PCI: imx6: Save and restore root port MSI control in suspend and resume

The imx6 PCI host controller suffers from a HW integration bug whereby
the MSI enable bit in the root port MSI capability enables/disables MSIs
interrupts for all downstream components in the PCI tree.

This requires, as implemented in

75cb8d2 ("PCI: imx: Enable MSI from downstream components")

that the root port MSI enable bit should be set in order for downstream
PCI devices MSIs to function.

The MSI enable bit programming might be lost during the suspend and
should be re-stored during resume.

Save the MSI control during suspend and restore it in resume.

Link: https://lore.kernel.org/r/1670479534-22154-1-git-send-email-hongxing.zhu@nxp.com
Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
[lpieralisi@kernel.org: commit log]
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>

drivers/pci/controller/dwc/pci-imx6.c

@@ -80,6 +80,7 @@ struct imx6_pcie {
 	struct clk		*pcie;
 	struct clk		*pcie_aux;
 	struct regmap		*iomuxc_gpr;
+	u16			msi_ctrl;
 	u32			controller_id;
 	struct reset_control	*pciephy_reset;
 	struct reset_control	*apps_reset;
@@ -1178,6 +1179,26 @@ static void imx6_pcie_pm_turnoff(struct imx6_pcie *imx6_pcie)
 	usleep_range(1000, 10000);
 }
 
+static void imx6_pcie_msi_save_restore(struct imx6_pcie *imx6_pcie, bool save)
+{
+	u8 offset;
+	u16 val;
+	struct dw_pcie *pci = imx6_pcie->pci;
+
+	if (pci_msi_enabled()) {
+		offset = dw_pcie_find_capability(pci, PCI_CAP_ID_MSI);
+		if (save) {
+			val = dw_pcie_readw_dbi(pci, offset + PCI_MSI_FLAGS);
+			imx6_pcie->msi_ctrl = val;
+		} else {
+			dw_pcie_dbi_ro_wr_en(pci);
+			val = imx6_pcie->msi_ctrl;
+			dw_pcie_writew_dbi(pci, offset + PCI_MSI_FLAGS, val);
+			dw_pcie_dbi_ro_wr_dis(pci);
+		}
+	}
+}
+
 static int imx6_pcie_suspend_noirq(struct device *dev)
 {
 	struct imx6_pcie *imx6_pcie = dev_get_drvdata(dev);
@@ -1186,6 +1207,7 @@ static int imx6_pcie_suspend_noirq(struct device *dev)
 	if (!(imx6_pcie->drvdata->flags & IMX6_PCIE_FLAG_SUPPORTS_SUSPEND))
 		return 0;
 
+	imx6_pcie_msi_save_restore(imx6_pcie, true);
 	imx6_pcie_pm_turnoff(imx6_pcie);
 	imx6_pcie_stop_link(imx6_pcie->pci);
 	imx6_pcie_host_exit(pp);
@@ -1205,6 +1227,7 @@ static int imx6_pcie_resume_noirq(struct device *dev)
 	ret = imx6_pcie_host_init(pp);
 	if (ret)
 		return ret;
+	imx6_pcie_msi_save_restore(imx6_pcie, false);
 	dw_pcie_setup_rc(pp);
 
 	if (imx6_pcie->link_is_up)