linux-kernel - Re: [PATCH 1/5] dt-bindings: net: qca,ar803x: Add IPQ5018 Internal GE PHY support

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Message-ID:
 <DS7PR19MB888391F2D9675F1067D551659D65A@DS7PR19MB8883.namprd19.prod.outlook.com>
Date: Mon, 26 May 2025 17:43:30 +0400
From: George Moussalem <george.moussalem@...look.com>
To: Andrew Lunn <andrew@...n.ch>
Cc: Heiner Kallweit <hkallweit1@...il.com>,
 Russell King <linux@...linux.org.uk>, "David S. Miller"
 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Florian Fainelli <f.fainelli@...il.com>,
 Philipp Zabel <p.zabel@...gutronix.de>,
 Bjorn Andersson <andersson@...nel.org>,
 Konrad Dybcio <konradybcio@...nel.org>,
 Michael Turquette <mturquette@...libre.com>, Stephen Boyd
 <sboyd@...nel.org>, netdev@...r.kernel.org, devicetree@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
 linux-clk@...r.kernel.org
Subject: Re: [PATCH 1/5] dt-bindings: net: qca,ar803x: Add IPQ5018 Internal GE
 PHY support

On 5/26/25 17:34, Andrew Lunn wrote:
>> I couldn't agree more but I just don't know what these values are exactly as
>> they aren't documented anywhere. I'm working off the downstream QCA-SSDK
>> codelinaro repo. My *best guess* for the MDAC value is that it halves the
>> amplitude and current for short cable length, but I don't know what
>> algorithm is used for error detection and correction.
>>
>> What I do know is that values must be written in a phy to phy link
>> architecture as the 'cable length' is short, a few cm at most. Without
>> setting these values, the link doesn't work.
>>
>> With the lack of proper documentation, I could move the values to the driver
>> itself and convert it to a boolean property such as qca,phy-to-phy-dac or
>> something..
> 
> Making it a boolean property is good. Please include in the binding
> the behaviour when the bool is not present.

Thanks, will do.

> 
> What you are really describing here is a sort cable, not phy-to-phy,
> since a PHY is always connected to another PHY. So i think the
> property name should be about the sort cable/distance.

The two common archictures across IPQ5018 boards are:
1. IPQ5018 PHY --> MDI --> RJ45 connector
2. IPQ5018 PHY --> MDI --> External PHY (ex. a PHY of a qca8337 switch)
Only in scenario 2 (phy to phy architecture), DAC values need to be set 
to accommodate for the short cable length. Nothing needs to be set in 
scenario 1 when connected directly to an RJ45 connector.

if not, qcom,phy-to-phy-dac, perhaps qcom,dac-short-cable or any other 
suggestions?

> 
> 	Andrew

Best regards,
George