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

[Vladimir Oltean]

On Wed, Aug 04, 2021 at 07:58:15PM +0530, Prasanna Vengateshan wrote:
> On Wed, 2021-08-04 at 13:46 +0300, Vladimir Oltean wrote:
> > The problem is that I have no clear migration path for the drivers I
> > maintain, like sja1105, and I suspect that others might be in the exact
> > same situation.
> > 
> > Currently, if the sja1105 needs to add internal delays in a MAC-to-MAC
> > (fixed-link) setup, it does that based on the phy-mode string. So
> > "rgmii-id" + "fixed-link" means for sja1105 "add RX and TX RGMII
> > internal delays", even though the documentation now says "the MAC should
> > not add the RX or TX delays in this case".
> > 
> > There are 2 cases to think about, old driver with new DT blob and new
> > driver with old DT blob. If breakage is involved, I am not actually very
> > interested in doing the migration, because even though the interpretation
> > of the phy-mode string is inconsistent between the phy-handle and fixed-link
> > case (which was deliberate), at least it currently does all that I need it to.
> > 
> > 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? 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?
> > If it is an error, should all MAC drivers check for it? And if it is an
> > error, does it not make migration even more difficult (adding an
> > rx-internal-delay-ps property to a MAC OF node which already uses
> > "rgmii-id" would be preferable to also having to change the "rgmii-id"
> > to "rgmii", because an old kernel might also need to work with that DT
> > blob, and that will ignore the new rx-internal-delay-ps property).
> 
> 
> Considering the PHY is responsible to add internal delays w.r.to phy-mode, "*-
> tx-internal-delay-ps" approach that i was applying to different connections as
> shown below by bringing up different examples.
> 
> 1) Fixed-link MAC-MAC: 
>        port@4 {
>             .....
>             phy-mode = "rgmii";
>             rx-internal-delay-ps = <xxx>;
>             tx-internal-delay-ps = <xxx>;
>             ethernet = <&ethernet>;
>             fixed-link {
>            	......
>             };
>           };
> 
> 2) Fixed-link MAC-Unknown:
>         port@5 {
>             ......
>             phy-mode = "rgmii-id";
>             rx-internal-delay-ps = <xxx>;
>             tx-internal-delay-ps = <xxx>;
>             fixed-link {
>            .	....
>             };
>           };
> 
> 3) Fixed-link :
>         port@5 {
>             ......
>             phy-mode = "rgmii-id";
>             fixed-link {
>               .....
>             };
>           };
> 
> From above examples,
> 	a) MAC node is responsible to add RGMII delay by parsing "*-internal-
> delay-ps" for (1) & (2). Its a known item in this discussion.
> 	b) Is rgmii-* to be ignored by the MAC in (2) and just apply the delays
> from MAC side? Because if its forced to have "rgmii", would it become just -
> >interface=*_MODE_RGMII and affects legacy?

Yes, I think the MAC would have to accept any "rgmii*" phy-mode in
fixed-link. The legacy behavior would be do to whatever it did before,
and the new behavior would be to NOT apply any MAC-level delays based on
the phy-mode value, but only based on the {rx,tx}-internal-delay-ps
properties if these are present, or fall back to the legacy behavior if
they aren't.

This way:
- New kernel with old DT blob falls back to legacy behavior
- New kernel with new DT blob finds the explicit {rx,tx}-internal-delay-ps
  properties and applies MAC-level delays only according to those, while
  accepting any phy-mode string
- Old kernel with new DT blob behaves the same as before, because it
  does not parse {rx,tx}-internal-delay-ps and we will not change its
  phy-mode.

> 	c) if MAC follows standard delay, then it needs to be validated against
> "*-internal-delay-ps", may be validating against single value and throw an
> error. Might be okay.

Drivers with no legacy might throw an error if:
- phy-mode == "rgmii-id" or "rgmii-rxid" and there is a non-zero rx-internal-delay-ps
- phy-mode == "rgmii-id" or "rgmii-txid" and there is a non-zero tx-internal-delay-ps

but considering that most drivers already have a legacy to support, I'm
not sure how useful that error will be.

> 	d) For 3), Neither MAC nor other side will apply delays. Expected.

In the "new" behavior, correct. In "legacy" behavior, they might have to.

> 3) MAC-PHY
> 
> 	i) &test3 {
> 		phy-handle = <&phy0>;
> 		phy-mode = "rgmii-id";
> 		phy0: ethernet-phy@xx {
> 			.....
> 			rx-internal-delay = <xxx>;
> 			tx-internal-delay = <xxx>;
> 		};
> 	  };
> 
> 	ii) &test4 {
> 		phy-handle = <&phy0>;
> 		phy-mode = "rgmii";
>         	rx-internal-delay-ps = <xxx>;
>         	tx-internal-delay-ps = <xxx>;
> 		phy0: ethernet-phy@xx {
> 			reg = <x>;
> 	        };
> 	     };
> 
> 
> For 3(i), I assume phy would apply internal delay values by checking its phydev-
> >interface.

PHY drivers have a phy_get_internal_delay() helper that takes into
consideration both the phy-mode value and the {rx,tx}-internal-delay
properties. In example 3(i), the {rx,tx}-internal-delay properties would
prevail as long as the PHY driver uses that helper.

> For 3(ii), MAC would apply the delays.
> 
> Overall, only (b) need a right decision? or any other items are missed?
> 
> 
> Prasanna V
>