PCI: mediatek-gen3: Add support for setting max-link-speed limit

Add support for respecting the max-link-speed devicetree property,
forcing a maximum speed (Gen) for a PCI-Express port.

Since the MediaTek PCIe Gen3 controllers also expose the maximum
supported link speed in the PCIE_BASE_CFG register, if property
max-link-speed is specified in devicetree, validate it against the
controller capabilities and proceed setting the limitations only
if the wanted Gen is lower than the maximum one that is supported
by the controller itself (otherwise it makes no sense!).

Link: https://lore.kernel.org/r/20241104114935.172908-2-angelogioacchino.delregno@collabora.com
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
[kwilczynski: change dev_dbg() to dev_info() and update message wording]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Fei Shao <fshao@chromium.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>

Author: AngeloGioacchino Del Regno

drivers/pci/controller/pcie-mediatek-gen3.c

@@ -28,7 +28,11 @@
 
 #include "../pci.h"
 
+#define PCIE_BASE_CFG_REG		0x14
+#define PCIE_BASE_CFG_SPEED		GENMASK(15, 8)
+
 #define PCIE_SETTING_REG		0x80
+#define PCIE_SETTING_GEN_SUPPORT	GENMASK(14, 12)
 #define PCIE_PCI_IDS_1			0x9c
 #define PCI_CLASS(class)		(class << 8)
 #define PCIE_RC_MODE			BIT(0)
@@ -125,6 +129,9 @@
 
 struct mtk_gen3_pcie;
 
+#define PCIE_CONF_LINK2_CTL_STS		(PCIE_CFG_OFFSET_ADDR + 0xb0)
+#define PCIE_CONF_LINK2_LCR2_LINK_SPEED	GENMASK(3, 0)
+
 /**
  * struct mtk_gen3_pcie_pdata - differentiate between host generations
  * @power_up: pcie power_up callback
@@ -160,6 +167,7 @@ struct mtk_msi_set {
  * @phy: PHY controller block
  * @clks: PCIe clocks
  * @num_clks: PCIe clocks count for this port
+ * @max_link_speed: Maximum link speed (PCIe Gen) for this port
  * @irq: PCIe controller interrupt number
  * @saved_irq_state: IRQ enable state saved at suspend time
  * @irq_lock: lock protecting IRQ register access
@@ -180,6 +188,7 @@ struct mtk_gen3_pcie {
 	struct phy *phy;
 	struct clk_bulk_data *clks;
 	int num_clks;
+	u8 max_link_speed;
 
 	int irq;
 	u32 saved_irq_state;
@@ -381,11 +390,27 @@ static int mtk_pcie_startup_port(struct mtk_gen3_pcie *pcie)
 	int err;
 	u32 val;
 
-	/* Set as RC mode */
+	/* Set as RC mode and set controller PCIe Gen speed restriction, if any */
 	val = readl_relaxed(pcie->base + PCIE_SETTING_REG);
 	val |= PCIE_RC_MODE;
+	if (pcie->max_link_speed) {
+		val &= ~PCIE_SETTING_GEN_SUPPORT;
+
+		/* Can enable link speed support only from Gen2 onwards */
+		if (pcie->max_link_speed >= 2)
+			val |= FIELD_PREP(PCIE_SETTING_GEN_SUPPORT,
+					  GENMASK(pcie->max_link_speed - 2, 0));
+	}
 	writel_relaxed(val, pcie->base + PCIE_SETTING_REG);
 
+	/* Set Link Control 2 (LNKCTL2) speed restriction, if any */
+	if (pcie->max_link_speed) {
+		val = readl_relaxed(pcie->base + PCIE_CONF_LINK2_CTL_STS);
+		val &= ~PCIE_CONF_LINK2_LCR2_LINK_SPEED;
+		val |= FIELD_PREP(PCIE_CONF_LINK2_LCR2_LINK_SPEED, pcie->max_link_speed);
+		writel_relaxed(val, pcie->base + PCIE_CONF_LINK2_CTL_STS);
+	}
+
 	/* Set class code */
 	val = readl_relaxed(pcie->base + PCIE_PCI_IDS_1);
 	val &= ~GENMASK(31, 8);
@@ -1004,9 +1029,21 @@ static void mtk_pcie_power_down(struct mtk_gen3_pcie *pcie)
 	reset_control_bulk_assert(pcie->soc->phy_resets.num_resets, pcie->phy_resets);
 }
 
+static int mtk_pcie_get_controller_max_link_speed(struct mtk_gen3_pcie *pcie)
+{
+	u32 val;
+	int ret;
+
+	val = readl_relaxed(pcie->base + PCIE_BASE_CFG_REG);
+	val = FIELD_GET(PCIE_BASE_CFG_SPEED, val);
+	ret = fls(val);
+
+	return ret > 0 ? ret : -EINVAL;
+}
+
 static int mtk_pcie_setup(struct mtk_gen3_pcie *pcie)
 {
-	int err;
+	int err, max_speed;
 
 	err = mtk_pcie_parse_port(pcie);
 	if (err)
@@ -1031,6 +1068,20 @@ static int mtk_pcie_setup(struct mtk_gen3_pcie *pcie)
 	if (err)
 		return err;
 
+	err = of_pci_get_max_link_speed(pcie->dev->of_node);
+	if (err) {
+		/* Get the maximum speed supported by the controller */
+		max_speed = mtk_pcie_get_controller_max_link_speed(pcie);
+
+		/* Set max_link_speed only if the controller supports it */
+		if (max_speed >= 0 && max_speed <= err) {
+			pcie->max_link_speed = err;
+			dev_info(pcie->dev,
+				 "maximum controller link speed Gen%d, overriding to Gen%u",
+				 max_speed, pcie->max_link_speed);
+		}
+	}
+
 	/* Try link up */
 	err = mtk_pcie_startup_port(pcie);
 	if (err)