[OpenWrt-Devel] [PATCH V2] b53: override CPU port state on BCM5301X with CPU port other than 8

[Rafał Miłecki]

On 15 April 2015 at 07:37, Ian Kent <raven at themaw.net> wrote:
> On Sun, 2015-04-12 at 19:01 +0200, Rafał Miłecki wrote:
>> Newer revisions (5+) of BCM53011 and probably all revs of BCM53012
>> require overriding CPU port to work. So far we were handling it only for
>> CPU port 8, but some devices may use e.g. port 5. In such case we need
>> to use recently defined GMII_PORT registers.
>> It was tested for regressions on BCM53011 revs 2 & 3. It was also
>> confirmed to fix switch on some internal Broadcom board.
>
> CPU port as 5 also allows the switch on my R8000 (BCM53012) rev 5 device
> to function.
>
> The puzzle is the the Vendor firmware uses port 8 as the cpu port (+ 5
> and 7 for some unknown purpose) for these higher rev switches. We
> clearly don't know what the relationship is between ports 5, 7 and 8 and
> I suspect there's more to it than just configuring the three ports
> properly. But my code to set ports 5, 7 and 8 when cpu is 8 might not be
> correct either, don't know yet.

I guess this is simply for performance reasons. Imagine there are two
1 Gbps devices connected to the router and that they want to "talk"
with router system (for whatever reason). To achieve max speeds we
need router system to Tx/Rx with 2 Gbps. This is impossible if switch
is connected to CPU with a single 1 Gbps port. By combining 2/3 ports
this should be possible (of course you're still limited by CPU
performance).
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