-----Original Message-----
From: Bjorn Helgaas <helgaas@kernel.org>
Sent: 2025年9月18日 6:24
To: Hongxing Zhu <hongxing.zhu@nxp.com>
Cc: Frank Li <frank.li@nxp.com>; jingoohan1@gmail.com;
l.stach@pengutronix.de; lpieralisi@kernel.org; kwilczynski@kernel.org;
mani@kernel.org; robh@kernel.org; bhelgaas@google.com;
shawnguo@kernel.org; s.hauer@pengutronix.de; kernel@pengutronix.de;
festevam@gmail.com; linux-pci@vger.kernel.org;
linux-arm-kernel@lists.infradead.org; imx@lists.linux.dev;
linux-kernel@vger.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:

quoted

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

quoted

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.