC to match: there is no PHY state machine for a
> fixed link, and lan78xx_reset() only sets the auto speed/duplex bits for
> the 7800. So the 7801 comes up at 10M/half, clocks RGMII TXC at 2.5 MHz
> instead of 125 MHz, and mangles everything it transmits.
>
> Fix it by programming MAC_CR to 1G/full in the fixed-PHY branch, to match
> fphy_status. Only that branch is touched, so boards with a real external
> PHY (and the 7800/7850) are unaffected.
>
> Mainline fixes this differently via the phylink conversion in v6.16,
> commit e110bc825897 ("net: usb: lan78xx: Convert to PHYLINK for improved
> PHY and MAC management"), which is far too large to backport, so this is
> a small fix for stable only.
Why is that too large? If we take this, we then diverge and any future
fixes will not apply here :(
Please backport exactly what is in the mainline tree, otherwise odds
are, this one-off fix will be found incorrect (we always get it
wrong...)
thanks,
greg k-h
prev parent reply other threads:[~2026-06-05 15:08 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-06-05 12:15 [PATCH 6.6.y] net: usb: lan78xx: program MAC_CR for LAN7801 fixed-PHY link Nguyen Minh Tien
2026-06-05 14:57 ` Greg KH [this message]
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