Re: [PATCH v3 net-next 05/10] net: dsa: microchip: add DSA support for microchip lan937x

On Sat, 2021-08-07 at 17:40 +0200, Andrew Lunn wrote:

EXTERNAL EMAIL: Do not click links or open attachments unless you know the
content is safe

quoted

I am not even clear what is the expected canonical behavior for a MAC
driver. It parses rx-internal-delay-ps and tx-internal-delay-ps, and
then what?

So best practices are based around a MAC-PHY link. phy-mode is passed
to the PHY, and the MAC does not act upon it. MAC rx-internal-delay-ps
and tx-internal-delay-ps can be used to fine tune the link. You can
use them to add and sometimes subtract small amounts of delay.

quoted

It treats all "rgmii*" phy-mode strings identically? Or is it an
error to have "rgmii-rxid" for phy-mode and non-zero
rx-internal-delay-ps?

I would say the first is correct, the second statement is false. You
should always be able to fine tune the link, independent of the PHY
mode.

We also have to consider the case when the PHY is not actually able to
implement the delay. It hopefully returns -EOPNOTSUPP for anything
other than "rgmii". You can then put the full 2ns delay into
tx-internal-delay-ps nd rx-internal-delay-ps.

I hope that using "*-internal-delay-ps" for Mac would be the right option.
Shall i include these changes as we discussed in next revision of the patch?