Re: [PATCH 3/4] arm64: dts: socfpga: agilex5: enable gmac2 on the Agilex5 dev kit

[Matthew Gerlach <matthew.gerlach@altera.com>]

On 7/14/25 3:50 PM, Andrew Lunn wrote:
> On Mon, Jul 14, 2025 at 03:29:21PM -0700, Matthew Gerlach wrote:
> > 
> > 
> > On 7/14/25 11:52 AM, Andrew Lunn wrote:
> > > On Mon, Jul 14, 2025 at 11:09:33AM -0700, Matthew Gerlach wrote:
> > > > > > On 7/14/25 10:25 AM, Andrew Lunn wrote:
> > > > > > +&gmac2 {
> > > > > > +	status = "okay";
> > > > > > +	phy-mode = "rgmii";	/* Delays implemented by the IO ring of the Agilex5 SOCFPGA. */
> > > > > > > Please could you explain in more details what this means.
> > > > > > > The normal meaning for 'rgmii' is that the PCB implements the
> > > delay. I
> > > > > just want to fully understand what this IO ring is, and if it is part
> > > > > of the PCB.
> > > > > The IO ring is the logic in the Agilex5 that controls the pins on
> > > the chip.
> > > > It is this logic that sits between the MAC IP in the Agilex5 and the pins
> > > > connected to the PCB that is inserting the necessary delays. Technically the
> > > > PCB is not implementing the delays, but the "wires" between the MAC and the
> > > > external pins of the Agilex5 are implementing the delay. It seems to me that
> > > > "rgmii" is a more accurate description of the hardware than "rgmii-id" in
> > > > this case.
> > > 
> > > Is this delay hard coded, physically impossible to be disabled? A
> > > syntheses option? Can it be changed at run time? Is the IO ring under
> > > the control of a pinctrl driver? Can i use the standard 'skew-delay'
> > > DT property to control this delay?
>
> > The delay is not hard coded. It is a synthesis option that can be disabled.
>
> Is there a register you can read which tells you if it is
> enabled/disabled?

There are a collection of registers that could be examined to determine 
if delay has been inserted by IO ring.

>
> > The delay in the IO ring can be disabled, but implementing the delay in the
> > IO ring allows for RGMII phys that don't implement the delay.
>
> All RGMII PHYs which Linux support have the ability to do delays. And
> we recommend the PHY does the delay, just to keep all systems the
> same. There are a few exceptions, mostly because the MAC has hard
> coded delays which cannot be disabled, but i guess 90% of systems have
> the PHY doing the 2ns delays.
>
> So, phy-mode should be set to 'rgmii-id', since the PCB does not add
> the delays.

Understood. The PCB is not adding delays; so I'll change the phy-mode to 
be rgmii-id.

>
> Ideally, you want to read from the IO ring if it is synthesised to do
> the 2ns delays. Assuming it is enabled, you then mask the phy-mode
> before connecting to the PHY, so avoiding double delays.
>
> 	Andrew
>

Currently, the registers in question are considered secure and only 
accessible by
Arm Trusted Firmware (ATF). I will investigate implementing this delay 
detecting
logic in a future patchset.

Thanks for the feedback,

Matthew Gerlach