Re: [PATCH net] net: stmmac: resume PHY before reopening the interface on MTU change

[Andrew Lunn <andrew@lunn.ch>]

On Tue, Jul 07, 2026 at 06:21:46PM +0200, Stefan Agner wrote:
> Since the referenced commit, changing the MTU on a running interface no
> longer disconnects and reconnects the PHY; __stmmac_release() merely
> stops phylink, which also suspends the PHY (BMCR power-down) when WoL
> is not enabled. __stmmac_open() then performs the DMA software reset in
> stmmac_hw_setup() before phylink_start() resumes the PHY again.
> 
> IEEE 802.3 22.2.4.1.5 allows a PHY to stop its receive clock while
> powered down, and stmmac requires a running receive clock for the DMA
> software reset to complete (the phylink config sets mac_requires_rxc).
> On such setups, e.g. the RK3566-based Home Assistant Green with an
> RTL8211F-VD PHY in RGMII mode, any runtime MTU change now times out and
> leaves the interface dead:
> 
>   rk_gmac-dwmac fe010000.ethernet end0: Failed to reset the dma
>   rk_gmac-dwmac fe010000.ethernet end0: stmmac_hw_setup: DMA engine initialization failed
>   rk_gmac-dwmac fe010000.ethernet end0: __stmmac_open: Hw setup failed
>   rk_gmac-dwmac fe010000.ethernet end0: failed reopening the interface after MTU change
> 
> In the field this is triggered by NetworkManager applying an MTU while
> activating the connection, breaking networking entirely.
> 
> Resume the PHY before reopening the interface, like stmmac_resume()
> does, to ensure the receive clock is running for the DMA software
> reset.
> 
> Fixes: db299a0c09e9 ("net: stmmac: move PHY handling out of __stmmac_open()/release()")
> Link: https://github.com/home-assistant/operating-system/issues/4858
> Assisted-by: Claude:claude-fable-5
> Tested-by: Stefan Agner <stefan@agner.ch>
> Signed-off-by: Stefan Agner <stefan@agner.ch>
> ---
> Note: phylink_prepare_resume()'s kernel-doc says it is to be called
> prior to phylink_resume(); here it is paired with phylink_start()
> (called from __stmmac_open()) instead, which phylink_resume() itself
> uses to restart the machinery. If preferred, I can extend the
> kernel-doc or introduce a more generically named helper.
> 
>  drivers/net/ethernet/stmicro/stmmac/stmmac_main.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> --- a/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> +++ b/drivers/net/ethernet/stmicro/stmmac/stmmac_main.c
> @@ -5884,6 +5884,15 @@
>  
>  		__stmmac_release(dev);
>  
> +		/* phylink_stop() in __stmmac_release() suspends the PHY.
> +		 * IEEE 802.3 allows PHYs to stop their receive clock while
> +		 * powered down, but the DMA software reset performed by
> +		 * stmmac_hw_setup() requires a running receive clock.
> +		 * Resume the PHY, as on system resume, to ensure its clocks
> +		 * are running before reopening the interface.
> +		 */
> +		phylink_prepare_resume(priv->phylink);
> +
>  		ret = __stmmac_open(dev, dma_conf);
>  		if (ret) {
>  			free_dma_desc_resources(priv, dma_conf);

I'm not convinced.

__stmmac_open() and __stmmac_release() should be opposites of each
other. If __stmmac_release() stops the clock, __stmmac_open() should
start the clock.

	Andrew