[PATCH v3] PCI: imx6: Update MPLLB bandwidth to improve i.MX95 Gen3 PCIe stability

[hongxing . zhu]

From: Richard Zhu <hongxing.zhu@xxxxxxx>

Bandwidth marginality was observed during i.MX95 Gen3 PCIe tests with
the default MPLLB_BANDWIDTH value. This margin degradation worsens
across voltage and temperature (VT) variations and different test
matrices, potentially causing link stability issues.

Testing with MPLLB_BANDWIDTH value of 140 (0x8c) shows significant
improvement in bandwidth margins across all VT conditions and test
scenarios.

Implement PHY register write helper function and configure:
- MPLLB_BW_OVRD_IN = 140 (0x8c) for improved bandwidth margin
- MPLLB_BW_OVRD_EN to enable the override

This ensures robust PCIe Gen3 performance across all operating
conditions.

Signed-off-by: Richard Zhu <hongxing.zhu@xxxxxxx>
---
Changes in v3:
Regarding Frank's comments, refine naming consistency and clarity.

Changes in v2:
Update the register name and bit definitions.
Don't move IMX95_PCIE_PHY_CR_PARA_SEL settings.
---
 drivers/pci/controller/dwc/pci-imx6.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/drivers/pci/controller/dwc/pci-imx6.c b/drivers/pci/controller/dwc/pci-imx6.c
index 53f3da6ab30d5..fadf14de10f7c 100644
--- a/drivers/pci/controller/dwc/pci-imx6.c
+++ b/drivers/pci/controller/dwc/pci-imx6.c
@@ -80,6 +80,17 @@
 #define IMX95_SID_MASK				GENMASK(5, 0)
 #define IMX95_MAX_LUT				32
 
+#define IMX95_PCIE_PHY_REG_ADDR			0x3008
+#define IMX95_PCIE_PHY_REG_EN			BIT(31)
+#define IMX95_PCIE_PHY_REG_ADDR_MASK		GENMASK(15, 0)
+#define IMX95_PCIE_PHY_REG_DATA			0x300c
+#define IMX95_PCIE_PHY_MPLLB_OVRD_IN		0x2004
+/* BIT(10): Override enable for mpllb_bandwidth[15:0] */
+#define IMX95_PCIE_PHY_MPLLB_OVRD_BW_EN		0x400
+/* Register offset: Override value for mpllb_bandwidth[15:0] */
+#define IMX95_PCIE_PHY_MPLLB_BW_IN		0x2005
+#define IMX95_PCIE_PHY_MPLLB_BW_VAL		0x8c
+
 #define IMX95_PCIE_RST_CTRL			0x3010
 #define IMX95_PCIE_COLD_RST			BIT(0)
 
@@ -269,6 +280,16 @@ static int imx95_pcie_select_ref_clk_src(struct imx_pcie *imx_pcie)
 	return 0;
 }
 
+static void imx95_pcie_phy_write(struct imx_pcie *imx_pcie, int addr, u16 data)
+{
+	udelay(200);
+	regmap_update_bits(imx_pcie->iomuxc_gpr, IMX95_PCIE_PHY_REG_ADDR,
+			   IMX95_PCIE_PHY_REG_EN, IMX95_PCIE_PHY_REG_EN);
+	regmap_update_bits(imx_pcie->iomuxc_gpr, IMX95_PCIE_PHY_REG_ADDR,
+			   IMX95_PCIE_PHY_REG_ADDR_MASK, addr);
+	regmap_write(imx_pcie->iomuxc_gpr, IMX95_PCIE_PHY_REG_DATA, data);
+}
+
 static int imx95_pcie_init_phy(struct imx_pcie *imx_pcie)
 {
 	/*
@@ -289,6 +310,11 @@ static int imx95_pcie_init_phy(struct imx_pcie *imx_pcie)
 			IMX95_PCIE_PHY_CR_PARA_SEL,
 			IMX95_PCIE_PHY_CR_PARA_SEL);
 
+	imx95_pcie_phy_write(imx_pcie, IMX95_PCIE_PHY_MPLLB_BW_IN,
+			     IMX95_PCIE_PHY_MPLLB_BW_VAL);
+	imx95_pcie_phy_write(imx_pcie, IMX95_PCIE_PHY_MPLLB_OVRD_IN,
+			     IMX95_PCIE_PHY_MPLLB_OVRD_BW_EN);
+
 	return 0;
 }
 
-- 
2.34.1