ethtool_ops.set_ringparam() is implemented using the primitive close /
update ring size / reopen sequence. Under memory pressure this does not
fly: we free our buffers at close and cannot reallocate new ones at
open. Also, it triggers a slow PHY reinit.

Instead, exploit the new context mechanism and improve our sequence to:
 - allocate a new context (including buffers) first
 - if it fails, early return without any impact to the interface
 - stop interface
 - update global state (bp, netdev, etc)
 - pass buffer pointers to the hardware
 - start interface
 - free old context.

The HW disable sequence is inspired by macb_reset_hw() but avoids
(1) setting NCR bit CLRSTAT and (2) clearing register PBUFRXCUT.

The HW re-enable sequence is inspired by macb_mac_link_up(), skipping
over register writes which would be redundant (because values have not
changed).

The generic context swapping parts are isolated into helper functions
macb_context_swap_start|end(), reusable by other operations 
(change_mtu,
set_channels, etc).

Introduce a new locking primitive (mac_cfg_lock mutex) to serialise 
swap
with phylink MAC callbacks. Avoid stopping phylink to avoid a slow PHY
retrain. Those callbacks grab phydev->lock if it exists so we could
imagine grabbing that from the swap op, but phydev->lock doesn't exist
in the SFP case.

AT91 EMAC is handled differently as their buffer management is separate
and they don't do NAPI. We refuse them (-EBUSY) to avoid implementing
context swapping for them.

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
---
 drivers/net/ethernet/cadence/macb.h      |   5 +
 drivers/net/ethernet/cadence/macb_main.c | 162 
+++++++++++++++++++++++++++++--
 2 files changed, 158 insertions(+), 9 deletions(-)

diff --git a/drivers/net/ethernet/cadence/macb.h 

b/drivers/net/ethernet/cadence/macb.h
index ac2f2d8065d7..93e513cf1fbb 100644

--- a/drivers/net/ethernet/cadence/macb.h
+++ b/drivers/net/ethernet/cadence/macb.h

@@ -1361,6 +1361,8 @@ struct macb {
 	struct macb_queue	queues[MACB_MAX_QUEUES];

 	spinlock_t		lock;
+	/* Serializes context swap against phylink MAC callbacks. */
+	struct mutex		mac_cfg_lock;
 	struct clk		*pclk;
 	struct clk		*hclk;
 	struct clk		*tx_clk;

@@ -1421,6 +1423,9 @@ struct macb {
 	struct delayed_work	tx_lpi_work;
 	u32			tx_lpi_timer;

+	/* ISR must not drive NAPI & BH mechanisms. Protected by bp->lock. */
+	bool			ctx_swap;
+
 	u32	rx_intr_mask;

 	struct macb_pm_data pm_data;

diff --git a/drivers/net/ethernet/cadence/macb_main.c 

b/drivers/net/ethernet/cadence/macb_main.c
index c832b6c1b98c..5792647eb0a6 100644

--- a/drivers/net/ethernet/cadence/macb_main.c
+++ b/drivers/net/ethernet/cadence/macb_main.c