[PATCH v1 3/3] PCI: imx6: reset link on resume

[Stefan Eichenberger <eichest@gmail.com>]

From: Stefan Eichenberger <stefan.eichenberger@toradex.com>

According to the https://www.nxp.com/docs/en/errata/IMX6DQCE.pdf errata,
the i.MX6Q PCIe controller does not support suspend/resume. So suspend
and resume was omitted. However, this does not seem to work because it
looks like the PCIe link is still expecting a reset. If we do not reset
the link, we end up with a frozen system after resume. The last message
we see is:
ath10k_pci 0000:01:00.0: Unable to change power state from D3hot to D0,
device inaccessible

Besides resetting the link, we also need to enable msi again, otherwise
DMA access will not work and we can still end up with a frozen system.
With these changes we can suspend and resume the system properly with a
PCIe device attached. This was tested with a Compex WLE900VX miniPCIe
Wifi module.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
---
 drivers/pci/controller/dwc/pci-imx6.c | 45 ++++++++++++++++++++++++++-
 1 file changed, 44 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/controller/dwc/pci-imx6.c b/drivers/pci/controller/dwc/pci-imx6.c
index f17561791e35a..751243f4c519e 100644
--- a/drivers/pci/controller/dwc/pci-imx6.c
+++ b/drivers/pci/controller/dwc/pci-imx6.c
@@ -1213,14 +1213,57 @@ static int imx6_pcie_suspend_noirq(struct device *dev)
 	return 0;
 }
 
+static int imx6_pcie_reset_link(struct imx6_pcie *imx6_pcie)
+{
+	int ret;
+
+	regmap_update_bits(imx6_pcie->iomuxc_gpr, IOMUXC_GPR1,
+			   IMX6Q_GPR1_PCIE_TEST_PD, 1 << 18);
+	regmap_update_bits(imx6_pcie->iomuxc_gpr, IOMUXC_GPR1,
+			   IMX6Q_GPR1_PCIE_REF_CLK_EN, 0 << 16);
+
+	/* Reset the PCIe device */
+	gpiod_set_value_cansleep(imx6_pcie->reset_gpiod, 1);
+
+	ret = imx6_pcie_enable_ref_clk(imx6_pcie);
+	if (ret) {
+		dev_err(imx6_pcie->pci->dev, "unable to enable pcie ref clock\n");
+		return ret;
+	}
+
+	imx6_pcie_deassert_reset_gpio(imx6_pcie);
+
+	/*
+	 * Setup the root complex again and enable msi. Without this PCIe will
+	 * not work in msi mode and drivers will crash if they try to access
+	 * the device memory area
+	 */
+	dw_pcie_setup_rc(&imx6_pcie->pci->pp);
+	if (pci_msi_enabled()) {
+		u32 val;
+		u8 offset = dw_pcie_find_capability(imx6_pcie->pci, PCI_CAP_ID_MSI);
+
+		val = dw_pcie_readw_dbi(imx6_pcie->pci, offset + PCI_MSI_FLAGS);
+		val |= PCI_MSI_FLAGS_ENABLE;
+		dw_pcie_writew_dbi(imx6_pcie->pci, offset + PCI_MSI_FLAGS, val);
+	}
+
+	return 0;
+}
+
 static int imx6_pcie_resume_noirq(struct device *dev)
 {
 	int ret;
 	struct imx6_pcie *imx6_pcie = dev_get_drvdata(dev);
 	struct dw_pcie_rp *pp = &imx6_pcie->pci->pp;
 
+	/*
+	 * Even though the i.MX6Q does not support suspend/resume, we need to
+	 * reset the link after resume or the memory mapped PCIe I/O space will
+	 * be inaccessible. This will cause the system to freeze.
+	 */
 	if (!(imx6_pcie->drvdata->flags & IMX6_PCIE_FLAG_SUPPORTS_SUSPEND))
-		return 0;
+		return imx6_pcie_reset_link(imx6_pcie);
 
 	ret = imx6_pcie_host_init(pp);
 	if (ret)
-- 
2.43.0