PCI: mediatek-gen3: Add support for restricting link width

Add support for restricting the port's link width by specifying
the num-lanes devicetree property in the PCIe node.

The setting is done in the GEN_SETTINGS register (in the driver
named as PCIE_SETTING_REG), where each set bit in [11:8] activates
a set of lanes (from bits 11 to 8 respectively, x16/x8/x4/x2).

Link: https://lore.kernel.org/r/20241104114935.172908-3-angelogioacchino.delregno@collabora.com
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
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

@@ -32,6 +32,7 @@
 #define PCIE_BASE_CFG_SPEED		GENMASK(15, 8)
 
 #define PCIE_SETTING_REG		0x80
+#define PCIE_SETTING_LINK_WIDTH		GENMASK(11, 8)
 #define PCIE_SETTING_GEN_SUPPORT	GENMASK(14, 12)
 #define PCIE_PCI_IDS_1			0x9c
 #define PCI_CLASS(class)		(class << 8)
@@ -168,6 +169,7 @@ struct mtk_msi_set {
  * @clks: PCIe clocks
  * @num_clks: PCIe clocks count for this port
  * @max_link_speed: Maximum link speed (PCIe Gen) for this port
+ * @num_lanes: Number of PCIe lanes 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
@@ -189,6 +191,7 @@ struct mtk_gen3_pcie {
 	struct clk_bulk_data *clks;
 	int num_clks;
 	u8 max_link_speed;
+	u8 num_lanes;
 
 	int irq;
 	u32 saved_irq_state;
@@ -401,6 +404,14 @@ static int mtk_pcie_startup_port(struct mtk_gen3_pcie *pcie)
 			val |= FIELD_PREP(PCIE_SETTING_GEN_SUPPORT,
 					  GENMASK(pcie->max_link_speed - 2, 0));
 	}
+	if (pcie->num_lanes) {
+		val &= ~PCIE_SETTING_LINK_WIDTH;
+
+		/* Zero means one lane, each bit activates x2/x4/x8/x16 */
+		if (pcie->num_lanes > 1)
+			val |= FIELD_PREP(PCIE_SETTING_LINK_WIDTH,
+					  GENMASK(fls(pcie->num_lanes >> 2), 0));
+	};
 	writel_relaxed(val, pcie->base + PCIE_SETTING_REG);
 
 	/* Set Link Control 2 (LNKCTL2) speed restriction, if any */
@@ -838,6 +849,7 @@ static int mtk_pcie_parse_port(struct mtk_gen3_pcie *pcie)
 	struct device *dev = pcie->dev;
 	struct platform_device *pdev = to_platform_device(dev);
 	struct resource *regs;
+	u32 num_lanes;
 
 	regs = platform_get_resource_byname(pdev, IORESOURCE_MEM, "pcie-mac");
 	if (!regs)
@@ -883,6 +895,14 @@ static int mtk_pcie_parse_port(struct mtk_gen3_pcie *pcie)
 		return pcie->num_clks;
 	}
 
+       ret = of_property_read_u32(dev->of_node, "num-lanes", &num_lanes);
+       if (ret == 0) {
+	       if (num_lanes == 0 || num_lanes > 16 || (num_lanes != 1 && num_lanes % 2))
+			dev_warn(dev, "invalid num-lanes, using controller defaults\n");
+	       else
+			pcie->num_lanes = num_lanes;
+       }
+
 	return 0;
 }