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

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Message-ID: <20250917222356.GA1879221@bhelgaas>
Date: Wed, 17 Sep 2025 17:23:56 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Richard Zhu <hongxing.zhu@....com>
Cc: 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.

> 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.