Re: [PATCH net-next] net: mvneta: Fix validation of 2.5G HSGMII without comphy

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Sun, Nov 15, 2020 at 01:41:51AM +0100, Andreas Färber wrote:
> Commit 1a642ca7f38992b086101fe204a1ae3c90ed8016 (net: ethernet: mvneta:
> Add 2500BaseX support for SoCs without comphy) added support for 2500BaseX.
> 
> In case a comphy is not provided, mvneta_validate()'s check
>   state->interface == PHY_INTERFACE_MODE_2500BASEX
> could never be true (it would've returned with empty bitmask before),
> so that 2500baseT_Full and 2500baseX_Full do net get added to the mask.

This makes me nervous. It was intentional that if there is no comphy
configured in DT for SoCs such as Armada 388, then there is no support
to switch between 1G and 2.5G speed. Unfortunately, the configuration
of the comphy is up to the board to do, not the SoC .dtsi, so we can't
rely on there being a comphy on Armada 388 systems.

So, one of the side effects of this patch is it incorrectly opens up
the possibility of allowing 2.5G support on Armada 388 without a comphy
configured.

We really need a better way to solve this; just relying on the lack of
comphy and poking at a register that has no effect on Armada 388 to
select 2.5G speed while allowing 1G and 2.5G to be arbitarily chosen
doesn't sound like a good idea to me.

Clearly there are differences in mvneta hardware in different SoCs.
Maybe they should have used different compatibles, so the driver can
know which variant of the hardware it is dealing with, rather than
relying on presence/lack of comphy.

-- 
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