Re: [PATCH net-next v5 14/15] net: macb: use context swapping in .set_ringparam()

[Nicolai Buchwitz]

On 24.7.2026 17:29, Théo Lebrun wrote:
> 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@xxxxxxxxxxx>
> ---

> [...]

Reviewed-by: Nicolai Buchwitz <nb@xxxxxxxxxxx>

Thanks
Nicolai