PCI: mediatek-gen3: Avoid PCIe resetting via PERST# for Airoha EN7581 SoC

Airoha EN7581 has a hw bug asserting/releasing PERST# signal causing
occasional PCIe link down issues. In order to overcome the problem,
PERST# signal is not asserted/released during device probe or
suspend/resume phase and the PCIe block is reset using
en7523_reset_assert() and en7581_pci_enable().

Introduce flags field in the mtk_gen3_pcie_pdata struct in order to
specify per-SoC capabilities.

Link: https://lore.kernel.org/r/20250109-pcie-en7581-rst-fix-v4-1-4a45c89fb143@kernel.org
Tested-by: Hui Ma <hui.ma@airoha.com>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>

Author: LorenzoBianconi

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

@@ -135,17 +135,26 @@ 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)
 
+enum mtk_gen3_pcie_flags {
+	SKIP_PCIE_RSTB	= BIT(0), /* Skip PERST# assertion during device
+				   * probing or suspend/resume phase to
+				   * avoid hw bugs/issues.
+				   */
+};
+
 /**
  * struct mtk_gen3_pcie_pdata - differentiate between host generations
  * @power_up: pcie power_up callback
  * @phy_resets: phy reset lines SoC data.
+ * @flags: pcie device flags.
  */
 struct mtk_gen3_pcie_pdata {
 	int (*power_up)(struct mtk_gen3_pcie *pcie);
 	struct {
 		const char *id[MAX_NUM_PHY_RESETS];
 		int num_resets;
 	} phy_resets;
+	u32 flags;
 };
 
 /**
@@ -440,22 +449,33 @@ static int mtk_pcie_startup_port(struct mtk_gen3_pcie *pcie)
 	val |= PCIE_DISABLE_DVFSRC_VLT_REQ;
 	writel_relaxed(val, pcie->base + PCIE_MISC_CTRL_REG);
 
-	/* Assert all reset signals */
-	val = readl_relaxed(pcie->base + PCIE_RST_CTRL_REG);
-	val |= PCIE_MAC_RSTB | PCIE_PHY_RSTB | PCIE_BRG_RSTB | PCIE_PE_RSTB;
-	writel_relaxed(val, pcie->base + PCIE_RST_CTRL_REG);
-
 	/*
-	 * Described in PCIe CEM specification sections 2.2 (PERST# Signal)
-	 * and 2.2.1 (Initial Power-Up (G3 to S0)).
-	 * The deassertion of PERST# should be delayed 100ms (TPVPERL)
-	 * for the power and clock to become stable.
+	 * Airoha EN7581 has a hw bug asserting/releasing PCIE_PE_RSTB signal
+	 * causing occasional PCIe link down. In order to overcome the issue,
+	 * PCIE_RSTB signals are not asserted/released at this stage and the
+	 * PCIe block is reset using en7523_reset_assert() and
+	 * en7581_pci_enable().
 	 */
-	msleep(100);
-
-	/* De-assert reset signals */
-	val &= ~(PCIE_MAC_RSTB | PCIE_PHY_RSTB | PCIE_BRG_RSTB | PCIE_PE_RSTB);
-	writel_relaxed(val, pcie->base + PCIE_RST_CTRL_REG);
+	if (!(pcie->soc->flags & SKIP_PCIE_RSTB)) {
+		/* Assert all reset signals */
+		val = readl_relaxed(pcie->base + PCIE_RST_CTRL_REG);
+		val |= PCIE_MAC_RSTB | PCIE_PHY_RSTB | PCIE_BRG_RSTB |
+		       PCIE_PE_RSTB;
+		writel_relaxed(val, pcie->base + PCIE_RST_CTRL_REG);
+
+		/*
+		 * Described in PCIe CEM specification revision 6.0.
+		 *
+		 * The deassertion of PERST# should be delayed 100ms (TPVPERL)
+		 * for the power and clock to become stable.
+		 */
+		msleep(PCIE_T_PVPERL_MS);
+
+		/* De-assert reset signals */
+		val &= ~(PCIE_MAC_RSTB | PCIE_PHY_RSTB | PCIE_BRG_RSTB |
+			 PCIE_PE_RSTB);
+		writel_relaxed(val, pcie->base + PCIE_RST_CTRL_REG);
+	}
 
 	/* Check if the link is up or not */
 	err = readl_poll_timeout(pcie->base + PCIE_LINK_STATUS_REG, val,
@@ -1242,10 +1262,12 @@ static int mtk_pcie_suspend_noirq(struct device *dev)
 		return err;
 	}
 
-	/* Pull down the PERST# pin */
-	val = readl_relaxed(pcie->base + PCIE_RST_CTRL_REG);
-	val |= PCIE_PE_RSTB;
-	writel_relaxed(val, pcie->base + PCIE_RST_CTRL_REG);
+	if (!(pcie->soc->flags & SKIP_PCIE_RSTB)) {
+		/* Assert the PERST# pin */
+		val = readl_relaxed(pcie->base + PCIE_RST_CTRL_REG);
+		val |= PCIE_PE_RSTB;
+		writel_relaxed(val, pcie->base + PCIE_RST_CTRL_REG);
+	}
 
 	dev_dbg(pcie->dev, "entered L2 states successfully");
 
@@ -1296,6 +1318,7 @@ static const struct mtk_gen3_pcie_pdata mtk_pcie_soc_en7581 = {
 		.id[2] = "phy-lane2",
 		.num_resets = 3,
 	},
+	.flags = SKIP_PCIE_RSTB,
 };
 
 static const struct of_device_id mtk_pcie_of_match[] = {