Re: [PATCH] net: stmmac: Avoid freeing and re-requesting IRQ during XDP set prog

[Daniel Thompson <daniel@riscstar.com>]

On Mon, Jul 06, 2026 at 05:17:58PM +0100, Daniel Thompson wrote:
> Currently stmmac will run a full cycle of IRQ tear down and setup when
> setting up a new XDP program. This makes tuning TSN systems difficult
> because whenever a new XDP program is installed then the irq threads will
> be stopped and restarted which will undo any thread tuning.
>
> The problem is avoided by removing stmmac_free_irq()/stmmac_request_irq()
> from stmmac_xdp_release()/stmmac_xdp_open().
>
> stmmac_free_irq() implicitly synchronizes interrupts and, with that
> removed, I was unable to prove that later actions in
> stmmac_xdp_release() are safe when there are concurrent interrupts. To
> avoid problems let's also move the code to disable DMA interrupts earlier
> in the sequence and explicitly sync the interrupts handler(s).
>
> Signed-off-by: Daniel Thompson <daniel@riscstar.com>
> <snip>
> @@ -7156,10 +7201,8 @@ int stmmac_xdp_open(struct net_device *dev)
>  	stmmac_reset_queues_param(priv);
>
>  	/* DMA CSR Channel configuration */
> -	for (chan = 0; chan < dma_csr_ch; chan++) {
> +	for (chan = 0; chan < dma_csr_ch; chan++)
>  		stmmac_init_chan(priv, priv->ioaddr, priv->plat->dma_cfg, chan);
> -		stmmac_disable_dma_irq(priv, priv->ioaddr, chan, 1, 1);
> -	}

Sashiko picked up that stmmac_init_chan() has the effect of enabling DMA
irqs, making this code unsafe.

Given stmmac_xdp_open() can only be called on a running interface (and that
stmmac_xdp_open() explicitly sets sph) then re-initializing with stmmac_init_chan()
should have no effect and we can drop that as well.

I'll double check with a code review across all the dwmac versions
before pushing out a v2!


Daniel.