Subject: Re: [net-next,08/14] net: enetc: add multiple command BD rings support

From: Simon Horman <[email-masked]>
Date: Wed, 18 Mar 2026 14:32:01 +0000

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---
net: enetc: add multiple command BD rings support

This commit adds support for multiple command BD rings in the NTMP
driver. When the NETC switch has multiple command BD rings and one is
busy, the driver can select an alternate ring to execute commands.

> diff --git a/drivers/net/ethernet/freescale/enetc/ntmp.c b/drivers/net/ethernet/freescale/enetc/ntmp.c
> index 9fc3422137fc..e23a32946762 100644
> --- a/drivers/net/ethernet/freescale/enetc/ntmp.c
> +++ b/drivers/net/ethernet/freescale/enetc/ntmp.c
> @@ -117,6 +117,25 @@ static void ntmp_clean_cbdr(struct netc_cbdr *cbdr)
>  	cbdr->next_to_clean = i;
>  }
>
> +static struct netc_cbdr *netc_select_cbdr(struct ntmp_user *user)
> +{
> +	int cpu, i;
> +
> +	for (i = 0; i < user->cbdr_num; i++) {
> +		if (spin_is_locked(&user->ring[i].ring_lock))
> +			continue;
> +
> +		return &user->ring[i];
> +	}
> +
> +	/* If all the command BDRs are busy now, we select
> +	 * one of them, but need to wait for a while to use.
> +	 */
> +	cpu = smp_processor_id();
> +
> +	return &user->ring[cpu % user->cbdr_num];
> +}
> +

Can this trigger a preemption warning? netc_select_cbdr() is called from
netc_xmit_ntmp_cmd() before spin_lock_bh() is acquired, and the callers
(ntmp_maft_add_entry, ntmp_fdbt_add_entry, etc.) run in process context
with preemption enabled.

Using smp_processor_id() in preemptible code triggers a
"BUG: using smp_processor_id() in preemptible code" warning under
CONFIG_DEBUG_PREEMPT.

Since the CPU value here is only used as a hint for ring selection and not
for correctness, would raw_smp_processor_id() be more appropriate?

>  static int netc_xmit_ntmp_cmd(struct ntmp_user *user, union netc_cbd *cbd)
>  {
>  	union netc_cbd *cur_cbd;
> @@ -125,10 +144,10 @@ static int netc_xmit_ntmp_cmd(struct ntmp_user *user, union netc_cbd *cbd)
>  	u16 status;
>  	u32 val;
>
> -	/* Currently only i.MX95 ENETC is supported, and it only has one
> -	 * command BD ring
> -	 */
> -	cbdr = &user->ring[0];
> +	if (user->cbdr_num == 1)
> +		cbdr = &user->ring[0];
> +	else
> +		cbdr = netc_select_cbdr(user);
>
>  	spin_lock_bh(&cbdr->ring_lock);