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

[Min Lin]

Hi Russell,


> -----Original Messages-----
> From: "Russell King (Oracle)" <linux@xxxxxxxxxxxxxxx>
> Send time:Tuesday, 27/01/2026 02:29:09
> To: "Min Lin" <linmin@xxxxxxxxxxxxxxxxxx>
> Cc: "Bo Gan" <ganboing@xxxxxxxxx>, "Andrew Lunn" <andrew@xxxxxxx>, "Krzysztof Kozlowski" <krzk@xxxxxxxxxx>, 李志 <lizhi2@xxxxxxxxxxxxxxxxxx>, devicetree@xxxxxxxxxxxxxxx, andrew+netdev@xxxxxxx, davem@xxxxxxxxxxxxx, edumazet@xxxxxxxxxx, kuba@xxxxxxxxxx, robh@xxxxxxxxxx, krzk+dt@xxxxxxxxxx, conor+dt@xxxxxxxxxx, netdev@xxxxxxxxxxxxxxx, pabeni@xxxxxxxxxx, mcoquelin.stm32@xxxxxxxxx, alexandre.torgue@xxxxxxxxxxx, linux-stm32@xxxxxxxxxxxxxxxxxxxxxxxxxxxx, linux-arm-kernel@xxxxxxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, ningyu@xxxxxxxxxxxxxxxxxx, pinkesh.vaghela@xxxxxxxxxxxxxx, weishangjuan@xxxxxxxxxxxxxxxxxx
> 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