linux-kernel - Re: [PATCH] arm64: dts: rockchip: align bindings to PCIe spec

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Message-ID: <sbulnlwz3vxyk3yw2c2tcsdvyu57cdvyixkpeq2okh4vn6yyod@4o4kltfb5u6n>
Date: Sat, 8 Nov 2025 23:12:54 +0100
From: Sebastian Reichel <sebastian.reichel@...labora.com>
To: Shawn Lin <shawn.lin@...k-chips.com>
Cc: Geraldo Nascimento <geraldogabriel@...il.com>, 
	Ye Zhang <ye.zhang@...k-chips.com>, Lorenzo Pieralisi <lpieralisi@...nel.org>, 
	Krzysztof Wilczyński <kwilczynski@...nel.org>, Manivannan Sadhasivam <mani@...nel.org>, 
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>, 
	Heiko Stuebner <heiko@...ech.de>, linux-pci@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Johan Jonker <jbx6244@...il.com>, linux-rockchip@...ts.infradead.org
Subject: Re: [PATCH] arm64: dts: rockchip: align bindings to PCIe spec

Hi,

On Fri, Nov 07, 2025 at 11:01:04AM +0800, Shawn Lin wrote:
> + Ye Zhang
> 
> 在 2025/11/07 星期五 10:43, Geraldo Nascimento 写道:
> > On Wed, Nov 05, 2025 at 04:56:36PM +0800, Shawn Lin wrote:
> > > 在 2025/11/05 星期三 16:18, Geraldo Nascimento 写道:
> > > > Hi Shawn, glad to hear from you.
> > > > 
> > > > Perhaps the following change is better? It resolves the issue
> > > > without the added complication of open drain. After you questioned
> > > > if open drain is actually part of the spec, I remembered that
> > > > GPIO_OPEN_DRAIN is actually (GPIO_SINGLE_ENDED | GPIO_LINE_OPEN_DRAIN)
> > > > so I decided to test with just GPIO_SINGLE_ENDED and it works.
> > 
> > Shawn,
> > 
> > I quote from the PCIe Mini Card Electromechanical Specification Rev 1.2
> > 
> > "3.4.1. Logic Signal Requirements
> > 
> > The 3.3V card logic levels for single-ended digital signals (WAKE#,
> > CLKREQ#, PERST#, and W_DISABLE#) are given in Table 3-7. [...]"
> > 
> > So while you are correct that PERST# is most definitely not Open Drain,
> > there's evidence on the spec that defines this signal as Single-Ended.
> > 
> 
> This's true. But I couldn't find any user in dts using either
> GPIO_SINGLE_ENDED or GPIO_OPEN_DRAIN for PCIe PERST#. I'm curious
> how these two flags affect actual behavior of chips. Ye, could you
> please help check it?

FWIW I assume single-ended in the spec means it's not differential
like all the highspeed signals on the PCIe connection. This says
nothing about open-drain, open-source or push-pull being used. The
kernel on the other hand has a very specific understanding of
GPIO_SINGLE_ENDED:

	if (flags & OF_GPIO_SINGLE_ENDED) {
		if (flags & OF_GPIO_OPEN_DRAIN)
			lflags |= GPIO_OPEN_DRAIN;
		else
			lflags |= GPIO_OPEN_SOURCE;
	}

I.e. it is the same as configuring open-source ;)

Greetings,

-- Sebastian

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