Hi Frank:
I'm very appreciated that you kindly help to add the explanations.

Best Regards
Richard Zhu

-----Original Message-----
From: Frank Li <frank.li@nxp.com>
Sent: 2025年9月9日 0:33
To: Manivannan Sadhasivam <mani@kernel.org>
Cc: Hongxing Zhu <hongxing.zhu@nxp.com>; l.stach@pengutronix.de;
lpieralisi@kernel.org; kwilczynski@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 v1 2/2] PCI: imx6: Add a method to handle CLKREQ#
override active low

On Mon, Sep 08, 2025 at 09:19:40PM +0530, Manivannan Sadhasivam wrote:

quoted

On Mon, Sep 08, 2025 at 11:26:11AM GMT, Frank Li wrote:

quoted

On Mon, Sep 08, 2025 at 11:36:02AM +0530, Manivannan Sadhasivam

wrote:

quoted

quoted

quoted

On Wed, Aug 20, 2025 at 04:10:48PM GMT, Richard Zhu wrote:

quoted

The CLKREQ# is an open drain, active low signal that is driven
low by the card to request reference clock.

Since the reference clock may be required by i.MX PCIe host too.

Add some info on why the refclk is needed by the host.

quoted

To make
sure this clock is available even when the CLKREQ# isn't driven
low by the card(e.x no card connected), force CLKREQ# override
active low for i.MX PCIe host during initialization.

CLKREQ# override is not a spec defined feature. So you need to
explain what it does first.

quoted

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.

Why do you need to depend on 'supports-clkreq' property? Don't you
already know if your platform supports CLKREQ# or not? None of the
upstream DTS has the 'supports-clkreq' property set and the NXP
binding also doesn't enable this property.

It is history reason. Supposed all the boards which supports L1SS
need set 'supports-clkreq' in dts. L1SS require board design use
open drain connect RC's clk-req and EP's clk-req together, which
come from one ECN of PCI spec.

But most M.2 slot now, which support L1SS, so most platform default
enable L1SS or default 'supports-clkreq' on.

Ideally, 'supports-clkreq' should use revert logic like 'clk-req-broken'.
but 'supports-clkreq' already come into stardard PCIe binding now.

One of i.MX95 boards use standard PCIe slot, PIN 12
12	CLKREQ#	Ground	Clock Request Signal[26]
which is reserved at old PCIe standard, so some old PCIe card float
this pin.

Ok. IIUC, i.MX platforms doesn't always support CLKREQ#, as the pin
might not be wired on some connectors. So if the driver turns off the
override, CLKREQ# will be driven high, but the endpoint wouldn't get a
chance to drive it low and it

CLKREQ# will be float and pull up by pull up resistor. The old endpoint (PCIe
standard slot) float this pin also because it is reversed at old PCIe standard.
So ref clock is off at that case.

quoted

won't receive the refclk.

Is my understanding correct?

Basic is correct with some small problem.

It is caused by two common PCIe problem.
- stadarnd PCIe slot define change PIN12 from reserved to CLKREQ#, which is
ECN, before ECN, all EP device float this pin. after ECN, EP device pull this
pin down.
- use logic [2] to design boards, which just impact L1SS, the basic function
should work.

Frank

quoted

I'm wondering in those cases, why can't you keep the CLKREQ# pin to be
in active low state by defining the initial pinctrl state in DT? Can't
you change the pinctrl state of CLKREQ#?

quoted

So I think most dts in kernel tree should add 'supports-clkreq'
property if they use M.2 and connect CLK_REQ# as below [1]
============================================
              VCC
              ---
               |
               R (10K)
               |
CLK_REQ# (RC)------ CLK_REQ#(EP)

NOT add supports-clkreq if connect as below [2]
==========================================

CLK_REQ# (RC)  ---> |---------|
                    | OR GATE | ---> control ref clock
CLK_REQ#(EP)   ---> |-------- |

quoted

So I'm wondering how you are suddenly using this property. The
property implies that when not set to true, CLKREQ# is not
supported by the platform. So when the driver starts using this
property, all the old DTS based platforms are not going to release
CLKREQ# from driving low, so L1SS will not be entered for them. Do you

really want it to happen?

quoted

quoted

Actually, some old board use [2]. we will add supports-clkreq if
board design use [1], so correct reflect board design.

Ok, thanks for clarifying.

- Mani

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