linux-kernel - RE: [PATCH v2 2/2] PCI: imx6: Add a method to handle CLKREQ# override active low

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Message-ID:
 <AS8PR04MB883309FC7D8F8052924179768C16A@AS8PR04MB8833.eurprd04.prod.outlook.com>
Date: Thu, 18 Sep 2025 03:04:38 +0000
From: Hongxing Zhu <hongxing.zhu@....com>
To: Bjorn Helgaas <helgaas@...nel.org>
CC: Frank Li <frank.li@....com>, "jingoohan1@...il.com"
	<jingoohan1@...il.com>, "l.stach@...gutronix.de" <l.stach@...gutronix.de>,
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	"robh@...nel.org" <robh@...nel.org>, "bhelgaas@...gle.com"
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	<kernel@...gutronix.de>, "festevam@...il.com" <festevam@...il.com>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
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	<linux-arm-kernel@...ts.infradead.org>, "imx@...ts.linux.dev"
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	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 2/2] PCI: imx6: Add a method to handle CLKREQ# override
 active low

> -----Original Message-----
> From: Bjorn Helgaas <helgaas@...nel.org>
> Sent: 2025年9月18日 6:24
> To: Hongxing Zhu <hongxing.zhu@....com>
> Cc: Frank Li <frank.li@....com>; jingoohan1@...il.com;
> l.stach@...gutronix.de; lpieralisi@...nel.org; kwilczynski@...nel.org;
> mani@...nel.org; robh@...nel.org; bhelgaas@...gle.com;
> shawnguo@...nel.org; s.hauer@...gutronix.de; kernel@...gutronix.de;
> festevam@...il.com; linux-pci@...r.kernel.org;
> linux-arm-kernel@...ts.infradead.org; imx@...ts.linux.dev;
> linux-kernel@...r.kernel.org
> Subject: Re: [PATCH v2 2/2] PCI: imx6: Add a method to handle CLKREQ#
> override active low
> 
> On Wed, Sep 17, 2025 at 05:37:51PM +0800, Richard Zhu wrote:
> > The CLKREQ# is an open drain, active low signal that is driven low by
> > the card to request reference clock.
> >
> > But the CLKREQ# maybe reserved on some old device, compliant with CEM
> > r3.0 or before. Thus, this signal wouldn't be driven low by these old
> > devices.
> 
> Can you include a citation to a relevant section in the CEM spec?
> Maybe the point is that CLKREQ# is an optional signal added in PCIe CEM r4.0,
> sec 2?
> 
> If that's accurate, we can add it when applying, no need to repost for that.
Hi Bjorn:
Thanks for your comments.
The Pin12 of connector is reserved refer to CEM spec r3.0 sec6.1 Connector
 Pinout. CLKREQ# is defined in CEM spec r4.0, but it's optional refer to CEM
 r4.0 sec2. And, the Pin12 of connector is defined as CLKREQ# refer to CEM spec
 r4.0 sec6.1 Connector Pinout. That's all.
Should I repost for that?

Best Regards
Richard Zhu
> 
> > Since the reference clock controlled by CLKREQ# may be required by
> > i.MX PCIe host too. To make sure this clock is ready even when the
> > CLKREQ# isn't driven low by the card(e.x old cards described above),
> > force CLKREQ# override active low for i.MX PCIe host during initialization.
> >
> > The CLKREQ# override can be cleared safely when supports-clkreq is
> > present and PCIe link is up later. Because the CLKREQ# would be driven
> > low by the card in this case.