PCI: Fix up L1SS capability for Intel Apollo Lake Root Port

On Google Coral and Reef family Chromebooks with Intel Apollo Lake SoC,
firmware clobbers the header of the L1 PM Substates capability and the
previous capability when returning from D3cold to D0.

Save those headers at enumeration-time and restore them at resume.

[bhelgaas: The main benefit is to make the lspci output after resume
correct.  Apparently there's little or no effect on power consumption.]

Link: https://lore.kernel.org/linux-pci/CAFJ_xbq0cxcH-cgpXLU4Mjk30+muWyWm1aUZGK7iG53yaLBaQg@mail.gmail.com/T/#u
Link: https://lore.kernel.org/r/20230411160213.4453-1-ron.lee@intel.comSigned-off-by: Ron Lee <ron.lee@intel.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

arch/x86/pci/fixup.c

@@ -824,3 +824,62 @@ static void rs690_fix_64bit_dma(struct pci_dev *pdev)
 DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_ATI, 0x7910, rs690_fix_64bit_dma);
 
 #endif
+
+/*
+ * When returning from D3cold to D0, firmware on some Google Coral and Reef
+ * family Chromebooks with Intel Apollo Lake SoC clobbers the headers of
+ * both the L1 PM Substates capability and the previous capability for the
+ * "Celeron N3350/Pentium N4200/Atom E3900 Series PCI Express Port B #1".
+ *
+ * Save those values at enumeration-time and restore them at resume.
+ */
+
+static u16 prev_cap, l1ss_cap;
+static u32 prev_header, l1ss_header;
+
+static void chromeos_save_apl_pci_l1ss_capability(struct pci_dev *dev)
+{
+	int pos = PCI_CFG_SPACE_SIZE, prev = 0;
+	u32 header, pheader = 0;
+
+	while (pos) {
+		pci_read_config_dword(dev, pos, &header);
+		if (PCI_EXT_CAP_ID(header) == PCI_EXT_CAP_ID_L1SS) {
+			prev_cap = prev;
+			prev_header = pheader;
+			l1ss_cap = pos;
+			l1ss_header = header;
+			return;
+		}
+
+		prev = pos;
+		pheader = header;
+		pos = PCI_EXT_CAP_NEXT(header);
+	}
+}
+
+static void chromeos_fixup_apl_pci_l1ss_capability(struct pci_dev *dev)
+{
+	u32 header;
+
+	if (!prev_cap || !prev_header || !l1ss_cap || !l1ss_header)
+		return;
+
+	/* Fixup the header of L1SS Capability if missing */
+	pci_read_config_dword(dev, l1ss_cap, &header);
+	if (header != l1ss_header) {
+		pci_write_config_dword(dev, l1ss_cap, l1ss_header);
+		pci_info(dev, "restore L1SS Capability header (was %#010x now %#010x)\n",
+			 header, l1ss_header);
+	}
+
+	/* Fixup the link to L1SS Capability if missing */
+	pci_read_config_dword(dev, prev_cap, &header);
+	if (header != prev_header) {
+		pci_write_config_dword(dev, prev_cap, prev_header);
+		pci_info(dev, "restore previous Capability header (was %#010x now %#010x)\n",
+			 header, prev_header);
+	}
+}
+DECLARE_PCI_FIXUP_FINAL(PCI_VENDOR_ID_INTEL, 0x5ad6, chromeos_save_apl_pci_l1ss_capability);
+DECLARE_PCI_FIXUP_RESUME(PCI_VENDOR_ID_INTEL, 0x5ad6, chromeos_fixup_apl_pci_l1ss_capability);