Re: [PATCH net-next 00/11] net: macb: implement context swapping

Nicolai Buchwitz
2026-04-02T11:35:41Z

On 1.4.2026 18:39, Théo Lebrun wrote:
> 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.

Thanks for your work, the context swapping approach probably
makes a lot of sense and will finally bring proper MTU change
support that I tried to patch earlier.

>
> 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

The series didn't apply cleanly on current net-next. The
base commit 321d1ee521de doesn't seem to be upstream yet, is
this based on your set_channels v4 series?

> change-id: 20260401-macb-context-bd0caf20414d
>
> Best regards,
> --
> Théo Lebrun <theo.lebrun@bootlin.com>

Thanks
Nicolai