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

On Tue,  7 Dec 2021 20:07:30 +0100
Holger Brunck [off-list ref] wrote:

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.)

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?

Marek