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

["Min Lin" <linmin@eswincomputing.com>]

Hi Russell, Krzysztof,


> -----Original Messages-----
> From: "Min Lin" <linmin@eswincomputing.com>
> Send time:Wednesday, 28/01/2026 13:48:59
> To: "Bo Gan" <ganboing@gmail.com>
> Cc: "Russell King (Oracle)" <linux@armlinux.org.uk>, "Andrew Lunn" <andrew@lunn.ch>, "Krzysztof Kozlowski" <krzk@kernel.org>, 李志 <lizhi2@eswincomputing.com>, devicetree@vger.kernel.org, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, robh@kernel.org, krzk+dt@kernel.org, conor+dt@kernel.org, netdev@vger.kernel.org, pabeni@redhat.com, mcoquelin.stm32@gmail.com, alexandre.torgue@foss.st.com, linux-stm32@st-md-mailman.stormreply.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, ningyu@eswincomputing.com, pinkesh.vaghela@einfochips.com, weishangjuan@eswincomputing.com
> Subject: Re: Re: [PATCH v1 1/2] dt-bindings: ethernet: eswin: add clock sampling control
> 
> Hi Bo Gan,
> 
> 
> > -----Original Messages-----
> > From: "Bo Gan" <ganboing@gmail.com>
> > Send time:Wednesday, 28/01/2026 10:38:28
> > To: "Min Lin" <linmin@eswincomputing.com>, "Russell King (Oracle)" <linux@armlinux.org.uk>
> > Cc: "Andrew Lunn" <andrew@lunn.ch>, "Krzysztof Kozlowski" <krzk@kernel.org>, 李志 <lizhi2@eswincomputing.com>, devicetree@vger.kernel.org, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, robh@kernel.org, krzk+dt@kernel.org, conor+dt@kernel.org, netdev@vger.kernel.org, pabeni@redhat.com, mcoquelin.stm32@gmail.com, alexandre.torgue@foss.st.com, linux-stm32@st-md-mailman.stormreply.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, ningyu@eswincomputing.com, pinkesh.vaghela@einfochips.com, weishangjuan@eswincomputing.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@armlinux.org.uk>
> > >> Send time:Tuesday, 27/01/2026 02:29:09
> > >> To: "Min Lin" <linmin@eswincomputing.com>
> > >> Cc: "Bo Gan" <ganboing@gmail.com>, "Andrew Lunn" <andrew@lunn.ch>, "Krzysztof Kozlowski" <krzk@kernel.org>, 李志 <lizhi2@eswincomputing.com>, devicetree@vger.kernel.org, andrew+netdev@lunn.ch, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, robh@kernel.org, krzk+dt@kernel.org, conor+dt@kernel.org, netdev@vger.kernel.org, pabeni@redhat.com, mcoquelin.stm32@gmail.com, alexandre.torgue@foss.st.com, linux-stm32@st-md-mailman.stormreply.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, ningyu@eswincomputing.com, pinkesh.vaghela@einfochips.com, weishangjuan@eswincomputing.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.
> > >>
> 
> > 
> > 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?
> 
> I agree with your approach.
> 


Do you think it would be reasonable for eth1 to have another different compatible
string, such as "eswin,eic7700-qos-eth-quirk"?

Thanks for your comments.

Regards,
Lin Min