linux-kernel - Re: [PATCH v2 1/4] dt-bindings: PCI: rcar-gen4-pci-host: Document optional aux clock

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Message-ID: <y246ietlwyaix7q3wulviy7wysqkwczspgtj26gducxbay4qrl@im3iht4kvhuj>
Date: Mon, 19 May 2025 18:30:36 +0100
From: Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>
To: Rob Herring <robh@...nel.org>
Cc: Marek Vasut <marek.vasut@...lbox.org>, 
	Marek Vasut <marek.vasut+renesas@...lbox.org>, linux-arm-kernel@...ts.infradead.org, 
	Niklas Söderlund <niklas.soderlund+renesas@...natech.se>, Krzysztof Wilczyński <kw@...ux.com>, 
	Rafał Miłecki <rafal@...ecki.pl>, Aradhya Bhatia <a-bhatia1@...com>, 
	Bjorn Helgaas <bhelgaas@...gle.com>, Conor Dooley <conor+dt@...nel.org>, 
	Geert Uytterhoeven <geert+renesas@...der.be>, Heiko Stuebner <heiko@...ech.de>, 
	Junhao Xie <bigfoot@...ssfun.cn>, Kever Yang <kever.yang@...k-chips.com>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Kuninori Morimoto <kuninori.morimoto.gx@...esas.com>, 
	Lorenzo Pieralisi <lpieralisi@...nel.org>, Magnus Damm <magnus.damm@...il.com>, 
	Neil Armstrong <neil.armstrong@...aro.org>, Yoshihiro Shimoda <yoshihiro.shimoda.uh@...esas.com>, 
	devicetree@...r.kernel.org, linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, 
	linux-renesas-soc@...r.kernel.org
Subject: Re: [PATCH v2 1/4] dt-bindings: PCI: rcar-gen4-pci-host: Document
 optional aux clock

On Mon, May 19, 2025 at 10:04:09AM -0500, Rob Herring wrote:
> On Thu, May 15, 2025 at 6:57 AM Manivannan Sadhasivam
> <manivannan.sadhasivam@...aro.org> wrote:
> >
> > On Mon, May 12, 2025 at 10:42:20PM +0200, Marek Vasut wrote:
> > > On 5/9/25 9:37 PM, Manivannan Sadhasivam wrote:
> > > > On Sun, Apr 06, 2025 at 04:45:21PM +0200, Marek Vasut wrote:
> > > > > Document 'aux' clock which are used to supply the PCIe bus. This
> > > > > is useful in case of a hardware setup, where the PCIe controller
> > > > > input clock and the PCIe bus clock are supplied from the same
> > > > > clock synthesiser, but from different differential clock outputs:
> > > >
> > > > How different is this clock from the 'reference clock'? I'm not sure what you
> > > > mean by 'PCIe bus clock' here. AFAIK, endpoint only takes the reference clock
> > > > and the binding already has 'ref' clock for that purpose. So I don't understand
> > > > how this new clock is connected to the endpoint device.
> > >
> > > See the ASCII art below , CLK_DIF0 is 'ref' clock that feeds the controller
> > > side, CLK_DIF1 is the bus (or 'aux') clock which feeds the bus (or endpoint)
> > > side. Both clock come from the same clock synthesizer, but from two separate
> > > clock outputs of the synthesizer.
> > >
> >
> > Okay. So separate refclks are suppplied to the host and endpoint here and no,
> > you should not call the other one as 'aux' clock, it is still the refclk. In
> > this case, you should describe the endpoint refclk in the PCIe bridge node:
> >
> >                 pcie@... {
> >                         clock = <refclk_host>;
> >                         ...
> >
> >                         pcie@0 {
> >                                 device_type = "pci";
> >                                 reg = <0x0 0x0 0x0 0x0 0x0>;
> >                                 bus-range = <0x01 0xff>;
> >                                 clock = <refclk_ep>;
> >                                 ...
> >                         };
> >                 };
> >
> >
> > and use the pwrctrl driver PCI_PWRCTRL_SLOT to enable it. Right now, the slot
> > pwrctrl driver is not handling the refclk, but I can submit a patch for that.
> 
> There's another discussion about PCIe clocks here[1]. Seems there's a
> variety of options here with spread-spectrum layered on top.
> 

The other discussion is separate IMO. It just concerns how the endpoint detects
local clock vs supplied clock.

- Mani

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