Re: [PATCH net-next v3 14/15] net: macb: use context swapping in .set_ringparam()

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

Hi Théo

On 1.7.2026 17:59, 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      |   2 +
>  drivers/net/ethernet/cadence/macb_main.c | 142 
> +++++++++++++++++++++++++++++--

> [...]

> +static void macb_context_swap_start(struct macb *bp)
> +{
> +	struct macb_queue *queue;
> +	unsigned long flags;
> +	unsigned int q;
> +	u32 ctrl;
> +
> +	mutex_lock(&bp->mac_cfg_lock);
> +
> +	/* Mask interrupts before disabling BH features. */
> +	spin_lock_irqsave(&bp->lock, flags);
> +	for (q = 0, queue = bp->queues; q < bp->num_queues; ++q, ++queue) {
> +		queue_writel(queue, IDR, -1);
> +		queue_readl(queue, ISR);
> +		macb_queue_isr_clear(bp, queue, -1);
> +	}
> +	spin_unlock_irqrestore(&bp->lock, flags);
> +
> +	/* Drain BH features. HW is still active and usable at this point. */
> +
> +	cancel_work_sync(&bp->hresp_err_bh_work);
> +	cancel_delayed_work_sync(&bp->tx_lpi_work);
> +
> +	for (q = 0, queue = bp->queues; q < bp->num_queues; ++q, ++queue) {
> +		napi_disable(&queue->napi_rx);
> +		napi_disable(&queue->napi_tx);
> +		cancel_work_sync(&queue->tx_error_task);
> +		netdev_tx_reset_queue(netdev_get_tx_queue(bp->netdev, q));
> +	}

Can this deadlock against a pending tx_error_task?

AFAIU macb_tx_error_task() does napi_disable(&queue->napi_tx) and later
napi_enable() on the same napi, and it can already be queued 
(macb_interrupt()
schedules it on a TX error) by the time the swap runs:

   swap_start: napi_disable(napi_tx)          /* sets SCHED, returns */
   worker:     tx_error_task: napi_disable(napi_tx)  /* spins on SCHED */
   swap_start: cancel_work_sync(tx_error_task)       /* waits on worker 
*/

napi_disable() spins until napi_enable() clears SCHED, but here the swap 
won't
re-enable until macb_context_swap_end(), and cancel_work_sync() is 
what's
holding it up. Nothing clears it.

Maybe cancel_work_sync() before the napi_disable() calls would work 
instead? IRQs
are masked just above, so AFAICT nothing can reschedule tx_error_task by 
then.

(Same path via .ndo_change_mtu() in the next patch.)

> [...]

Thanks
Nicolai