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.