Subject: Re: [PATCH net-next v5 2/5] net: cadence: macb: implement EEE TX LPI support

From: Jakub Kicinski <[email-masked]>
Date: Mon, 2 Mar 2026 18:15:22 -0800

On Fri, 27 Feb 2026 16:06:07 +0100 Nicolai Buchwitz wrote:
> +static bool macb_tx_lpi_set(struct macb *bp, bool enable)
> +{
> +	unsigned long flags;
> +	u32 old, ncr;
> +
> +	spin_lock_irqsave(&bp->lock, flags);

we should optimize this function for the past path caller.
xmit path does:

+	macb_tx_lpi_wake(bp);
+
 	spin_lock(&bp->lock);

So it immediately takes that lock again, can we move the lpi_wake()
call under the spin_lock, and make sure other callers also take that
lock? I think you can add a lockdep assert to make sure spin lock is
held

> +	ncr = macb_readl(bp, NCR);
> +	old = ncr;
> +	if (enable)
> +		ncr |= GEM_BIT(TXLPIEN);
> +	else
> +		ncr &= ~GEM_BIT(TXLPIEN);
> +	if (old != ncr)
> +		macb_writel(bp, NCR, ncr);
> +	spin_unlock_irqrestore(&bp->lock, flags);
> +
> +	return old != ncr;
> +}
> +
> +static bool macb_tx_all_queues_idle(struct macb *bp)
> +{
> +	unsigned int q;
> +
> +	for (q = 0; q < bp->num_queues; q++) {
> +		struct macb_queue *queue = &bp->queues[q];
> +
> +		if (queue->tx_head != queue->tx_tail)

Does not not need tx_ptr_lock technically?

> +			return false;
> +	}
> +	return true;
> +}
> +
> +static void macb_tx_lpi_work_fn(struct work_struct *work)
> +{
> +	struct macb *bp = container_of(work, struct macb, tx_lpi_work.work);
> +
> +	if (bp->eee_active && macb_tx_all_queues_idle(bp))
> +		macb_tx_lpi_set(bp, true);
> +}
> +
> +static void macb_tx_lpi_schedule(struct macb *bp)
> +{
> +	if (bp->eee_active)
> +		mod_delayed_work(system_wq, &bp->tx_lpi_work,
> +				 usecs_to_jiffies(bp->tx_lpi_timer));
> +}
> +
> +/* Wake from LPI before transmitting. The MAC must deassert TXLPIEN
> + * and wait for the PHY to exit LPI before any frame can be sent.
> + * IEEE 802.3az Tw_sys is ~17us for 1000BASE-T, ~30us for 100BASE-TX;
> + * we use a conservative 50us.
> + */
> +static void macb_tx_lpi_wake(struct macb *bp)
> +{
> +	if (!macb_tx_lpi_set(bp, false))

Does this lpi_set() not have a relatively high cost, even if eee_active
is disabled? Reading registers is usually pretty slow. Can we add 
a eee_active check here as well to short cut the lpi check? 
If we do we probably want to make sure that the code paths setting
eee_active are also under bp->lock, otherwise this new check will be
racy.
-- 
pw-bot: cr