RE: [v3 2/2] dsa: mv88e6xxx: make serdes SGMII/Fiber output amplitude configurable

[Holger Brunck <holger.brunck@hitachienergy.com>]

Hi Marek,

> > The mv88e6352, mv88e6240 and mv88e6176  have a serdes interface. This
> > patch allows to configure the output swing to a desired value in the
> > devicetree node of the port. As the chips only supports eight
> > dedicated values we return EINVAL if the value in the DTS does not match.
> >
> > CC: Andrew Lunn <andrew@lunn.ch>
> > CC: Jakub Kicinski <kuba@kernel.org>
> > CC: Marek Behún <kabel@kernel.org>
> > Signed-off-by: Holger Brunck <holger.brunck@hitachienergy.com>
> 
> Holger, Andrew,
> 
> there is another issue with this, which I only realized yesterday. What if the
> different amplitude needs to be set only for certain SerDes modes?
> 
> I am bringing this up because I discovered that on Turris Mox we need to
> increase SerDes output amplitude when A3720 SOC is connected directly to
> 88E6141 switch, but only for 2500base-x mode. For 1000base-x, the default
> amplitude is okay. (Also when the SOC is connected to 88E6190, the amplitude
> does not need to be changed at all.)
> 

on my board I have a fixed link connected with SGMII and there is no dedicated
value given in the manual.

> I plan to solve this in the comphy driver, not in device-tree.
> 
> But if the solution is to be done in DTS, shouldn't there be a possibility to define
> the amplitude for a specific serdes mode only?
> 
> For example
>   serdes-2500base-x-tx-amplitude-millivolt
> or
>   serdes-tx-amplitude-millivolt-2500base-x
> or
>   serdes-tx-amplitude-millivolt,2500base-x
> ?
> 
> What do you think?
> 

in the data sheet for the MV6352 I am using there are no dedicated values stated for
different modes at all, they can be chosen arbitrary. So in my case I think it makes
sense to keep it as it is for now. Other driver may have other needs and may enhance
this later on.
 
Best regards
Holger