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

Hi Marek,

quoted

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 <redacted>

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