PCI: qcom: Add equalization settings for 16.0 GT/s

During high data transmission rates such as 16.0 GT/s, there is an
increased risk of signal loss due to poor channel quality and
interference. This can impact receiver's ability to capture signals
accurately.

Hence, as signal compensation is achieved through appropriate lane
equalization, apply lane equalization settings at both transmitter
and receiver which results in an increase in the PCIe signal strength.

While at it, modify the pcie-tegra194 driver to make use of the
common GEN3_EQ_CONTROL_OFF definitions in pcie-designware.h.

Link: https://lore.kernel.org/linux-pci/20240911-pci-qcom-gen4-stability-v7-3-743f5c1fd027@linaro.org
Tested-by: Johan Hovold <johan+linaro@kernel.org>
Signed-off-by: Shashank Babu Chinta Venkata <quic_schintav@quicinc.com>
[mani: dropped the code refactoring and minor changes]
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
[kwilczynski: commit log]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

1 files changed, 47 insertions, 0 deletions

diff --git a/drivers/pci/controller/dwc/pcie-qcom-common.c b/drivers/pci/controller/dwc/pcie-qcom-common.c
new file mode 100644
index 000000000000..596a35449de1
--- /dev/null
+++ b/drivers/pci/controller/dwc/pcie-qcom-common.c
@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Copyright (c) 2024 Qualcomm Innovation Center, Inc. All rights reserved.
+ */
+
+#include <linux/pci.h>
+
+#include "pcie-designware.h"
+#include "pcie-qcom-common.h"
+
+void qcom_pcie_common_set_16gt_equalization(struct dw_pcie *pci)
+{
+	u32 reg;
+
+	/*
+	 * GEN3_RELATED_OFF register is repurposed to apply equalization
+	 * settings at various data transmission rates through registers namely
+	 * GEN3_EQ_*. The RATE_SHADOW_SEL bit field of GEN3_RELATED_OFF
+	 * determines the data rate for which these equalization settings are
+	 * applied.
+	 */
+	reg = dw_pcie_readl_dbi(pci, GEN3_RELATED_OFF);
+	reg &= ~GEN3_RELATED_OFF_GEN3_ZRXDC_NONCOMPL;
+	reg &= ~GEN3_RELATED_OFF_RATE_SHADOW_SEL_MASK;
+	reg |= FIELD_PREP(GEN3_RELATED_OFF_RATE_SHADOW_SEL_MASK,
+			  GEN3_RELATED_OFF_RATE_SHADOW_SEL_16_0GT);
+	dw_pcie_writel_dbi(pci, GEN3_RELATED_OFF, reg);
+
+	reg = dw_pcie_readl_dbi(pci, GEN3_EQ_FB_MODE_DIR_CHANGE_OFF);
+	reg &= ~(GEN3_EQ_FMDC_T_MIN_PHASE23 |
+		GEN3_EQ_FMDC_N_EVALS |
+		GEN3_EQ_FMDC_MAX_PRE_CUSROR_DELTA |
+		GEN3_EQ_FMDC_MAX_POST_CUSROR_DELTA);
+	reg |= FIELD_PREP(GEN3_EQ_FMDC_T_MIN_PHASE23, 0x1) |
+		FIELD_PREP(GEN3_EQ_FMDC_N_EVALS, 0xd) |
+		FIELD_PREP(GEN3_EQ_FMDC_MAX_PRE_CUSROR_DELTA, 0x5) |
+		FIELD_PREP(GEN3_EQ_FMDC_MAX_POST_CUSROR_DELTA, 0x5);
+	dw_pcie_writel_dbi(pci, GEN3_EQ_FB_MODE_DIR_CHANGE_OFF, reg);
+
+	reg = dw_pcie_readl_dbi(pci, GEN3_EQ_CONTROL_OFF);
+	reg &= ~(GEN3_EQ_CONTROL_OFF_FB_MODE |
+		GEN3_EQ_CONTROL_OFF_PHASE23_EXIT_MODE |
+		GEN3_EQ_CONTROL_OFF_FOM_INC_INITIAL_EVAL |
+		GEN3_EQ_CONTROL_OFF_PSET_REQ_VEC);
+	dw_pcie_writel_dbi(pci, GEN3_EQ_CONTROL_OFF, reg);
+}
+EXPORT_SYMBOL_GPL(qcom_pcie_common_set_16gt_equalization);