-----Original Message-----
From: Bjorn Helgaas <helgaas@kernel.org>
Sent: 2025年9月26日 6:04
To: Hongxing Zhu <hongxing.zhu@nxp.com>
Cc: Frank Li <frank.li@nxp.com>; jingoohan1@gmail.com;
l.stach@pengutronix.de; lpieralisi@kernel.org; kwilczynski@kernel.org;
mani@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 v5 2/2] PCI: imx6: Add a method to handle CLKREQ#
override active low

On Tue, Sep 23, 2025 at 03:39:13PM +0800, Richard Zhu wrote:

quoted

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.

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 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?

Hi Bjorn:
Thanks for your kindly concerns.
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.

Best Regards
Richard Zhu

quoted

Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
---
 drivers/pci/controller/dwc/pci-imx6.c | 43
++++++++++++++++++++++++++-
 1 file changed, 42 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/controller/dwc/pci-imx6.c

b/drivers/pci/controller/dwc/pci-imx6.c
index 80e48746bbaf..6b03b1111d06 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;

@@ -239,6 +243,16 @@ static unsigned int imx_pcie_grp_offset(const

struct imx_pcie *imx_pcie)

quoted

 	return imx_pcie->controller_id == 1 ? IOMUXC_GPR16 : IOMUXC_GPR14;
}

+static void  imx95_pcie_clkreq_override(struct imx_pcie *imx_pcie,
+bool enable) {
+	regmap_update_bits(imx_pcie->iomuxc_gpr, IMX95_PCIE_SS_RW_REG_1,
+			   IMX95_PCIE_CLKREQ_OVERRIDE_EN,
+			   enable ? IMX95_PCIE_CLKREQ_OVERRIDE_EN : 0);
+	regmap_update_bits(imx_pcie->iomuxc_gpr, IMX95_PCIE_SS_RW_REG_1,
+			   IMX95_PCIE_CLKREQ_OVERRIDE_VAL,
+			   enable ? IMX95_PCIE_CLKREQ_OVERRIDE_VAL : 0); }
+
 static int imx95_pcie_init_phy(struct imx_pcie *imx_pcie)  {
 	/*

@@ -685,7 +699,7 @@ static int imx6q_pcie_enable_ref_clk(struct

imx_pcie *imx_pcie, bool enable)

quoted

 	return 0;
 }

-static int imx8mm_pcie_enable_ref_clk(struct imx_pcie *imx_pcie, bool
enable)
+static void imx8mm_pcie_clkreq_override(struct imx_pcie *imx_pcie,
+bool enable)
 {
 	int offset = imx_pcie_grp_offset(imx_pcie);

@@ -695,6 +709,11 @@ static int imx8mm_pcie_enable_ref_clk(struct

imx_pcie *imx_pcie, bool enable)

quoted

 	regmap_update_bits(imx_pcie->iomuxc_gpr, offset,
 			   IMX8MQ_GPR_PCIE_CLK_REQ_OVERRIDE_EN,
 			   enable ? IMX8MQ_GPR_PCIE_CLK_REQ_OVERRIDE_EN : 0);
+}
+
+static int imx8mm_pcie_enable_ref_clk(struct imx_pcie *imx_pcie, bool
+enable) {
+	imx8mm_pcie_clkreq_override(imx_pcie, enable);
 	return 0;
 }

@@ -1298,6 +1317,16 @@ static void imx_pcie_host_exit(struct

dw_pcie_rp *pp)

quoted

 		regulator_disable(imx_pcie->vpcie);
 }

+static void imx8mm_pcie_clr_clkreq_override(struct imx_pcie
+*imx_pcie) {
+	imx8mm_pcie_clkreq_override(imx_pcie, false); }
+
+static void imx95_pcie_clr_clkreq_override(struct imx_pcie *imx_pcie)
+{
+	imx95_pcie_clkreq_override(imx_pcie, false); }
+
 static void imx_pcie_host_post_init(struct dw_pcie_rp *pp)  {
 	struct dw_pcie *pci = to_dw_pcie_from_pp(pp); @@ -1322,6 +1351,12

@@

quoted

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);
+	}
 }

 /*

@@ -1745,6 +1780,8 @@ static int imx_pcie_probe(struct platform_device

*pdev)

quoted

 	pci->max_link_speed = 1;
 	of_property_read_u32(node, "fsl,max-link-speed",
&pci->max_link_speed);

+	imx_pcie->supports_clkreq =
+		of_property_read_bool(node, "supports-clkreq");
 	imx_pcie->vpcie = devm_regulator_get_optional(&pdev->dev, "vpcie");
 	if (IS_ERR(imx_pcie->vpcie)) {
 		if (PTR_ERR(imx_pcie->vpcie) != -ENODEV) @@ -1873,6 +1910,7 @@
static const struct imx_pcie_drvdata drvdata[] = {
 		.mode_mask[1] = IMX8MQ_GPR12_PCIE2_CTRL_DEVICE_TYPE,
 		.init_phy = imx8mq_pcie_init_phy,
 		.enable_ref_clk = imx8mm_pcie_enable_ref_clk,
+		.clr_clkreq_override = imx8mm_pcie_clr_clkreq_override,
 	},
 	[IMX8MM] = {
 		.variant = IMX8MM,

@@ -1883,6 +1921,7 @@ static const struct imx_pcie_drvdata drvdata[] = {
 		.mode_off[0] = IOMUXC_GPR12,
 		.mode_mask[0] = IMX6Q_GPR12_DEVICE_TYPE,
 		.enable_ref_clk = imx8mm_pcie_enable_ref_clk,
+		.clr_clkreq_override = imx8mm_pcie_clr_clkreq_override,
 	},
 	[IMX8MP] = {
 		.variant = IMX8MP,

@@ -1893,6 +1932,7 @@ static const struct imx_pcie_drvdata drvdata[] = {
 		.mode_off[0] = IOMUXC_GPR12,
 		.mode_mask[0] = IMX6Q_GPR12_DEVICE_TYPE,
 		.enable_ref_clk = imx8mm_pcie_enable_ref_clk,
+		.clr_clkreq_override = imx8mm_pcie_clr_clkreq_override,
 	},
 	[IMX8Q] = {
 		.variant = IMX8Q,

@@ -1913,6 +1953,7 @@ static const struct imx_pcie_drvdata drvdata[] = {
 		.core_reset = imx95_pcie_core_reset,
 		.init_phy = imx95_pcie_init_phy,
 		.wait_pll_lock = imx95_pcie_wait_for_phy_pll_lock,
+		.clr_clkreq_override = imx95_pcie_clr_clkreq_override,
 	},
 	[IMX8MQ_EP] = {
 		.variant = IMX8MQ_EP,
--

2.37.1