Re: [PATCH v5 2/2] PCI: imx6: Add a method to handle CLKREQ# override active low

[Bjorn Helgaas <helgaas@kernel.org>]

On Fri, Sep 26, 2025 at 03:23:43AM +0000, Hongxing Zhu wrote:
> > -----Original Message-----
> > From: Bjorn Helgaas <helgaas@kernel.org>

> > On Fri, Sep 26, 2025 at 02:19:37AM +0000, Hongxing Zhu wrote:
> > > > -----Original Message-----
> > > > From: Bjorn Helgaas <helgaas@kernel.org> On Tue, Sep 23, 2025 at
> > > > 03:39:13PM +0800, Richard Zhu wrote:
> > > > > The CLKREQ# is an open drain, active low signal that is driven low
> > > > > by the card to request reference clock. It's an optional signal
> > > > > added in PCIe CEM r4.0, sec 2. Thus, this signal wouldn't be
> > > > > driven low if it's reserved.
> > > > >
> > > > > 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 the scenario 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 at this time.
> > > >
> > > > What happens if we clear the CLKREQ# override (so the host doesn't
> > > > assert it), and the link is up but the card never asserts CLKREQ#
> > > > (since it's an optional signal)?
> > > >
> > > > Does the i.MX host still work?
> > >
> > > The CLKREQ# override active low only be cleared when link is up
> > > and supports-clkreq is present. In the other words, there is a
> > > remote endpoint  device, and the CLKREQ# would be driven active
> > > low by this endpoint device.
> > 
> > Assume an endpoint designed to CEM r2.0.  CLKREQ# doesn't exist in
> > CEM r2.0, so even if the endpoint is present and the link is up,
> > the endpoint will not assert CLKREQ#.
> > 
> > Will the i.MX host still work?
> > 
> > IIUC, CLKREQ# is required for ASPM L1 PM Substates.  Maybe the
> > CLKREQ# override should only be cleared if the endpoint advertises
> > L1 PM Substates support?
>
> CLKREQ# override active low only be cleared when the endpoint
> advertises that it has L1 PM Substates support or it always drives
> CLKREQ# low.

What?  That's not what the patch does.  It calls .clr_clkreq_override()
whenever the link is up and devicetree contains 'support-clkreq'.

A device advertises L1 PM Substates support by putting the L1 PM
Substates Capability in its config space.

> > > > > static void imx_pcie_host_post_init(struct dw_pcie_rp *pp)
> > > > >  		dw_pcie_writel_dbi(pci, GEN3_RELATED_OFF, val);
> > > > >  		dw_pcie_dbi_ro_wr_dis(pci);
> > > > >  	}
> > > > > +
> > > > > +	/* Clear CLKREQ# override if supports_clkreq is true and link is up */
> > > > > +	if (dw_pcie_link_up(pci) && imx_pcie->supports_clkreq) {
> > > > > +		if (imx_pcie->drvdata->clr_clkreq_override)
> > > > > +			imx_pcie->drvdata->clr_clkreq_override(imx_pcie);
> > > > > +	}