On Thu, Mar 26, 2026 at 08:17:29AM +0000, Hongxing Zhu wrote:

quoted

-----Original Message-----
From: Franz Schnyder <redacted>
Sent: 2026年3月26日 16:00
To: Hongxing Zhu <hongxing.zhu@nxp.com>
Cc: Franz Schnyder <redacted>;
linux-pci@vger.kernel.org; linux-arm-kernel@lists.infradead.org;
imx@lists.linux.dev; linux-kernel@vger.kernel.org; Francesco Dolcini
[off-list ref]; Manivannan Sadhasivam [off-list ref]; Frank
Li [off-list ref]
Subject: Clear CLKREQ# override breaks functionality

Hi Richard,

While integrating the `supports-clkreq` DT property on our iMX95-based
SoM, we had failures in our CI on one of the two M.2 PCIe slots on our
development board.
The failing slot uses a card that does not advertise L1 PM substates.
This issue comes from commit a152a90f5390 ("PCI: imx6: Clear CLKREQ#
override if 'supports-clkreq' DT property is available"), which clears the
CLKREQ# override based only on the DT property.

It seems that clearing the CLKREQ# override should happen only when the
driver knows that the downstream device advertises L1 PM Substates.
Otherwise CLKREQ# should remain asserted to keep compatibility with cards
that do not support L1 PM Substates.

Thoughts?

Hi Franz:
No, CLKREQ# is not dependent on L1 PM Substates capabilities.

According to the PCI Express Card Electromechanical Specification r6.0,
Section 2 (Auxiliary Signals):

"CLKREQ# (optional) signal is an open drain, active low signal that is driven
low by the card to request that the PCI Express reference clock be available
(active clock state) to allow the PCI Express interface to send/receive data."

When the 'supports-clkreq' property is present, it indicates that the endpoint
device supports CLKREQ# signaling and will actively drive the signal low when
it needs the reference clock.

Therefore, the RC (Root Complex) controller can safely clear any CLKREQ#
active-low override settings, allowing the endpoint to control the clock
request through proper CLKREQ# signaling.

If the endpoint devices on your platform can't drive the CLKREQ# signal to
low, remove 'supports-clkreq' property from your board's device tree file.

Best Regards
Richard Zhu

quoted

Kind Regards

Franz

Hi Richard,

Thanks for the explanation. I managed to find a PCIe specification, and
I can now see that my assumption was wrong. The L1 PM Substates mechanism
requires CLKREQ#, but not vice versa. To be safe with cards which lack
support, we'll just remove the 'supports-clkreq' property for now.

Kind regards

Franz