linux-kernel - Re: [PATCH 3/3] net: marvell: prestera: force good base mac

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Date: Mon, 11 Mar 2024 15:29:51 +0100
From: Przemek Kitszel <przemyslaw.kitszel@...el.com>
To: Elad Nachman <enachman@...vell.com>, <taras.chornyi@...ision.eu>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<pabeni@...hat.com>, <andrew@...n.ch>, <kory.maincent@...tlin.com>,
	<thomas.petazzoni@...tlin.com>, <miquel.raynal@...tlin.com>,
	<netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] net: marvell: prestera: force good base mac

On 3/11/24 14:51, Elad Nachman wrote:
> From: Elad Nachman <enachman@...vell.com>
> 
> Since each switchport MAC address uses the switch base mac address
> and adds the physical port number to it,
> Force the last byte of the switch base mac address to be at
> least 128, so when adding to it, we will not wrap around the

to be at *most* 128

> previous (more significant) mac address byte, resulting in a
> warning message.
> 
> Signed-off-by: Elad Nachman <enachman@...vell.com>
> ---
>   drivers/net/ethernet/marvell/prestera/prestera_main.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/ethernet/marvell/prestera/prestera_main.c b/drivers/net/ethernet/marvell/prestera/prestera_main.c
> index bcaa8ea27b49..e17b1a24fe18 100644
> --- a/drivers/net/ethernet/marvell/prestera/prestera_main.c
> +++ b/drivers/net/ethernet/marvell/prestera/prestera_main.c
> @@ -859,7 +859,9 @@ static int prestera_switch_set_base_mac_addr(struct prestera_switch *sw)
>   	if (sw->np)
>   		ret = of_get_mac_address(sw->np, sw->base_mac);
>   	if (!is_valid_ether_addr(sw->base_mac) || ret) {
> -		eth_random_addr(sw->base_mac);
> +		do {
> +			eth_random_addr(sw->base_mac);
> +		} while (sw->base_mac[5] > 0x80);

Instead of this loop, that uses 6 bytes of random data at each step,
I would just fix the last byte.

Either by calling get_random_u8() in a loop, or perhaps better, just
toggle of MSB unconditionally:
sw->base_mac[5] &= ~0x80; // or '&= 127'

what would change your condition in commit message to "to be at most
127", but I think that should be fine, granted simpler code.

>   		dev_info(prestera_dev(sw), "using random base mac address\n");
>   	}
>