netdev - RE: [PATCH v4 net-next 08/15] net/sched: mqprio: allow offloading drivers to request queue count validation

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Message-ID: <AM9PR04MB83977621774954ABED31037896D39@AM9PR04MB8397.eurprd04.prod.outlook.com>
Date:   Mon, 30 Jan 2023 18:37:02 +0000
From:   Claudiu Manoil <claudiu.manoil@....com>
To:     Vladimir Oltean <vladimir.oltean@....com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Vinicius Costa Gomes <vinicius.gomes@...el.com>,
        Kurt Kanzenbach <kurt@...utronix.de>,
        Jacob Keller <jacob.e.keller@...el.com>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Jiri Pirko <jiri@...nulli.us>
Subject: RE: [PATCH v4 net-next 08/15] net/sched: mqprio: allow offloading
 drivers to request queue count validation

> -----Original Message-----
> From: Vladimir Oltean <vladimir.oltean@....com>
> Sent: Monday, January 30, 2023 7:32 PM
[...]
> Subject: [PATCH v4 net-next 08/15] net/sched: mqprio: allow offloading drivers
> to request queue count validation
>

[...]

> +static int mqprio_validate_queue_counts(struct net_device *dev,
> +					const struct tc_mqprio_qopt *qopt)
> +{
> +	int i, j;
> +
> +	for (i = 0; i < qopt->num_tc; i++) {
> +		unsigned int last = qopt->offset[i] + qopt->count[i];
> +
> +		/* Verify the queue count is in tx range being equal to the
> +		 * real_num_tx_queues indicates the last queue is in use.
> +		 */
> +		if (qopt->offset[i] >= dev->real_num_tx_queues ||
> +		    !qopt->count[i] ||
> +		    last > dev->real_num_tx_queues)
> +			return -EINVAL;
> +
> +		/* Verify that the offset and counts do not overlap */
> +		for (j = i + 1; j < qopt->num_tc; j++) {
> +			if (last > qopt->offset[j])
> +				return -EINVAL;
> +		}
> +	}
> +
> +	return 0;
> +}
> +

Not related to this series, but the above O(n^2) code snippet....
If last[i] := offset[i] + count[i] and last[i] <= offset[i+1],
then offset[i] + count[i] <= offset[i+1] for every i := 0, num_tc - 1.

In other words, it's enough to check that last[i] <= offset[i+1] to make
sure there's no interval overlap, and it's O(n).