[PATCH v4 2/4] PCI: of: Add API to retrieve equalization presets from device tree

[Krishna Chaitanya Chundru <krishna.chundru@oss.qualcomm.com>]

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@oss.qualcomm.com>
---
 drivers/pci/of.c  | 47 +++++++++++++++++++++++++++++++++++++++++++++++
 drivers/pci/pci.h | 24 +++++++++++++++++++++++-
 2 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/of.c b/drivers/pci/of.c
index dacea3fc5128..7aa17c0042c5 100644
--- a/drivers/pci/of.c
+++ b/drivers/pci/of.c
@@ -826,3 +826,50 @@ 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);
+
+/**
+ * of_pci_get_equalization_presets - Parses the "eq-presets-ngts" property.
+ *
+ * @dev: Device containing the properties.
+ * @presets: Pointer to store the parsed data.
+ * @num_lanes: Maximum number of lanes supported.
+ *
+ * If the property is present read and store the data in the preset structure
+ * assign default value 0xff to indicate property is not present.
+ *
+ * If the property is not found or is invalid, returns 0.
+ */
+int of_pci_get_equalization_presets(struct device *dev,
+				    struct pci_eq_presets *presets,
+				    int num_lanes)
+{
+	char name[20];
+	int ret;
+
+	presets->eq_presets_8gts[0] = 0xff;
+	if (of_property_present(dev->of_node, "eq-presets-8gts")) {
+		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 = 0; i < EQ_PRESET_TYPE_MAX; i++) {
+		presets->eq_presets_Ngts[i][0] = 0xff;
+		snprintf(name, sizeof(name), "eq-presets-%dgts", 8 << (i + 1));
+		if (of_property_present(dev->of_node, name)) {
+			ret = of_property_read_u8_array(dev->of_node, name,
+							presets->eq_presets_Ngts[i],
+							num_lanes);
+			if (ret) {
+				dev_err(dev, "Error %s %d\n", name, ret);
+				return ret;
+			}
+		}
+	}
+
+	return 0;
+}
+EXPORT_SYMBOL_GPL(of_pci_get_equalization_presets);
diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
index 14d00ce45bfa..3a8c04e3b30d 100644
--- a/drivers/pci/pci.h
+++ b/drivers/pci/pci.h
@@ -7,6 +7,8 @@
 /* Number of possible devfns: 0.0 to 1f.7 inclusive */
 #define MAX_NR_DEVFNS 256
 
+#define MAX_NR_LANES 16
+
 #define PCI_FIND_CAP_TTL	48
 
 #define PCI_VSEC_ID_INTEL_TBT	0x1234	/* Thunderbolt */
@@ -732,6 +734,18 @@ static inline u64 pci_rebar_size_to_bytes(int size)
 
 struct device_node;
 
+enum equalization_preset_type {
+	EQ_PRESET_TYPE_16GTS,
+	EQ_PRESET_TYPE_32GTS,
+	EQ_PRESET_TYPE_64GTS,
+	EQ_PRESET_TYPE_MAX
+};
+
+struct pci_eq_presets {
+	u16 eq_presets_8gts[MAX_NR_LANES];
+	u8 eq_presets_Ngts[EQ_PRESET_TYPE_MAX][MAX_NR_LANES];
+};
+
 #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 +760,9 @@ 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 +809,12 @@ 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