Subject: Re: [PATCH v4 net-next 04/15] net: enetc: ensure we always have a minimum number of TXQs for stack

From: Simon Horman <[email-masked]>
Date: Wed, 1 Feb 2023 14:43:44 +0100

On Mon, Jan 30, 2023 at 07:31:34PM +0200, Vladimir Oltean wrote:
> Currently it can happen that an mqprio qdisc is installed with num_tc 8,
> and this will reserve 8 (out of 8) TXQs for the network stack. Then we
> can attach an XDP program, and this will crop 2 TXQs, leaving just 6 for
> mqprio. That's not what the user requested, and we should fail it.
> 
> On the other hand, if mqprio isn't requested, we still give the 8 TXQs
> to the network stack (with hashing among a single traffic class), but
> then, cropping 2 TXQs for XDP is fine, because the user didn't
> explicitly ask for any number of TXQs, so no expectations are violated.
> 
> Simply put, the logic that mqprio should impose a minimum number of TXQs
> for the network never existed. Let's say (more or less arbitrarily) that
> without mqprio, the driver expects a minimum number of TXQs equal to the
> number of CPUs (on NXP LS1028A, that is either 1, or 2). And with mqprio,
> mqprio gives the minimum required number of TXQs.
> 
> Signed-off-by: Vladimir Oltean <[email-masked]>

The nit below notwithstanding,

Reviewed-by: Simon Horman <[email-masked]>

...

> diff --git a/drivers/net/ethernet/freescale/enetc/enetc.h b/drivers/net/ethernet/freescale/enetc/enetc.h
> index 1fe8dfd6b6d4..e21d096c5a90 100644
> --- a/drivers/net/ethernet/freescale/enetc/enetc.h
> +++ b/drivers/net/ethernet/freescale/enetc/enetc.h
> @@ -369,6 +369,9 @@ struct enetc_ndev_priv {
>  
>  	struct psfp_cap psfp_cap;
>  
> +	/* Minimum number of TX queues required by the network stack */
> +	unsigned int min_num_stack_tx_queues;
> +

It is probably not important.
But I do notice there are several holes in struct enetc_ndev_priv
that would fit this field.

>  	struct phylink *phylink;
>  	int ic_mode;
>  	u32 tx_ictt;
> -- 
> 2.34.1