netdev - Re: [Uclinux-dist-devel] [PATCH 1/2] net: dsa: introduce STPID switch tagging handling code

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Message-ID: <AANLkTinPtd2xXzq0MpR517OsbZWUZjdp25x8FndhQ2CP@mail.gmail.com>
Date:	Wed, 21 Jul 2010 12:08:58 -0400
From:	Mike Frysinger <vapier.adi@...il.com>
To:	Lennert Buytenhek <buytenh@...tstofly.org>
Cc:	Karl Beldan <karl.beldan@...il.com>, netdev@...r.kernel.org,
	uclinux-dist-devel@...ckfin.uclinux.org,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [Uclinux-dist-devel] [PATCH 1/2] net: dsa: introduce STPID switch 
	tagging handling code

On Wed, Jul 21, 2010 at 11:35, Lennert Buytenhek wrote:
> On Wed, Jul 21, 2010 at 11:29:30AM -0400, Mike Frysinger wrote:
>> >> +     source_port = dsa_header[1] & 0x03;
>> >> +     if (source_port >= DSA_MAX_PORTS || ds->ports[source_port] == NULL)
>> >> +             goto out_drop;
>> >> +
>> >> +     if (((dsa_header[0] & ETH_P_8021QH) == ETH_P_8021QH) &&
>> >
>> > This is bogus -- what it does is:
>> >
>> >        if ((dsa_header[0] & 0x81) == 0x81)
>> >
>> > It doesn't look like you need to mask here at all.
>>
>> where does it say dsa_header[0] will always have 0x81 set ?
>
> Eh?
>
> This code is checking whether the packet has a STPID tag on it or not.
> A STPID tag exists if the first 12 nibbles are 0x810.
>
> You are checking whether the first 8 nibbles of this are equal to 0x81
> by doing:
>
>        if ((byte & 0x81) == 0x81)
>
> What if the first byte is 0x93?  Or 0xc5?

that was my point.  should it be masking or doing a raw compare ?

i have nfc as i had nothing to do with the creation of this code.  i
dont know the first thing about VLAN tags or anything else at that
level.
-mike
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