imx: 6.6: add a patch to work around a PCI init hang on IMX8M{M,P}

The act of attempting link at gen1 then trying to link at higher speeds
causes a hang with the specific PCIe switch used on the Gateworks Venice
boards. Work around this by linking at the highest speed first as is
common with all other PCI controller drivers.

Signed-off-by: Tim Harvey <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/17717
Signed-off-by: Robert Marko <[email protected]>

target/linux/imx/patches-6.6/600-PCI-imx6-Start-link-at-max-gen-first-for-IMX8MM-and-IMX8MP.patch

@@ -0,0 +1,119 @@
+From cf983e4a04eecb5be93af7b53cb10805ee448998 Mon Sep 17 00:00:00 2001
+From: Tim Harvey <[email protected]>
+Date: Mon, 21 Aug 2023 09:20:17 -0700
+Subject: [PATCH] PCI: imx6: Start link at max gen first for IMX8MM and IMX8MP
+
+commit fa33a6d87eac ("PCI: imx6: Start link in Gen1 before negotiating
+for Gen2 mode") started link negotiation at Gen1 before attempting
+faster speeds in order to work around an issue with a particular switch
+on an IMX6Q SoC.
+
+This behavior is not the norm for PCI link negotiation and it has been
+found to cause issues in other cases:
+- IMX8MM with PI7C9X2G608GP switch: various endpoints (such as qca988x)
+  will fail to link more than 50% of the time
+- IMX8MP with PI7C9X2G608GP switch: occasionally will fail to link with
+  switch and cause a CPU hang about 30% of the time
+
+Disable this behavior for IMX8MM and IMX8MP.
+
+Signed-off-by: Tim Harvey <[email protected]>
+---
+ drivers/pci/controller/dwc/pci-imx6.c | 53 ++++++++++++++-------------
+ 1 file changed, 27 insertions(+), 26 deletions(-)
+
+--- a/drivers/pci/controller/dwc/pci-imx6.c
++++ b/drivers/pci/controller/dwc/pci-imx6.c
+@@ -60,6 +60,7 @@ enum imx6_pcie_variants {
+ #define IMX6_PCIE_FLAG_IMX6_PHY			BIT(0)
+ #define IMX6_PCIE_FLAG_IMX6_SPEED_CHANGE	BIT(1)
+ #define IMX6_PCIE_FLAG_SUPPORTS_SUSPEND		BIT(2)
++#define IMX6_PCIE_FLAG_GEN1_LAST		BIT(3)
+ 
+ struct imx6_pcie_drvdata {
+ 	enum imx6_pcie_variants variant;
+@@ -876,26 +877,28 @@ static int imx6_pcie_start_link(struct d
+ 	u32 tmp;
+ 	int ret;
+ 
+-	/*
+-	 * Force Gen1 operation when starting the link.  In case the link is
+-	 * started in Gen2 mode, there is a possibility the devices on the
+-	 * bus will not be detected at all.  This happens with PCIe switches.
+-	 */
+-	dw_pcie_dbi_ro_wr_en(pci);
+-	tmp = dw_pcie_readl_dbi(pci, offset + PCI_EXP_LNKCAP);
+-	tmp &= ~PCI_EXP_LNKCAP_SLS;
+-	tmp |= PCI_EXP_LNKCAP_SLS_2_5GB;
+-	dw_pcie_writel_dbi(pci, offset + PCI_EXP_LNKCAP, tmp);
+-	dw_pcie_dbi_ro_wr_dis(pci);
++	if (!(imx6_pcie->drvdata->flags & IMX6_PCIE_FLAG_GEN1_LAST)) {
++		/*
++		 * Force Gen1 operation when starting the link.  In case the link is
++		 * started in Gen2 mode, there is a possibility the devices on the
++		 * bus will not be detected at all.  This happens with PCIe switches.
++		 */
++		dw_pcie_dbi_ro_wr_en(pci);
++		tmp = dw_pcie_readl_dbi(pci, offset + PCI_EXP_LNKCAP);
++		tmp &= ~PCI_EXP_LNKCAP_SLS;
++		tmp |= PCI_EXP_LNKCAP_SLS_2_5GB;
++		dw_pcie_writel_dbi(pci, offset + PCI_EXP_LNKCAP, tmp);
++		dw_pcie_dbi_ro_wr_dis(pci);
++	}
+ 
+ 	/* Start LTSSM. */
+ 	imx6_pcie_ltssm_enable(dev);
+ 
+-	ret = dw_pcie_wait_for_link(pci);
+-	if (ret)
+-		goto err_reset_phy;
++	if ((pci->link_gen > 1) && !(imx6_pcie->drvdata->flags & IMX6_PCIE_FLAG_GEN1_LAST)) {
++		ret = dw_pcie_wait_for_link(pci);
++		if (ret)
++			goto err_reset_phy;
+ 
+-	if (pci->link_gen > 1) {
+ 		/* Allow faster modes after the link is up */
+ 		dw_pcie_dbi_ro_wr_en(pci);
+ 		tmp = dw_pcie_readl_dbi(pci, offset + PCI_EXP_LNKCAP);
+@@ -929,18 +932,14 @@ static int imx6_pcie_start_link(struct d
+ 				goto err_reset_phy;
+ 			}
+ 		}
+-
+-		/* Make sure link training is finished as well! */
+-		ret = dw_pcie_wait_for_link(pci);
+-		if (ret)
+-			goto err_reset_phy;
+-	} else {
+-		dev_info(dev, "Link: Only Gen1 is enabled\n");
+ 	}
+ 
++	ret = dw_pcie_wait_for_link(pci);
++	if (ret)
++		goto err_reset_phy;
++
+ 	imx6_pcie->link_is_up = true;
+-	tmp = dw_pcie_readw_dbi(pci, offset + PCI_EXP_LNKSTA);
+-	dev_info(dev, "Link up, Gen%i\n", tmp & PCI_EXP_LNKSTA_CLS);
++
+ 	return 0;
+ 
+ err_reset_phy:
+@@ -1505,12 +1504,14 @@ static const struct imx6_pcie_drvdata dr
+ 	},
+ 	[IMX8MM] = {
+ 		.variant = IMX8MM,
+-		.flags = IMX6_PCIE_FLAG_SUPPORTS_SUSPEND,
++		.flags = IMX6_PCIE_FLAG_SUPPORTS_SUSPEND |
++			 IMX6_PCIE_FLAG_GEN1_LAST,
+ 		.gpr = "fsl,imx8mm-iomuxc-gpr",
+ 	},
+ 	[IMX8MP] = {
+ 		.variant = IMX8MP,
+-		.flags = IMX6_PCIE_FLAG_SUPPORTS_SUSPEND,
++		.flags = IMX6_PCIE_FLAG_SUPPORTS_SUSPEND |
++			 IMX6_PCIE_FLAG_GEN1_LAST,
+ 		.gpr = "fsl,imx8mp-iomuxc-gpr",
+ 	},
+ 	[IMX8MQ_EP] = {