linux-kernel - Re: [PATCH v2 2/2] arm64: dts: qcom: qcs615-ride: Enable ethernet node

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Message-ID: <2556b02c-f884-40c2-a0d4-0c87da6e5332@quicinc.com>
Date: Wed, 27 Nov 2024 14:17:15 +0800
From: Yijie Yang <quic_yijiyang@...cinc.com>
To: Andrew Lunn <andrew@...n.ch>,
        Konrad Dybcio
	<konrad.dybcio@....qualcomm.com>
CC: Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio
	<konradybcio@...nel.org>, Rob Herring <robh@...nel.org>,
        Krzysztof Kozlowski
	<krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Richard Cochran
	<richardcochran@...il.com>,
        <linux-arm-msm@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <netdev@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] arm64: dts: qcom: qcs615-ride: Enable ethernet
 node



On 2024-11-22 21:19, Andrew Lunn wrote:
>>>>>    +&ethernet {
>>>>> +    status = "okay";
>>>>> +
>>>>> +    pinctrl-0 = <&ethernet_defaults>;
>>>>> +    pinctrl-names = "default";
>>>>> +
>>>>> +    phy-handle = <&rgmii_phy>;
>>>>> +    phy-mode = "rgmii";
>>>>
>>>> That is unusual. Does the board have extra long clock lines?
>>>
>>> Do you mean to imply that using RGMII mode is unusual? While the EMAC controller supports various modes, due to hardware design limitations, only RGMII mode can be effectively implemented.
>>
>> Is that a board-specific issue, or something that concerns the SoC itself?
> 
> Lots of developers gets this wrong.... Searching the mainline list for
> patchs getting it wrong and the explanation i have given in the past
> might help.
> 
> The usual setting here is 'rgmmii-id', which means something needs to
> insert a 2ns delay on the clock lines. This is not always true, a very
> small number of boards use extra long clock likes on the PCB to add
> the needed 2ns delay.
> 
> Now, if 'rgmii' does work, it means something else is broken
> somewhere. I will let you find out what.

The 'rgmii' does function correctly, but it does not necessarily mean 
that a time delay is required at the board level. The EPHY can also 
compensate for the time skew.

> 
>>>>> +    max-speed = <1000>;
>>>>
>>>> Why do you have this property? It is normally used to slow the MAC
>>>> down because of issues at higher speeds.
>>>
>>> According to the databoot, the EMAC in RGMII mode can support speeds of up to 1Gbps.
>>
>> I believe the question Andrew is asking is "how is that effectively
>> different from the default setting (omitting the property)?"
> 
> Correct. If there are no issues at higher speeds, you don't need
> this. phylib will ask the PHY what it is capable of, and limit the
> negotiated speeds to its capabilities. Occasionally you do see an
> RGMII PHY connected to a MII MAC, because a RGMII PHY is cheaper...
> 
> 	Andrew

It does unnecessary, I will remove it.

-- 
Best Regards,
Yijie