netdev - Re: [PATCH 2/3] net: stmmac: qcom-ethqos: Enable RX programmable swap on qcs615

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Message-ID: <ebf3238c-12ec-4da7-bcdf-594bbe070a82@lunn.ch>
Date: Tue, 21 Jan 2025 15:34:19 +0100
From: Andrew Lunn <andrew@...n.ch>
To: Yijie Yang <quic_yijiyang@...cinc.com>
Cc: Krzysztof Kozlowski <krzk@...nel.org>, Vinod Koul <vkoul@...nel.org>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"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>,
	Bjorn Andersson <andersson@...nel.org>,
	Konrad Dybcio <konradybcio@...nel.org>,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Jose Abreu <joabreu@...opsys.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>, netdev@...r.kernel.org,
	linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/3] net: stmmac: qcom-ethqos: Enable RX programmable
 swap on qcs615

> > Maybe you should go read the RGMII standard, and then think about how
> > your hardware actually works.
> > 
> > RGMII always has a variable clock, with different clock speeds for
> > 10/100/1G. So your board design is just plain normal, not
> > special. Does the standard talk about different delays for different
> > speeds? As you say, other drivers apply the same delay for all
> > speeds. Why should your hardware be special?
> > 
> > RGMII has been around for 25 years. Do you really think your RGMII
> > implementation needs something special which no other implementation
> > has needed in the last 25 years?
> 
> I do not intend to violate the regulations of the RGMII standard and aim to
> maintain the same delay across all speeds. But the RX programming swap bit
> can only introduce a delay of 180 degrees. Should I assume the 1G speed
> clock to calculate and determine if this bit should be enabled for all
> speeds?

Lets rewind a bit.

The RGMII standard specified which edge you sample on. Since it is
defined, no other driver has a configuration like this, they just
setup there hardware to be standards compliant.

Why do you need the ability to break the standard, and sample on the
wrong edge?

I can think of two reasons:

1) You have a PHY which is broken, it also samples on the wrong
edge. This is a workaround for that broken PHY.

2) You have a board with a clock line driver inserted between the
RGMII output pins and the PHY, and this line driver includes a NOT?
This line driver is causing the clocking to break the RGMII
standard. You are working around this broken board design by getting
the MAC to invert the clock.

Is there a third reason?

Lets first understand the details of why you need to be able to invert
the clock.

	Andrew