MACB has a pretty primitive approach to buffer management. They are all
stored in `struct macb *bp`. On operations that require buffer realloc
(set_ringparam & change_mtu ATM), the only option is to close the
interface, change our global state and re-open the interface.

Two issues:
- It doesn't fly on memory pressured systems; we free our precious
  buffers and don't manage to reallocate fully, meaning our machine
  just lost its network access.
- Anecdotally, it is pretty slow because it implies a full PHY reinit.

Instead, we shall:
 - 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 newly allocated buffer pointers to the hardware
 - start interface
 - free old context

This is what we implement here. Both .set_ringparam() and
.ndo_change_mtu() are covered by this series. In the future,
at least .set_channels() [0], XDP [1] and XSK [2] would benefit.

The change is super intrusive so conflicts will be major. Sorry!

Thanks,
Have a nice day,
Théo

[0]: https://lore.kernel.org/netdev/20260317-macb-set-channels-v4-0-1bd4f4ffcfca@bootlin.com/
[1]: https://lore.kernel.org/netdev/20260323221047.2749577-1-pvalerio@redhat.com/
[2]: https://lore.kernel.org/netdev/20260304-macb-xsk-v1-0-ba2ebe2bdaa3@bootlin.com/

Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
---
Théo Lebrun (11):
      net: macb: unify device pointer naming convention
      net: macb: unify `struct macb *` naming convention
      net: macb: unify queue index variable naming convention and types
      net: macb: enforce reverse christmas tree (RCT) convention
      net: macb: allocate tieoff descriptor once across device lifetime
      net: macb: introduce macb_context struct for buffer management
      net: macb: avoid macb_init_rx_buffer_size() modifying state
      net: macb: make `struct macb` subset reachable from macb_context struct
      net: macb: introduce macb_context_alloc() helper
      net: macb: use context swapping in .set_ringparam()
      net: macb: use context swapping in .ndo_change_mtu()

 drivers/net/ethernet/cadence/macb.h      |  119 +-
 drivers/net/ethernet/cadence/macb_main.c | 1731 +++++++++++++++++-------------
 drivers/net/ethernet/cadence/macb_pci.c  |   46 +-
 drivers/net/ethernet/cadence/macb_ptp.c  |   26 +-
 4 files changed, 1090 insertions(+), 832 deletions(-)
---
base-commit: 321d1ee521de1362c22adadbc0ce066050a17783
change-id: 20260401-macb-context-bd0caf20414d

Best regards,
--  
Théo Lebrun <theo.lebrun@bootlin.com>