RE: Clear CLKREQ# override breaks functionality

[Hongxing Zhu]

> -----Original Message-----
> From: Franz Schnyder <fra.schnyder@xxxxxxxxx>
> Sent: 2026年3月26日 16:00
> To: Hongxing Zhu <hongxing.zhu@xxxxxxx>
> Cc: Franz Schnyder <franz.schnyder@xxxxxxxxxxx>;
> linux-pci@xxxxxxxxxxxxxxx; linux-arm-kernel@xxxxxxxxxxxxxxxxxxx;
> imx@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Francesco Dolcini
> <francesco@xxxxxxxxxx>; Manivannan Sadhasivam <mani@xxxxxxxxxx>; Frank
> Li <frank.li@xxxxxxx>
> 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
> 
> Kind Regards
> 
> Franz