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

[Hongxing Zhu]

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