linux-kernel - Re: [PATCH v1 1/2] dt-bindings: ethernet: eswin: add clock sampling control

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Message-ID: <973f2b4b-bf8d-4a77-a27a-47495bc21ff8@gmail.com>
Date: Tue, 27 Jan 2026 18:38:28 -0800
From: Bo Gan <ganboing@...il.com>
To: Min Lin <linmin@...incomputing.com>,
 "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Andrew Lunn <andrew@...n.ch>, Krzysztof Kozlowski <krzk@...nel.org>,
 李志 <lizhi2@...incomputing.com>,
 devicetree@...r.kernel.org, andrew+netdev@...n.ch, davem@...emloft.net,
 edumazet@...gle.com, kuba@...nel.org, robh@...nel.org, krzk+dt@...nel.org,
 conor+dt@...nel.org, netdev@...r.kernel.org, pabeni@...hat.com,
 mcoquelin.stm32@...il.com, alexandre.torgue@...s.st.com,
 linux-stm32@...md-mailman.stormreply.com,
 linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
 ningyu@...incomputing.com, pinkesh.vaghela@...fochips.com,
 weishangjuan@...incomputing.com
Subject: Re: [PATCH v1 1/2] dt-bindings: ethernet: eswin: add clock sampling
 control

Hi Min, Russell, Krzysztof,

On 1/26/26 22:14, Min Lin wrote:
> Hi Russell,
> 
> 
>> -----Original Messages-----
>> From: "Russell King (Oracle)" <linux@...linux.org.uk>
>> Send time:Tuesday, 27/01/2026 02:29:09
>> To: "Min Lin" <linmin@...incomputing.com>
>> Cc: "Bo Gan" <ganboing@...il.com>, "Andrew Lunn" <andrew@...n.ch>, "Krzysztof Kozlowski" <krzk@...nel.org>, 李志 <lizhi2@...incomputing.com>, devicetree@...r.kernel.org, andrew+netdev@...n.ch, davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, robh@...nel.org, krzk+dt@...nel.org, conor+dt@...nel.org, netdev@...r.kernel.org, pabeni@...hat.com, mcoquelin.stm32@...il.com, alexandre.torgue@...s.st.com, linux-stm32@...md-mailman.stormreply.com, linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, ningyu@...incomputing.com, pinkesh.vaghela@...fochips.com, weishangjuan@...incomputing.com
>> Subject: Re: Re: [PATCH v1 1/2] dt-bindings: ethernet: eswin: add clock sampling control
>>
>> On Mon, Jan 26, 2026 at 11:10:12AM +0800, Min Lin wrote:
>>> Due to chip backend reasons, there is already a ~4-5ns skew between the RX
>>> clock and data of the eth1 MAC controller inside the silicon.
>>
>> Let's analyse this.
>>
>> 	TXC / RXC	TXC / RXC
>> Speed	Clock rate	Clock period
>> 1G	125MHz		8ns
>> 100M	25MHz		40ns
>> 10M	2.5MHz		400ns
>>
>> The required skew for TXC and RXC at the receiver is specified to be
>> between 1 and 2.6ns irrespective of the speed. The edge of the clock
>> is also important: the rising edge indicates the lower 4 bits, and
>> the falling edge indicates the upper 4 bits.
>>
>> At 1G speed, with a "4 to 5ns" skew in the chip. If this is accurate,
>> then inverting the clock and adding 1ns of additional skew by some
>> means (PCB trace, or at the MAC or PHY) will give the required clock
>> at the receiver.
>>
> 
> Yes, that's exactly the case.
> 
>> The timing table in the RGMII standard (3.3) allows for Tcyc (the
>> clock rate) to be scaled, but there is no allowance for scaling
>> TskewR (the required 1 to 2.6ns skew.) This skew parameter is
>> fixed.
>>
>> So, at the other speeds, you are completely unable to meet the timing
>> specification, whether irrespective of the clock inversion. In effect,
>> the only speed that you can meet the specification is 1G.
>>
> 
> The timing table in the RGMII standard(3.3) says the max value of Tskew
> for 10/100 is unspecified.
> Quotation:"note1: ...,For 10/100 the Max value is unspecified."
> 
> I think for 10/100, the "4 to 5ns" skew in the chip doesn't break the
> standard. At 10/100 speeds, it meets the timing specification without
> having to to add clock inversion.
> In practice, it works at 10/100 speeds in the rgmii-id phy mode.
> 
>> Thus, I think this is something that needs a lot more than just "do
>> we need to invert the clock". You also need to prevent 10M and 100M
>> being supported IMHO.
>>
> 
> Regards,
> Lin Min

I had an offline discussion with Yao Zi and others regarding this. We feel
like the proper way for ESWIN to deal with this broken eth1 is to have a
different compatible string just for eth1, where it can be associated with
platform data with quirks to do eswin,rx-clk-invert at 1G. The property is
therefore not required to be exposed in DT. (Pretend it conforms to spec
for 1G). Need confirmation for 10M/100M, though. I double checked Lin Min's
claim, and indeed the spec says "For 10/100 the Max value is unspecified":
https://community.nxp.com/pwmxy87654/attachments/pwmxy87654/imx-processors/20655/1/RGMIIv2_0_final_hp.pdf

Thoughts?

Bo