PCI/ASPM: Ignore L1 PM Substates if device lacks capability

187f91d ("PCI/ASPM: Remove struct aspm_register_info.l1ss_cap")
inadvertently removed a check for existence of the L1 PM Substates (L1SS)
Capability before reading it.

If there is no L1SS Capability, this means we mistakenly read PCI_COMMAND
and PCI_STATUS (config address 0x04) and interpret that as the PCI_L1SS_CAP
register, so we may incorrectly configure L1SS.

Make sure the L1SS Capability exists before trying to read it.

Fixes: 187f91d ("PCI/ASPM: Remove struct aspm_register_info.l1ss_cap")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Bjorn Helgaas <[email protected]>
Reviewed-by: Kuppuswamy Sathyanarayanan <[email protected]>

Author: bjorn-helgaas

drivers/pci/pcie/aspm.c

@@ -560,6 +560,9 @@ static void aspm_l1ss_init(struct pcie_link_state *link)
 	u32 parent_l1ss_cap, child_l1ss_cap;
 	u32 parent_l1ss_ctl1 = 0, child_l1ss_ctl1 = 0;
 
+	if (!parent->l1ss || !child->l1ss)
+		return;
+
 	/* Setup L1 substate */
 	pci_read_config_dword(parent, parent->l1ss + PCI_L1SS_CAP,
 			      &parent_l1ss_cap);