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 in __stmmac_open() before the hardware setup, making it
the counterpart of the phylink_stop() in __stmmac_release(), like
stmmac_resume() already does for the same reason. phylink_start() also
resumes the PHY, but only after stmmac_hw_setup(), and it cannot be
moved before the hardware setup since it may bring the link up
immediately from a workqueue, racing with the initialization (see the
comment in stmmac_resume()). For the regular ndo_open path the PHY has
just been attached and is not suspended, in which case
phylink_prepare_resume() does nothing.

Fixes: db299a0c09e9 ("net: stmmac: move PHY handling out of __stmmac_open()/release()")
Link: https://github.com/home-assistant/operating-system/issues/4858
Signed-off-by: Stefan Agner <stefan@agner.ch>
---
Changes in v2:
- Move the PHY resume from stmmac_change_mtu() into __stmmac_open() so
  that it also counters the PHY suspend caused by __stmmac_release()
  (suggested by Andrew Lunn), placed before stmmac_reset_queues_param()
  to match the ordering used in stmmac_resume()

 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

@@ -4147,6 +4147,15 @@
 			dma_conf->tx_queue[i].tbs = priv->dma_conf.tx_queue[i].tbs;
 	memcpy(&priv->dma_conf, dma_conf, sizeof(*dma_conf));
 
+	/* The PHY is suspended when the interface is reopened without
+	 * disconnecting the PHY, e.g. on MTU change. IEEE 802.3 allows PHYs
+	 * to stop their receive clock while powered down, but the DMA
+	 * software reset in stmmac_hw_setup() requires a running receive
+	 * clock, and phylink_start() below resumes the PHY only after the
+	 * hardware setup. Resume a suspended PHY here first.
+	 */
+	phylink_prepare_resume(priv->phylink);
+
 	stmmac_reset_queues_param(priv);
 
 	ret = stmmac_hw_setup(dev);

-- 
2.49.0