[PATCH 2/2] Documentation: PCI: Document aer_inject's aer_mask_override parameter

[Priyank Rathod]

aer_inject has had the aer_mask_override module parameter since commit
457d9d088b02 ("PCI: aer-inject: Override PCIe AER Mask Registers"), but
the AER HOWTO never mentioned it.  Describe what it is for (injecting
errors that the device masks, in particular those masked by default)
and how it behaves now that it works: the device's Mask registers are
left alone, the injected bits are only reported as unmasked while they
are pending, and the hidden Mask bits stay set in writes until the
module is unloaded.

Signed-off-by: Priyank Rathod <rathodpriyank@xxxxxxxxxx>
---
 Documentation/PCI/pcieaer-howto.rst | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/Documentation/PCI/pcieaer-howto.rst b/Documentation/PCI/pcieaer-howto.rst
index 90fdfddd3ae5..71a9c2377b3b 100644
--- a/Documentation/PCI/pcieaer-howto.rst
+++ b/Documentation/PCI/pcieaer-howto.rst
@@ -265,3 +265,26 @@ from:
 
 More information about aer-inject can be found in the document in
 its source code.
+
+Injecting masked errors
+-----------------------
+
+By default aer_inject refuses, with -EINVAL, to inject an error that the
+device masks in its Correctable or Uncorrectable Error Mask register,
+because the AER driver would ignore it just like a real masked error.
+To exercise the handling of such errors anyway, e.g., of those that are
+masked by default like Header Log Overflow or Poisoned TLP Egress
+Blocked, load the module with::
+
+    modprobe aer_inject aer_mask_override=1
+
+or boot with aer_inject.aer_mask_override=1 if it is built in.
+
+The device's Mask registers are not modified.  Instead, as long as the
+injected error bits are pending in the emulated Status registers, reads
+of the Mask registers through the PCI core report them as unmasked, so
+the AER driver handles the injected error as if the device had it
+unmasked.  The Mask register bits hidden this way are kept set in
+writes to the Mask registers until aer_inject is unloaded, so that a
+value saved through the emulation (e.g., by pci_save_state()) cannot
+unmask them in the device.

-- 
2.56.0.385.gd3acb90ef8-goog