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

[Stefan Agner <stefan@agner.ch>]

On 2026-07-07 20:50, Andrew Lunn wrote:
> 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.

Hm, I see. __stmmac_release() calls phylink_stop(). But from what I can
tell we can't simply move phylink_start() in __stmmac_open() since it
does too much. So we need to use phylink_prepare_resume() in
__stmmac_open(), so there is still some asymmetry. I'll send a v2.

--
Stefan