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SubjectRe: [PATCH 1/3] net: stmmac: dwmac-meson8b: Fix the RGMII TX delay on Meson8b/8m2 SoCs
Hi Andrew,

On Thu, Dec 26, 2019 at 1:01 PM Andrew Lunn <andrew@lunn.ch> wrote:
>
> > the MAC is not capable of generating an RX delay (at least as far as I know).
>
> So that immediately means rgmii is invalid as a phy-mode, since the
> documentation implies the MAC needs to add RX delay.
things turned out even more confusing thanks to your persistence (keep
reading, it will get better though) :-)

I have tested the following on my Odroid-C1 which has an RTL8211F PHY.
With patch #1 from this series I knew that the following was working:
- phy-mode = "rgmii" and 2ns TX delay on the MAC (RX delay is
seemingly not configured anywhere)
- phy-mode = "rgmii-txid" (again, the RX delay is seemingly not
configured anywhere)

with the patch to change the RX delay on the RTL8211F PHY I decided to
try out phy-mode = "rgmii-id": this broke Ethernet.
then I looked at the MAC registers and spotted that bits 13
(adj_enable) and 14 (adj_setup) are set (first time I'm noticing
this). unsetting them makes phy-mode = "rgmii-id" work!
I also confirmed the opposite case: unsetting bit 13 and 14 breaks
Ethernet with phy-mode = "rgmii-txid".

so it seems that there *is* a way to configure the RX delay on Meson8b
and Meson8m2 SoCs (at least).
I will spin up a RfC patch to discuss this with the Amlogic team and
because I don't know what these bits do exactly

> > it's mostly "broken" (high TX packet loss, slow TX speeds) for the two
> > supported boards with an RGMII PHY (meson8b-odroidc1.dts and
> > meson8m2-mxiii-plus.dts)
> > examples on the many ways it was broken will follow - feel free to
> > skip this part
>
> That is actually good. If it never worked, we don't need to worry
> about breaking it! We can spend our time getting this correct, and not
> have to worry about backwards compatibility, etc.
ACK

> > > What we normally say is make the MAC add no delays, and pass the
> > > correct configuration to the PHY so it adds the delay. But due to the
> > > strapping pin on the rtl8211f, we are in a bit of a grey area. I would
> > > suggest the MAC adds no delay, phy-mode is set to rmgii-id, the PHY
> > > driver adds TX delay in software, we assume the strapping pin is set
> > > to add RX delay, and we add a big fat comment in the DT.
> > >
> > > For the Micrel PHY, we do the same, plus add the vendor properties to
> > > configure the clock skew.
> > >
> > > But as i said, we are in a bit of a grey area. We can consider other
> > > options, but everything needs to be self consistent, between what the
> > > MAC is doing, what the PHY is doing, and what phy-mode is set to in
> > > DT.
>
> > do you think it's worth the effort to get clarification from Realtek
> > on the RX delay behavior (and whether there's a register to control
> > it)?
>
> You can ask. There are also datasheet here:
>
> http://files.pine64.org/doc/datasheet/rock64/RTL8211F-CG-Realtek.pdf
> https://datasheet.lcsc.com/szlcsc/1909021205_Realtek-Semicon-RTL8211F-CG_C187932.pdf
>
> It looks like both RX and TX delay can be controlled via
> strapping. But the register for controlling the TX delay is not
> documented.
I checked the mails I got from Realtek I while ago and they even
included the RX delay bits!
I even sent a patch two years ago but I must have dropped it at some
point (maybe I assumed that it wasn't relevant anymore - I don't
remember): [0]

> > you mentioned that there was breakage earlier this year, so I'm not sure anymore
> > (that leaves me thinking: asking them is still useful to get out of
> > this grey area)
>
> It was an Atheros PHY with breakage.
>
> If we can fully control the RX and TX delays, that would be great. It
> would also be useful if there was a way to determine how the PHY has
> been strapped. If we cannot fully control the delays but we can find
> out what delays it is using, we can check the requested configuration
> against the strapped configuration, and warn if they are different.
I am currently testing whether the pin strapping configuration can be
read back by the RX and TX delay registers
my Odroid-C1 has both strapped to GND which means off
but my Khadas VIM2 has TX delay strapped to ETH_VDDIO which means on
(RX delay is still strapped to GND)
once I am done testing I will send patches for the RTL8211F PHY driver


Martin


[0] https://patchwork.ozlabs.org/patch/843946/

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