PCI: imx6: Fix suspend/resume support on i.MX6QDL

The suspend/resume functionality is currently broken on the i.MX6QDL
platform, as documented in the NXP errata (ERR005723):

  https://www.nxp.com/docs/en/errata/IMX6DQCE.pdf

This patch addresses the issue by sharing most of the suspend/resume
sequences used by other i.MX devices, while avoiding modifications to
critical registers that disrupt the PCIe functionality. It targets the
same problem as the following downstream commit:

  nxp-imx/linux-imx@4e92355

Unlike the downstream commit, this patch also resets the connected PCIe
device if possible. Without this reset, certain drivers, such as ath10k
or iwlwifi, will crash on resume. The device reset is also done by the
driver on other i.MX platforms, making this patch consistent with
existing practices.

Upon resuming, the kernel will hang and display an error. Here's an
example of the error encountered with the ath10k driver:

  ath10k_pci 0000:01:00.0: Unable to change power state from D3hot to D0, device inaccessible
  Unhandled fault: imprecise external abort (0x1406) at 0x0106f944

Without this patch, suspend/resume will fail on i.MX6QDL devices if a
PCIe device is connected.

Link: https://lore.kernel.org/r/20241030103250.83640-1-eichest@gmail.com
Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
[kwilczynski: commit log, added tag for stable releases]
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Acked-by: Richard Zhu <hongxing.zhu@nxp.com>
Cc: stable@vger.kernel.org

Author: eichenberger

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

@@ -82,6 +82,11 @@ enum imx_pcie_variants {
 #define IMX_PCIE_FLAG_HAS_SERDES		BIT(6)
 #define IMX_PCIE_FLAG_SUPPORT_64BIT		BIT(7)
 #define IMX_PCIE_FLAG_CPU_ADDR_FIXUP		BIT(8)
+/*
+ * Because of ERR005723 (PCIe does not support L2 power down) we need to
+ * workaround suspend resume on some devices which are affected by this errata.
+ */
+#define IMX_PCIE_FLAG_BROKEN_SUSPEND		BIT(9)
 
 #define imx_check_flag(pci, val)	(pci->drvdata->flags & val)
 
@@ -1237,9 +1242,19 @@ static int imx_pcie_suspend_noirq(struct device *dev)
 		return 0;
 
 	imx_pcie_msi_save_restore(imx_pcie, true);
-	imx_pcie_pm_turnoff(imx_pcie);
-	imx_pcie_stop_link(imx_pcie->pci);
-	imx_pcie_host_exit(pp);
+	if (imx_check_flag(imx_pcie, IMX_PCIE_FLAG_BROKEN_SUSPEND)) {
+		/*
+		 * The minimum for a workaround would be to set PERST# and to
+		 * set the PCIE_TEST_PD flag. However, we can also disable the
+		 * clock which saves some power.
+		 */
+		imx_pcie_assert_core_reset(imx_pcie);
+		imx_pcie->drvdata->enable_ref_clk(imx_pcie, false);
+	} else {
+		imx_pcie_pm_turnoff(imx_pcie);
+		imx_pcie_stop_link(imx_pcie->pci);
+		imx_pcie_host_exit(pp);
+	}
 
 	return 0;
 }
@@ -1253,14 +1268,32 @@ static int imx_pcie_resume_noirq(struct device *dev)
 	if (!(imx_pcie->drvdata->flags & IMX_PCIE_FLAG_SUPPORTS_SUSPEND))
 		return 0;
 
-	ret = imx_pcie_host_init(pp);
-	if (ret)
-		return ret;
-	imx_pcie_msi_save_restore(imx_pcie, false);
-	dw_pcie_setup_rc(pp);
+	if (imx_check_flag(imx_pcie, IMX_PCIE_FLAG_BROKEN_SUSPEND)) {
+		ret = imx_pcie->drvdata->enable_ref_clk(imx_pcie, true);
+		if (ret)
+			return ret;
+		ret = imx_pcie_deassert_core_reset(imx_pcie);
+		if (ret)
+			return ret;
+		/*
+		 * Using PCIE_TEST_PD seems to disable MSI and powers down the
+		 * root complex. This is why we have to setup the rc again and
+		 * why we have to restore the MSI register.
+		 */
+		ret = dw_pcie_setup_rc(&imx_pcie->pci->pp);
+		if (ret)
+			return ret;
+		imx_pcie_msi_save_restore(imx_pcie, false);
+	} else {
+		ret = imx_pcie_host_init(pp);
+		if (ret)
+			return ret;
+		imx_pcie_msi_save_restore(imx_pcie, false);
+		dw_pcie_setup_rc(pp);
 
-	if (imx_pcie->link_is_up)
-		imx_pcie_start_link(imx_pcie->pci);
+		if (imx_pcie->link_is_up)
+			imx_pcie_start_link(imx_pcie->pci);
+	}
 
 	return 0;
 }
@@ -1485,7 +1518,9 @@ static const struct imx_pcie_drvdata drvdata[] = {
 	[IMX6Q] = {
 		.variant = IMX6Q,
 		.flags = IMX_PCIE_FLAG_IMX_PHY |
-			 IMX_PCIE_FLAG_IMX_SPEED_CHANGE,
+			 IMX_PCIE_FLAG_IMX_SPEED_CHANGE |
+			 IMX_PCIE_FLAG_BROKEN_SUSPEND |
+			 IMX_PCIE_FLAG_SUPPORTS_SUSPEND,
 		.dbi_length = 0x200,
 		.gpr = "fsl,imx6q-iomuxc-gpr",
 		.clk_names = imx6q_clks,