Re: [PATCH v2 05/10] ARM: dts: aspeed: system1: Add RGMII support

[Ninad Palsule <ninad@linux.ibm.com>]

Hello Andrew & Jacky,

On 1/8/25 11:52, Andrew Lunn wrote:
>>> Does the mac0 TX clock have an extra long clock line on the PCB?
>>>
>>> Does the mac1 TX and RX clocks have extra long clock lines on the PCB?
>>>
>>> Anything but rgmii-id is in most cases wrong, so you need a really
>>> good explanation why you need to use something else. Something that
>>> shows you understand what is going on, and why what you have is
>>> correct.
>> Here I'll add some explanation.
>>
>> In our design, we hope the TX and RX RGMII delay are configured by our MAC side.
>> We can control the TX/RX RGMII delay on MAC step by step, it is not a setting to delay to 2 ns.
>> We are not sure the all target PHYs are support for RX internal delay.
>>
>> But ast2600 MAC1/2 delay cell cannot cover range to 2 ns, MAC 3/4 can do that.
>> Therefore, when using ast2600 MAC1/2, please enable the RX internal delay on the PHY side
>> to make up for the part we cannot cover.
>>
>> Summarize our design and we recommend.
>> AST2600 MAC1/2: rgmii-rxid
>> (RGMII with internal RX delay provided by the PHY, the MAC should not add an RX delay in this
>> case)
>> AST2600 MAC3/4: rgmii
>> (RX and TX delays are added by the MAC when required)
>>
>> rgmii and rgmii-rxid are referred from ethernet-controller.yaml file.
> O.K, so you have the meaning of phy-mode wrong. phy-mode effectively
> described the PCB. Does the PCB implement the 2ns delay via extra long
> clock lines or not. If the PCB has long clock lines, phy-mode is
> 'rgmii'. If the PCB does not have long clock lines, 'rgmii-id' is
> used, meaning either the MAC or the PHY needs to add the delays.
>
> The MAC driver is the one that reads the phy-mode from the DT, and
> then it decides what to do. 95% of linux MAC drivers simply pass it
> direct to the PHY. If the phy-mode is 'rgmii', the PHY does nothing,
> because the PCB has added the delays. If it is rgmii-id, it adds
> delays in both directions. This works, because nearly very RGMII PHY
> on the market does support RGMII delays.
>
> There is however a very small number of MAC drivers which do things
> differently. Renesas produced an RDK with a PHY which could not do
> delays in the PHY, and so were forced to do the delays in the
> MAC. Please look at how the ravb driver works. If the PCB does not add
> the delays, rmgii-id, the MAC driver adds the delays. It then masks
> the phy-mode it passes to of_phy_connect() back to 'rgmii', so that
> the PHY does not add any delays. If the PCB did add the delays,
> 'rgmii', the MAC driver does not add delays, and it passed rgmii to
> the PHY driver, and it also does not add delays.
>
> So, your MAC driver is broken. It is not correctly handling the
> phy-mode. First question is, how many boards are there in mainline
> which have broken phy-modes. If this is the first board, we can fix
> the MAC driver. If there are already boards in mainline we have to be
> much more careful when fixing this, so as not to regress boards which
> are already merged.

There are around 11 boards in Aspeed SOC with phy-mode set to "rgmii" 
(some of them are mac0&1 and others are mac2&3). "rgmii-rxid" is only mine.

No one in aspeed SOC using "rgmii-id".

>
> Humm, interesting. Looking at ftgmac100.c, i don't see where you
> configure the RGMII delays in the MAC?
>
> 	  Andrew
>