netdev - Re: [PATCH v3 1/2] dt-bindings: net: Add ICSSG Ethernet Driver bindings

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Date:   Fri, 6 Jan 2023 13:15:12 +0200
From:   Roger Quadros <rogerq@...nel.org>
To:     Andrew Lunn <andrew@...n.ch>
Cc:     Md Danish Anwar <danishanwar@...com>,
        "Andrew F. Davis" <afd@...com>, Tero Kristo <t-kristo@...com>,
        Suman Anna <s-anna@...com>, YueHaibing <yuehaibing@...wei.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "David S. Miller" <davem@...emloft.net>, nm@...com,
        ssantosh@...nel.org, srk@...com, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org, netdev@...r.kernel.org,
        linux-omap@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 1/2] dt-bindings: net: Add ICSSG Ethernet Driver
 bindings

On 05/01/2023 19:13, Andrew Lunn wrote:
>>>> On 23/12/2022 16:28, Andrew Lunn wrote:
>>>>>> +        ethernet-ports {
>>>>>> +            #address-cells = <1>;
>>>>>> +            #size-cells = <0>;
>>>>>> +            pruss2_emac0: port@0 {
>>>>>> +                reg = <0>;
>>>>>> +                phy-handle = <&pruss2_eth0_phy>;
>>>>>> +                phy-mode = "rgmii-rxid";
>>>>>
>>>>> That is unusual. Where are the TX delays coming from?
>>>>
>>>> >From the below property
>>>>
>>>> +                ti,syscon-rgmii-delay = <&scm_conf 0x4120>;
>>>>
>>>> The TX delay can be enabled/disabled from within the ICSSG block.
>>>>
>>>> If this property exists and PHY mode is neither PHY_INTERFACE_MODE_RGMII_ID
>>>> nor PHY_INTERFACE_MODE_RGMII_TXID then the internal delay is enabled.
>>>>
>>>> This logic is in prueth_config_rgmiidelay() function in the introduced driver.
>>>
>>> What nearly every other MAC driver does is pass the phy-mode to the
>>> PHY and lets the PHY add the delays. I would recommend you do that,
>>> rather than be special and different.
>>
>>
>> If I remember right we couldn't disable MAC TX delay on some earlier silicon
>> so had to take this route. I don't remember why we couldn't disable it though.
>>
>> In more recent Silicon Manuals I do see that MAC TX delay can be enabled/disabled.
>> If this really is the case then we should change to
>>
>>  phy-mode = "rgmii-id";
>>
>> And let PHY handle the TX+RX delays.
> 
> DT describes the board. PHY mode indicates what delays the board
> requires, because the board itself is not performing the delays by
> using extra long lines. So typically, phy-mode is rgmii-id, indicating
> delays need to be added somewhere in both directions.
> 
> Who adds the delays is then between the MAC and the PHY. In most
> cases, the MAC does nothing, and passes phy-mode to the PHY and the
> PHY does it.
> 
> But it is also possible for the MAC to do the delay. So if you cannot
> actually disable the TX delay in the MAC, that is O.K. But you need to
> modify phy-mode you pass to the PHY to indicate the MAC is doing the
> delay, otherwise the PHY will additionally do the delay. So your DT
> will contain rgmii-id, because that is what the board requires, but
> the MAC will pass rmgii-rxid to the PHY, since that is what the PHY
> needs to add.

Thanks for the explanation. :)

cheers,
-roger