Re: [PATCH v1 2/2] PCI: imx6: Add a method to handle CLKREQ# override active low

[Bjorn Helgaas <helgaas@kernel.org>]

On Wed, Aug 20, 2025 at 05:02:28PM -0400, Frank Li wrote:
> On Wed, Aug 20, 2025 at 03:35:36PM -0500, Bjorn Helgaas wrote:
> > On Wed, Aug 20, 2025 at 04:10:48PM +0800, Richard Zhu wrote:
> > > 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. 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.

What is the effect of refclk not being available when no card is
connected?  I guess something is wrong with the i.MX PCIe host?  Maybe
register accesses don't work?  Something else?

How would a user know that something is wrong when the slot is empty?
Presumably this patch fixes whatever is wrong in that case.

> > > 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.
> > >
> > > Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
> > > ---
> > >  drivers/pci/controller/dwc/pci-imx6.c | 35 +++++++++++++++++++++++++++
> > >  1 file changed, 35 insertions(+)
> > >
> > > diff --git a/drivers/pci/controller/dwc/pci-imx6.c b/drivers/pci/controller/dwc/pci-imx6.c
> > > index 80e48746bbaf6..a73632b47e2d3 100644
> > > --- a/drivers/pci/controller/dwc/pci-imx6.c
> > > +++ b/drivers/pci/controller/dwc/pci-imx6.c
> > > @@ -52,6 +52,8 @@
> > >  #define IMX95_PCIE_REF_CLKEN			BIT(23)
> > >  #define IMX95_PCIE_PHY_CR_PARA_SEL		BIT(9)
> > >  #define IMX95_PCIE_SS_RW_REG_1			0xf4
> > > +#define IMX95_PCIE_CLKREQ_OVERRIDE_EN		BIT(8)
> > > +#define IMX95_PCIE_CLKREQ_OVERRIDE_VAL		BIT(9)
> > >  #define IMX95_PCIE_SYS_AUX_PWR_DET		BIT(31)
> > >
> > >  #define IMX95_PE0_GEN_CTRL_1			0x1050
> > > @@ -136,6 +138,7 @@ struct imx_pcie_drvdata {
> > >  	int (*enable_ref_clk)(struct imx_pcie *pcie, bool enable);
> > >  	int (*core_reset)(struct imx_pcie *pcie, bool assert);
> > >  	int (*wait_pll_lock)(struct imx_pcie *pcie);
> > > +	void (*clr_clkreq_override)(struct imx_pcie *pcie);
> > >  	const struct dw_pcie_host_ops *ops;
> > >  };
> > >
> > > @@ -149,6 +152,7 @@ struct imx_pcie {
> > >  	struct gpio_desc	*reset_gpiod;
> > >  	struct clk_bulk_data	*clks;
> > >  	int			num_clks;
> > > +	bool			supports_clkreq;
> > >  	struct regmap		*iomuxc_gpr;
> > >  	u16			msi_ctrl;
> > >  	u32			controller_id;
> > > @@ -267,6 +271,13 @@ static int imx95_pcie_init_phy(struct imx_pcie *imx_pcie)
> > >  			   IMX95_PCIE_REF_CLKEN,
> > >  			   IMX95_PCIE_REF_CLKEN);
> > >
> > > +	/* Force CLKREQ# low by override */
> > > +	regmap_update_bits(imx_pcie->iomuxc_gpr,
> > > +			   IMX95_PCIE_SS_RW_REG_1,
> > > +			   IMX95_PCIE_CLKREQ_OVERRIDE_EN |
> > > +			   IMX95_PCIE_CLKREQ_OVERRIDE_VAL,
> > > +			   IMX95_PCIE_CLKREQ_OVERRIDE_EN |
> > > +			   IMX95_PCIE_CLKREQ_OVERRIDE_VAL);
> > >  	return 0;
> > >  }
> > >
> > > @@ -1298,6 +1309,18 @@ static void imx_pcie_host_exit(struct dw_pcie_rp *pp)
> > >  		regulator_disable(imx_pcie->vpcie);
> > >  }
> > >
> > > +static void imx8mm_pcie_clr_clkreq_override(struct imx_pcie *imx_pcie)
> > > +{
> > > +	imx8mm_pcie_enable_ref_clk(imx_pcie, false);
> > > +}
> > > +
> > > +static void imx95_pcie_clr_clkreq_override(struct imx_pcie *imx_pcie)
> > > +{
> > > +	regmap_update_bits(imx_pcie->iomuxc_gpr, IMX95_PCIE_SS_RW_REG_1,
> > > +			   IMX95_PCIE_CLKREQ_OVERRIDE_EN |
> > > +			   IMX95_PCIE_CLKREQ_OVERRIDE_VAL, 0);
> > > +}
> > > +
> > >  static void imx_pcie_host_post_init(struct dw_pcie_rp *pp)
> > >  {
> > >  	struct dw_pcie *pci = to_dw_pcie_from_pp(pp);
> > > @@ -1322,6 +1345,12 @@ 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);
> >
> > It seems racy to clear the override when the link is up.
> >
> > Since it sounds like the i.MX host requires refclock all the time, why
> > not keep the override permanently?
> 
> It will Lost l1ss support if enable override permanetly. I think other
> platform have similar issues (at least dwc controller). Most platform use
> m.2 socket, which pull down clk_req by EP side devices.
> 
> Only standard PCIe slots,  which clkreq have not connect by EP devices
> because stardard PCIe slots add #clkreq signal happen recent years (maybe
> after miniPCIe slot spec). Some old PCIe devices have not connect it.
> 
> Even latest PCIe devices, I saw have jumper in PCIe card to controller
> #clkreq.
> 
> > Obviously it must be ok to keep the override for a while, because
> > there is some interval between the link coming up and the call of
> > .clr_clkreq_override().
> >
> > Would something bad happen if we *never* called
> > .clr_clkreq_override()?
> 
> clock will always running, lost L1SS power save feature.

Thanks!  Let's include a little of this back story in the next rev of
the commit log and the comment near the .clr_clkreq_override() call.

Bjorn