[RFC PATCH] PCI: Work around Pericom PI7C9X3G606GPC Port 4 BAR erratum

[Nirmoy Das]

The Pericom PI7C9X3G606GPC PCIe switch has an erratum where downstream
Port 4 retains a default MSI-X table and PBA decode when BAR 0 is zero.
Memory reads that match this window are dropped with an Unsupported
Request completion, which may cause the SoC to report a timeout.

The workaround is to make Port 4 BAR 0 mirror BAR 0 of the immediate
upstream port. Firmware may establish this at boot, but PCI resource
assignment can move upstream BAR 0 without updating Port 4.

For a 64-bit BAR, also mirror BAR 1 while memory decoding is disabled,
matching the PCI core update sequence. Port 4 BAR 0 may read back as zero
after a successful write, so do not use readback to validate the update.

Signed-off-by: Nirmoy Das <nirmoyd@xxxxxxxxxx>
---
 drivers/pci/quirks.c | 92 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 92 insertions(+)

diff --git a/drivers/pci/quirks.c b/drivers/pci/quirks.c
index b09f27f7846fc..fa8098d77adb5 100644
--- a/drivers/pci/quirks.c
+++ b/drivers/pci/quirks.c
@@ -6264,6 +6264,98 @@ DECLARE_PCI_FIXUP_ENABLE(PCI_VENDOR_ID_PERICOM, 0xb404,
 DECLARE_PCI_FIXUP_RESUME(PCI_VENDOR_ID_PERICOM, 0xb404,
 			 pci_fixup_pericom_acs_store_forward);
 
+#define PCI_DEVICE_ID_PERICOM_PI7C9X3G606GPC 0xc008
+
+/*
+ * Pericom PI7C9X3G606GPC switch erratum E15 -
+ * Downstream Port 4 BAR 0 must mirror the immediate upstream port BAR 0
+ *
+ * Port 4 uses BAR 0 for its MSI-X table and PBA. Firmware may program this
+ * mirror at boot, but Linux resource assignment can move the upstream BAR
+ * and leave Port 4 with a stale mirror.
+ *
+ * Diodes confirmed Tile0/P4 appears to Linux as device 4, function 0 on the
+ * bus below the upstream port. Match that downstream function and re-apply
+ * the mirror after resource assignment and early resume.
+ */
+static void pci_fixup_pericom_pi7c9x3g606gpc_bar0_mirror(struct pci_dev *pdev)
+{
+	struct pci_dev *upstream;
+	bool bar0_64, disable_mem;
+	u16 cmd = 0;
+	u32 bar = 0, bar1 = 0, upstream_bar = 0, upstream_bar1 = 0;
+
+	if (pci_pcie_type(pdev) != PCI_EXP_TYPE_DOWNSTREAM)
+		return;
+
+	if (PCI_SLOT(pdev->devfn) != 4 || PCI_FUNC(pdev->devfn))
+		return;
+
+	upstream = pci_upstream_bridge(pdev);
+	if (!upstream || upstream->vendor != PCI_VENDOR_ID_PERICOM ||
+	    upstream->device != PCI_DEVICE_ID_PERICOM_PI7C9X3G606GPC ||
+	    pci_pcie_type(upstream) != PCI_EXP_TYPE_UPSTREAM)
+		return;
+
+	pci_read_config_dword(upstream, PCI_BASE_ADDRESS_0, &upstream_bar);
+	if (upstream_bar & PCI_BASE_ADDRESS_SPACE_IO)
+		return;
+
+	bar0_64 = (upstream_bar & PCI_BASE_ADDRESS_MEM_TYPE_MASK) ==
+		  PCI_BASE_ADDRESS_MEM_TYPE_64;
+	if (bar0_64)
+		pci_read_config_dword(upstream, PCI_BASE_ADDRESS_1,
+				      &upstream_bar1);
+
+	if (!(upstream_bar & PCI_BASE_ADDRESS_MEM_MASK) &&
+	    (!bar0_64 || !upstream_bar1)) {
+		pci_warn(pdev, "skipping PI7C9X3G606GPC BAR 0 mirror workaround because upstream BAR 0 is unassigned\n");
+		return;
+	}
+
+	pci_read_config_dword(pdev, PCI_BASE_ADDRESS_0, &bar);
+	if (bar0_64) {
+		pci_read_config_dword(pdev, PCI_BASE_ADDRESS_1, &bar1);
+		if (bar == upstream_bar && bar1 == upstream_bar1)
+			return;
+	} else {
+		if (bar == upstream_bar)
+			return;
+	}
+
+	/*
+	 * Port 4 BAR 0 may read back as zero even after a successful write.
+	 * If BAR 0 is configured as 64-bit, BAR 1 is the upper half.
+	 * Disable memory decoding while updating both dwords, matching PCI
+	 * core's 64-bit BAR update sequence.
+	 */
+	disable_mem = bar0_64 && !pdev->mmio_always_on;
+	if (disable_mem) {
+		pci_read_config_word(pdev, PCI_COMMAND, &cmd);
+		pci_write_config_word(pdev, PCI_COMMAND,
+				      cmd & ~PCI_COMMAND_MEMORY);
+	}
+
+	pci_write_config_dword(pdev, PCI_BASE_ADDRESS_0, upstream_bar);
+	if (bar0_64)
+		pci_write_config_dword(pdev, PCI_BASE_ADDRESS_1, upstream_bar1);
+	if (disable_mem)
+		pci_write_config_word(pdev, PCI_COMMAND, cmd);
+
+	if (bar0_64)
+		pci_info(pdev, "wrote upstream BAR 0/1 %#x/%#x to Port 4 BAR 0/1 for PI7C9X3G606GPC BAR 0 mirror workaround\n",
+			 upstream_bar, upstream_bar1);
+	else
+		pci_info(pdev, "wrote upstream BAR 0 %#x to Port 4 BAR 0 for PI7C9X3G606GPC BAR 0 mirror workaround\n",
+			 upstream_bar);
+}
+DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_PERICOM,
+			PCI_DEVICE_ID_PERICOM_PI7C9X3G606GPC,
+			pci_fixup_pericom_pi7c9x3g606gpc_bar0_mirror);
+DECLARE_PCI_FIXUP_RESUME_EARLY(PCI_VENDOR_ID_PERICOM,
+			       PCI_DEVICE_ID_PERICOM_PI7C9X3G606GPC,
+			       pci_fixup_pericom_pi7c9x3g606gpc_bar0_mirror);
+
 static void nvidia_ion_ahci_fixup(struct pci_dev *pdev)
 {
 	pdev->dev_flags |= PCI_DEV_FLAGS_HAS_MSI_MASKING;

base-commit: dc59e4fea9d83f03bad6bddf3fa2e52491777482
-- 
2.43.0