netdev - Re: [PATCH net-next 09/12] net: dsa: tag_ocelot: create separate tagger for Seville

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Date:   Mon, 15 Feb 2021 15:19:31 +0200
From:   Vladimir Oltean <olteanv@...il.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     kbuild@...ts.01.org, "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        lkp@...el.com, kbuild-all@...ts.01.org,
        Andrew Lunn <andrew@...n.ch>,
        Florian Fainelli <f.fainelli@...il.com>,
        Vivien Didelot <vivien.didelot@...il.com>,
        Richard Cochran <richardcochran@...il.com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH net-next 09/12] net: dsa: tag_ocelot: create separate
 tagger for Seville

Hi Dan,

On Mon, Feb 15, 2021 at 04:00:04PM +0300, Dan Carpenter wrote:
> db->index is less than db->num_ports which 32 or less but sometimes it
> comes from the device tree so who knows.

The destination port mask is copied into a 12-bit field of the packet,
starting at bit offset 67 and ending at 56:

static inline void ocelot_ifh_set_dest(void *injection, u64 dest)
{
	packing(injection, &dest, 67, 56, OCELOT_TAG_LEN, PACK, 0);
}

So this DSA tagging protocol supports at most 12 bits, which is clearly
less than 32. Attempting to send to a port number > 12 will cause the
packing() call to truncate way before there will be 32-bit truncation
due to type promotion of the BIT(port) argument towards u64.

> The ocelot_ifh_set_dest() function takes a u64 though and that
> suggests that BIT() should be changed to BIT_ULL().

I understand that you want to silence the warning, which fundamentally
comes from the packing() API which works with u64 values and nothing of
a smaller size. So I can send a patch which replaces BIT(port) with
BIT_ULL(port), even if in practice both are equally fine.