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

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Message-ID: <aQ_HhfK0pbOE1m1R@geday>
Date: Sat, 8 Nov 2025 19:43:17 -0300
From: Geraldo Nascimento <geraldogabriel@...il.com>
To: Sebastian Reichel <sebastian.reichel@...labora.com>
Cc: Shawn Lin <shawn.lin@...k-chips.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

On Sat, Nov 08, 2025 at 11:12:54PM +0100, Sebastian Reichel wrote:
> Hi,

Hi Sebastian,

> 
> 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

yes, I agree. It was an oversight on my part to assume open-drain on
PERST# was part of the spec just because many cores implement it that
way. Kudos to Shawn for correcting me.

> 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 ;)

Yup, I had noticed that. This works because the reset value of PMU GRF
register PMUGRF_GPIO0B_P sets the relevant PERST# GPIO (GPIO0-12) on my
board to pull-down, which can work with Open Source/Emitter. If we set
the GPIO to Open Drain/Collector we must on the other hand set that pin
to pull-up. Either way it works.

I've been investigating why that GPIO isn't properly working as
Push-Pull for my board (Rock PI N10) but so far I'm clueless.

Thank you,
Geraldo Nascimento

> 
> Greetings,
> 
> -- Sebastian