Subject: Re: [PATCH net-next 1/5] ethtool: Add RSS indirection table resize helpers

From: Jakub Kicinski <[email-masked]>
Date: Tue, 3 Mar 2026 16:03:34 -0800

On Tue,  3 Mar 2026 19:15:29 +0100 Björn Töpel wrote:
> +/**
> + * ethtool_rxfh_indir_can_resize - Check if an indirection table can be resized
> + * @tbl: indirection table
> + * @old_size: current number of entries in the table
> + * @new_size: desired number of entries
> + *
> + * Validate that @tbl can be resized from @old_size to @new_size without
> + * data loss. Read-only; does not modify the table.
> + *
> + * Return: 0 if resize is possible, -%EINVAL otherwise.
> + */
> +int ethtool_rxfh_indir_can_resize(const u32 *tbl, u32 old_size, u32 new_size)

maybe pass it extack so that all the callers don't have to set it
themslves?

> +{
> +	if (old_size == 0 || new_size == 0)
> +		return -EINVAL;

defensive programming?

> +	if (new_size == old_size)
> +		return 0;
> +
> +	if (new_size < old_size) {
> +		if (old_size % new_size != 0)

please try not to compare things to zero in C, it reads as double
negation

BTW I guess that this expression to cover both branches would be
unreadable right? :D

	if (new_size + old_size % min(new_size, old_size))
		return -EINVAL;

> +			return -EINVAL;
> +		if (!ethtool_rxfh_indir_is_periodic(tbl, old_size, new_size))
> +			return -EINVAL;
> +		return 0;
> +	}
> +
> +	if (new_size % old_size != 0)
> +		return -EINVAL;
> +	return 0;
> +}
> +EXPORT_SYMBOL(ethtool_rxfh_indir_can_resize);
> +
> +/* Resize without validation; caller must have called can_resize first */
> +static void __ethtool_rxfh_indir_resize(u32 *tbl, u32 old_size, u32 new_size)
> +{
> +	u32 i;
> +
> +	/* Unfold (grow): replicate existing pattern; fold is a no-op on the data */
> +	for (i = old_size; i < new_size; i++)
> +		tbl[i] = tbl[i % old_size];
> +}

> +/**
> + * ethtool_rxfh_contexts_resize_all - Resize all RSS context indirection tables
> + * @dev: network device
> + * @new_indir_size: new indirection table size
> + *
> + * Resize the indirection table of every non-default RSS context to
> + * @new_indir_size. Intended to be called from drivers when the
> + * device's indirection table size changes (e.g. on channel count
> + * change). An %ETHTOOL_MSG_RSS_NTF is sent for each resized context.
> + *
> + * All contexts are validated before any are modified, so either all
> + * contexts are resized or none are.
> + *
> + * Return: 0 on success, negative errno on failure.
> + */
> +int ethtool_rxfh_contexts_resize_all(struct net_device *dev, u32 new_indir_size)
> +{
> +	struct ethtool_rxfh_context *ctx;
> +	unsigned long context;
> +	int ret;
> +
> +	if (dev->ethtool_ops->rxfh_indir_space == 0 ||

!dev..

> +	    new_indir_size > dev->ethtool_ops->rxfh_indir_space)

extack needed?

> +		return -EINVAL;
> +
> +	scoped_guard(mutex, &dev->ethtool->rss_lock) {
> +		xa_for_each(&dev->ethtool->rss_ctx, context, ctx) {
> +			ret = ethtool_rxfh_indir_can_resize(ethtool_rxfh_context_indir(ctx),

80 char limit width limit is still king in netdev
please shrink the length of the helpers you're adding?
And/or take the locks in a normal way..

> +							    ctx->indir_size, new_indir_size);
> +			if (ret)
> +				return ret;
> +		}
> +
> +		xa_for_each(&dev->ethtool->rss_ctx, context, ctx) {
> +			__ethtool_rxfh_indir_resize(ethtool_rxfh_context_indir(ctx),
> +						    ctx->indir_size, new_indir_size);
> +			ctx->indir_size = new_indir_size;
> +		}
> +	}
> +
> +	xa_for_each(&dev->ethtool->rss_ctx, context, ctx)
> +		ethtool_rss_notify(dev, ETHTOOL_MSG_RSS_NTF, context);

Why a separate loop ?

> +	return 0;
> +}
> +EXPORT_SYMBOL(ethtool_rxfh_contexts_resize_all);