Re: [PATCH v2 net-next 06/21] net: usb: aqc111: Introduce link management

Hi Andrew, Florian,

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So the point is that MAC firmware is managing SERDES and system interface link.

Linux can manage that SERDES link between the MAC and the PHY. There
are two ways this can go:

1) You use phylib. When the PHY reports link, the adjust_link callback
in the MAC is called. The phydev structure contains information about
how you should configure the SERDES, SGMII, 2500Base-X, 5000Base-X. It
works, but it is not so nice.

2) phylink gives you a much nicer API to do the same. Again, the PHY
reports the link is up. phylink will then tell the MAC how to
configure its end of the SERDES. The problem with phylink is that it
expects a DT building. You don't have that, since this is a USB
device. But you also don't need a lot of the features of phylink like
SFPs, the i2c bus for the SFPs, GPIOs etc. So it should not be to hard
to make this work without device tree.

By using core linux code, we avoid bugs in firmware which nobody can
fix. The Linux core code should be well tested and supported, but
phylink is rather new, so might still have some corner cases.

I also cannot imaging parts of the PHY driver will not be re-usable
for other Aquantia PHYs. I have a board with an AQCS109 under my desk
waiting for me to do something with it. I really would like a better
PHY driver for it than the kernel currently has. Hopefully there is
some code reuse possibilities here.

Even if the firmware is helping, there is still value in using PHYLINK
to report things to Linux as Andrew is saying in that you get a lot of
things for free: auto-negotiation results, link_ksetttings_{get,set} etc.

Thanks for the detailed explanation, I agree separating phy logic would
really improve things.

But just in time we've got a new updates from HW management/system teams,
It looks like they are deprecating direct phy access firmware mode,
so the only thing we have to support in this driver is FW based interface
for the link management.

Production dongles will always have firmware fully controlling all the phy.
Thus, I think in next series we'll just cut off all the direct phy
access code.

I agree with Andrew we eventually need a better generic Aquantia PHYs support
in linux, hope to look into this matter soon.

Regards,
  Igor