Re: [PATCH net-next 05/11] net: macb: allocate tieoff descriptor once across device lifetime

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

On 1.4.2026 18:39, Théo Lebrun wrote:
> The tieoff descriptor is a RX DMA descriptor ring of size one. It gets
> configured onto queues for Wake-on-LAN during system-wide suspend when
> hardware does not support disabling individual queues
> (MACB_CAPS_QUEUE_DISABLE).
> 
> MACB/GEM driver allocates it alongside the main RX ring
> inside macb_alloc_consistent() at open. Free is done by
> macb_free_consistent() at close.
> 
> Change to allocate once at probe and free on probe failure or device
> removal. This makes the tieoff descriptor lifetime much longer,
> avoiding repeating coherent buffer allocation on each open/close cycle.
> 
> Main benefit: we dissociate its lifetime from the main ring's lifetime.
> That way there is less work to be doing on resources (re)alloc. This
> currently happens on close/open, but will soon also happen on context
> swap operations (set_ringparam, change_mtu, set_channels, etc).
> 
> Signed-off-by: Théo Lebrun <theo.lebrun@bootlin.com>
> ---
>  drivers/net/ethernet/cadence/macb_main.c | 70 
> ++++++++++++++++----------------

> [...]

> 
> +static int macb_alloc_tieoff(struct macb *bp)
> +{
> +	/* Tieoff is a workaround in case HW cannot disable queues, for PM. 
> */
> +	if (bp->caps & MACB_CAPS_QUEUE_DISABLE)
> +		return 0;
> +
> +	bp->rx_ring_tieoff = dma_alloc_coherent(&bp->pdev->dev,
> +						macb_dma_desc_get_size(bp),
> +						&bp->rx_ring_tieoff_dma,
> +						GFP_KERNEL);
> +	if (!bp->rx_ring_tieoff)
> +		return -ENOMEM;
> +
> +	return 0;
> +}

The old macb_init_tieoff() that wrote WRAP+USED into the
descriptor is deleted but its work is not replicated here.
dma_alloc_coherent zeroes the memory, so RX_USED=0 and the
hardware will treat it as a valid receive buffer pointing to
DMA address 0 during suspend.

Shouldn't this have a macb_set_addr() + ctrl=0 after the
allocation?

> [...]

Thanks
Nicolai