Documentation: PCI: Amend error recovery doc with pci_save_state() rules

After recovering from a PCI error through reset, affected devices are in
D0_uninitialized state and need to be brought into D0_active state by
re-initializing their Config Space registers (PCIe r7.0 sec 5.3.1.1).

To facilitate that, the PCI core provides pci_restore_state() and
pci_save_state() helpers.  Document rules governing their usage.

As Bjorn notes, so far no file in "Documentation/ includes anything about
the idea of a driver using pci_save_state() to capture the state it wants
to restore after an error", even though it is a common pattern in drivers.
So that's obviously a gap that should be closed.

Reported-by: Bjorn Helgaas <helgaas@kernel.org>
Closes: https://lore.kernel.org/r/20251113161556.GA2284238@bhelgaas/
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rafael J. Wysocki (Intel) <rafael@kernel.org>
Link: https://patch.msgid.link/077596ba70202be0e43fdad3bb9b93d356cbe4ec.1763746079.git.lukas@wunner.de

Author: l1k

Documentation/PCI/pci-error-recovery.rst

@@ -326,6 +326,21 @@ be recovered, there is nothing more that can be done;  the platform
 will typically report a "permanent failure" in such a case.  The
 device will be considered "dead" in this case.
 
+Drivers typically need to call pci_restore_state() after reset to
+re-initialize the device's config space registers and thereby
+bring it from D0\ :sub:`uninitialized` into D0\ :sub:`active` state
+(PCIe r7.0 sec 5.3.1.1).  The PCI core invokes pci_save_state()
+on enumeration after initializing config space to ensure that a
+saved state is available for subsequent error recovery.
+Drivers which modify config space on probe may need to invoke
+pci_save_state() afterwards to record those changes for later
+error recovery.  When going into system suspend, pci_save_state()
+is called for every PCI device and that state will be restored
+not only on resume, but also on any subsequent error recovery.
+In the unlikely event that the saved state recorded on suspend
+is unsuitable for error recovery, drivers should call
+pci_save_state() on resume.
+
 Drivers for multi-function cards will need to coordinate among
 themselves as to which driver instance will perform any "one-shot"
 or global device initialization. For example, the Symbios sym53cxx2