linux-kernel - [PATCH v3 2/4] PCI: of: Add API to retrieve equalization presets from device tree

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Message-Id: <20241223-preset_v2-v3-2-a339f475caf5@oss.qualcomm.com>
Date: Mon, 23 Dec 2024 12:21:15 +0530
From: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
To: Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>, Jingoo Han <jingoohan1@...il.com>,
        Manivannan Sadhasivam <manivannan.sadhasivam@...aro.org>,
        Lorenzo Pieralisi <lpieralisi@...nel.org>,
        Krzysztof Wilczyński <kw@...ux.com>
Cc: linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        konrad.dybcio@....qualcomm.com, quic_mrana@...cinc.com,
        quic_vbadigan@...cinc.com, Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konradybcio@...nel.org>,
        Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
Subject: [PATCH v3 2/4] PCI: of: Add API to retrieve equalization presets
 from device tree

PCIe equalization presets are predefined settings used to optimize
signal integrity by compensating for signal loss and distortion in
high-speed data transmission.

As per PCIe spec 6.0.1 revision section 8.3.3.3 & 4.2.4 for data rates
of 8.0 GT/s, 16.0 GT/s, 32.0 GT/s, and 64.0 GT/s, there is a way to
configure lane equalization presets for each lane to enhance the PCIe
link reliability. Each preset value represents a different combination
of pre-shoot and de-emphasis values. For each data rate, different
registers are defined: for 8.0 GT/s, registers are defined in section
7.7.3.4; for 16.0 GT/s, in section 7.7.5.9, etc. The 8.0 GT/s rate has
an extra receiver preset hint, requiring 16 bits per lane, while the
remaining data rates use 8 bits per lane.

Based on the number of lanes and the supported data rate, this function
reads the device tree property and stores in the presets structure.

Signed-off-by: Krishna Chaitanya Chundru <krishna.chundru@....qualcomm.com>
---
 drivers/pci/of.c  | 45 +++++++++++++++++++++++++++++++++++++++++++++
 drivers/pci/pci.h | 17 +++++++++++++++--
 2 files changed, 60 insertions(+), 2 deletions(-)

diff --git a/drivers/pci/of.c b/drivers/pci/of.c
index dacea3fc5128..99e0e7ae12e9 100644
--- a/drivers/pci/of.c
+++ b/drivers/pci/of.c
@@ -826,3 +826,48 @@ u32 of_pci_get_slot_power_limit(struct device_node *node,
 	return slot_power_limit_mw;
 }
 EXPORT_SYMBOL_GPL(of_pci_get_slot_power_limit);
+
+int of_pci_get_equalization_presets(struct device *dev,
+				    struct pci_eq_presets *presets,
+				    int num_lanes)
+{
+	char name[20];
+	void **preset;
+	void *temp;
+	int ret;
+
+	if (of_property_present(dev->of_node, "eq-presets-8gts")) {
+		presets->eq_presets_8gts = devm_kzalloc(dev, sizeof(u16) * num_lanes, GFP_KERNEL);
+		if (!presets->eq_presets_8gts)
+			return -ENOMEM;
+
+		ret = of_property_read_u16_array(dev->of_node, "eq-presets-8gts",
+						 presets->eq_presets_8gts, num_lanes);
+		if (ret) {
+			dev_err(dev, "Error reading eq-presets-8gts %d\n", ret);
+			return ret;
+		}
+	}
+
+	for (int i = 1; i < sizeof(struct pci_eq_presets) / sizeof(void *); i++) {
+		snprintf(name, sizeof(name), "eq-presets-%dgts", 8 << i);
+		if (of_property_present(dev->of_node, name)) {
+			temp = devm_kzalloc(dev, sizeof(u8) * num_lanes, GFP_KERNEL);
+			if (!temp)
+				return -ENOMEM;
+
+			ret = of_property_read_u8_array(dev->of_node, name,
+							temp, num_lanes);
+			if (ret) {
+				dev_err(dev, "Error %s %d\n", name, ret);
+				return ret;
+			}
+
+			preset = (void **)((u8 *)presets + i * sizeof(void *));
+			*preset = temp;
+		}
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(of_pci_get_equalization_presets);
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 14d00ce45bfa..82362d58bedc 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -731,7 +731,12 @@ static inline u64 pci_rebar_size_to_bytes(int size)
 }
 
 struct device_node;
-
+struct pci_eq_presets {
+	void *eq_presets_8gts;
+	void *eq_presets_16gts;
+	void *eq_presets_32gts;
+	void *eq_presets_64gts;
+};
 #ifdef CONFIG_OF
 int of_pci_parse_bus_range(struct device_node *node, struct resource *res);
 int of_get_pci_domain_nr(struct device_node *node);
@@ -746,7 +751,9 @@ void pci_set_bus_of_node(struct pci_bus *bus);
 void pci_release_bus_of_node(struct pci_bus *bus);
 
 int devm_of_pci_bridge_init(struct device *dev, struct pci_host_bridge *bridge);
-
+int of_pci_get_equalization_presets(struct device *dev,
+				    struct pci_eq_presets *presets,
+				    int num_lanes);
 #else
 static inline int
 of_pci_parse_bus_range(struct device_node *node, struct resource *res)
@@ -793,6 +800,12 @@ static inline int devm_of_pci_bridge_init(struct device *dev, struct pci_host_br
 	return 0;
 }
 
+static inline int of_pci_get_equalization_presets(struct device *dev,
+						  struct pci_eq_presets *presets,
+						  int num_lanes)
+{
+	return 0;
+}
 #endif /* CONFIG_OF */
 
 struct of_changeset;

-- 
2.34.1