Re: [PATCH net-next v2 4/7] usbnet: smsc95xx: Avoid link settings race on interrupt reception

[Andrew Lunn <andrew@lunn.ch>]

On Tue, May 03, 2022 at 03:15:04PM +0200, Lukas Wunner wrote:
> When a PHY interrupt is signaled, the SMSC LAN95xx driver updates the
> MAC full duplex mode and PHY flow control registers based on cached data
> in struct phy_device:
> 
>   smsc95xx_status()                 # raises EVENT_LINK_RESET
>     usbnet_deferred_kevent()
>       smsc95xx_link_reset()         # uses cached data in phydev
> 
> Simultaneously, phylib polls link status once per second and updates
> that cached data:
> 
>   phy_state_machine()
>     phy_check_link_status()
>       phy_read_status()
>         lan87xx_read_status()
>           genphy_read_status()      # updates cached data in phydev
> 
> If smsc95xx_link_reset() wins the race against genphy_read_status(),
> the registers may be updated based on stale data.
> 
> E.g. if the link was previously down, phydev->duplex is set to
> DUPLEX_UNKNOWN and that's what smsc95xx_link_reset() will use, even
> though genphy_read_status() may update it to DUPLEX_FULL afterwards.
> 
> PHY interrupts are currently only enabled on suspend to trigger wakeup,
> so the impact of the race is limited, but we're about to enable them
> perpetually.
> 
> Avoid the race by delaying execution of smsc95xx_link_reset() until
> phy_state_machine() has done its job and calls back via
> smsc95xx_handle_link_change().
> 
> Signaling EVENT_LINK_RESET on wakeup is not necessary because phylib
> picks up link status changes through polling.  So drop the declaration
> of a ->link_reset() callback.
> 
> Note that the semicolon on a line by itself added in smsc95xx_status()
> is a placeholder for a function call which will be added in a subsequent
> commit.  That function call will actually handle the INT_ENP_PHY_INT_
> interrupt.
> 
> Tested-by: Oleksij Rempel <o.rempel@pengutronix.de> # LAN9514/9512/9500
> Tested-by: Ferry Toth <fntoth@gmail.com> # LAN9514
> Signed-off-by: Lukas Wunner <lukas@wunner.de>

Reviewed-by: Andrew Lunn <andrew@lunn.ch>

    Andrew