PCI: qcom: Add RX lane margining settings for 16.0 GT/s

Add RX lane margining settings for 16.0 GT/s (GEN 4) data rate.

These settings improve link stability while operating at high date
rates and helps to improve signal quality.

Link: https://lore.kernel.org/linux-pci/[e-mail]
Tested-by: Johan Hovold <[e-mail]>
Signed-off-by: Shashank Babu Chinta Venkata <[e-mail]>
[mani: dropped the code refactoring and minor changes]
Signed-off-by: Manivannan Sadhasivam <[e-mail]>
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <[e-mail]>
Reviewed-by: Johan Hovold <[e-mail]>
Reviewed-by: Manivannan Sadhasivam <[e-mail]>

drivers/pci/controller/dwc/pcie-designware.h

···
 #define PCIE_PL_CHK_REG_ERR_ADDR			0xB28
 
 /*
+ * 16.0 GT/s (Gen 4) lane margining register definitions
+ */
+#define GEN4_LANE_MARGINING_1_OFF		0xB80
+#define MARGINING_MAX_VOLTAGE_OFFSET		GENMASK(29, 24)
+#define MARGINING_NUM_VOLTAGE_STEPS		GENMASK(22, 16)
+#define MARGINING_MAX_TIMING_OFFSET		GENMASK(13, 8)
+#define MARGINING_NUM_TIMING_STEPS		GENMASK(5, 0)
+
+#define GEN4_LANE_MARGINING_2_OFF		0xB84
+#define MARGINING_IND_ERROR_SAMPLER		BIT(28)
+#define MARGINING_SAMPLE_REPORTING_METHOD	BIT(27)
+#define MARGINING_IND_LEFT_RIGHT_TIMING		BIT(26)
+#define MARGINING_IND_UP_DOWN_VOLTAGE		BIT(25)
+#define MARGINING_VOLTAGE_SUPPORTED		BIT(24)
+#define MARGINING_MAXLANES			GENMASK(20, 16)
+#define MARGINING_SAMPLE_RATE_TIMING		GENMASK(13, 8)
+#define MARGINING_SAMPLE_RATE_VOLTAGE		GENMASK(5, 0)
+/*
  * iATU Unroll-specific register definitions
  * From 4.80 core version the address translation will be made by unroll
  */

drivers/pci/controller/dwc/pcie-qcom-common.c

···
 	dw_pcie_writel_dbi(pci, GEN3_EQ_CONTROL_OFF, reg);
 }
 EXPORT_SYMBOL_GPL(qcom_pcie_common_set_16gt_equalization);
+
+void qcom_pcie_common_set_16gt_lane_margining(struct dw_pcie *pci)
+{
+	u32 reg;
+
+	reg = dw_pcie_readl_dbi(pci, GEN4_LANE_MARGINING_1_OFF);
+	reg &= ~(MARGINING_MAX_VOLTAGE_OFFSET |
+		MARGINING_NUM_VOLTAGE_STEPS |
+		MARGINING_MAX_TIMING_OFFSET |
+		MARGINING_NUM_TIMING_STEPS);
+	reg |= FIELD_PREP(MARGINING_MAX_VOLTAGE_OFFSET, 0x24) |
+		FIELD_PREP(MARGINING_NUM_VOLTAGE_STEPS, 0x78) |
+		FIELD_PREP(MARGINING_MAX_TIMING_OFFSET, 0x32) |
+		FIELD_PREP(MARGINING_NUM_TIMING_STEPS, 0x10);
+	dw_pcie_writel_dbi(pci, GEN4_LANE_MARGINING_1_OFF, reg);
+
+	reg = dw_pcie_readl_dbi(pci, GEN4_LANE_MARGINING_2_OFF);
+	reg |= MARGINING_IND_ERROR_SAMPLER |
+		MARGINING_SAMPLE_REPORTING_METHOD |
+		MARGINING_IND_LEFT_RIGHT_TIMING |
+		MARGINING_VOLTAGE_SUPPORTED;
+	reg &= ~(MARGINING_IND_UP_DOWN_VOLTAGE |
+		MARGINING_MAXLANES |
+		MARGINING_SAMPLE_RATE_TIMING |
+		MARGINING_SAMPLE_RATE_VOLTAGE);
+	reg |= FIELD_PREP(MARGINING_MAXLANES, pci->num_lanes) |
+		FIELD_PREP(MARGINING_SAMPLE_RATE_TIMING, 0x3f) |
+		FIELD_PREP(MARGINING_SAMPLE_RATE_VOLTAGE, 0x3f);
+	dw_pcie_writel_dbi(pci, GEN4_LANE_MARGINING_2_OFF, reg);
+}
+EXPORT_SYMBOL_GPL(qcom_pcie_common_set_16gt_lane_margining);

drivers/pci/controller/dwc/pcie-qcom-common.h

···
 struct dw_pcie;
 
 void qcom_pcie_common_set_16gt_equalization(struct dw_pcie *pci);
+void qcom_pcie_common_set_16gt_lane_margining(struct dw_pcie *pci);
 
 #endif

drivers/pci/controller/dwc/pcie-qcom-ep.c

···
 		goto err_disable_resources;
 	}
 
-	if (pcie_link_speed[pci->max_link_speed] == PCIE_SPEED_16_0GT)
+	if (pcie_link_speed[pci->max_link_speed] == PCIE_SPEED_16_0GT) {
 		qcom_pcie_common_set_16gt_equalization(pci);
+		qcom_pcie_common_set_16gt_lane_margining(pci);
+	}
 
 	/*
 	 * The physical address of the MMIO region which is exposed as the BAR

drivers/pci/controller/dwc/pcie-qcom.c

···
 {
 	struct qcom_pcie *pcie = to_qcom_pcie(pci);
 
-	if (pcie_link_speed[pci->max_link_speed] == PCIE_SPEED_16_0GT)
+	if (pcie_link_speed[pci->max_link_speed] == PCIE_SPEED_16_0GT) {
 		qcom_pcie_common_set_16gt_equalization(pci);
+		qcom_pcie_common_set_16gt_lane_margining(pci);
+	}
 
 	/* Enable Link Training state machine */
 	if (pcie->cfg->ops->ltssm_enable)