Re: [PATCH net-next v4 14/15] net: macb: use context swapping in .set_ringparam()

[Nicolai Buchwitz <nb@tipi-net.de>]

Hi Théo

On 17.7.2026 21:48, 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@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

> [...]

> +
> +	for (q = 0, queue = bp->queues; q < bp->num_queues; ++q, ++queue) {
> +		/* Must be done before NAPI is disabled. */
> +		cancel_work_sync(&queue->tx_error_task);
> +
> +		napi_disable(&queue->napi_rx);
> +		napi_disable(&queue->napi_tx);
> +		netdev_tx_reset_queue(netdev_get_tx_queue(bp->netdev, q));
> +	}
> +
> +	/* Must be done after napi_tx is disabled. */
> +	cancel_delayed_work_sync(&bp->tx_lpi_work);
> +
> +	/* Can finally disable software Tx; need to wait until napi_tx and
> +	 * tx_error_task cannot be scheduled as either might wakeup Tx.
> +	 */
> +	netif_tx_disable(bp->netdev);

Shouldn't netdev_tx_reset_queue() come after netif_tx_disable()? Tx is
still running here, so an xmit right after the reset adds bytes to the
DQL that never get completed (macb_free() frees the old skbs without
netdev_tx_completed_queue()). Not sure, but that could leave the queue
stopped by BQL forever?

> [...]

Thanks
Nicolai