[dpdk-dev] [PATCH v4] doc: add known issue about legacy intr mode for ixgbe

[Xiaoyun Li <xiaoyun.li@intel.com>]

When using uio_pci_generic module or using legacy interrupt mode of
igb_uio or vfio, X550 cannot get interrupts. Because the Interrupt
Status bit is not implemented, then the irq cannot be handled correctly
and cannot report the event fd to DPDK apps.

Add this hw limitation and details into ixgbe known issue.

Signed-off-by: Xiaoyun Li <xiaoyun.li@intel.com>
---
v4:
 * Clarify that the vfio workaround only works when the intx is not shared
   with other devices.
v3:
 * Correct a misspelling.
v2:
 * Correct the link of X550 spec update.
 * Polish the known issue title.
---
 doc/guides/nics/ixgbe.rst | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/doc/guides/nics/ixgbe.rst b/doc/guides/nics/ixgbe.rst
index 16d6390..1c294b0 100644
--- a/doc/guides/nics/ixgbe.rst
+++ b/doc/guides/nics/ixgbe.rst
@@ -200,6 +200,33 @@ There is no RTE API to add a VF's MAC address from the PF. On ixgbe, the
 ``rte_eth_dev_mac_addr_add()`` function can be used to add a VF's MAC address,
 as a workaround.
 
+X550 does not support legacy interrupt mode
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Desccription
+^^^^^^^^^^^^
+X550 cannot get interrupts if using ``uio_pci_generic`` module or using legacy
+interrupt mode of ``igb_uio`` or ``vfio``. Because the errata of X550 states
+that the Interrupt Status bit is not implemented. The errata is the item #22
+from `X550 spec update <https://www.intel.com/content/dam/www/public/us/en/
+documents/specification-updates/ethernet-x550-spec-update.pdf>`_
+
+Implication
+^^^^^^^^^^^
+When using ``uio_pci_generic`` module or using legacy interrupt mode of
+``igb_uio`` or ``vfio``, the Interrupt Status bit would be checked if the
+interrupt is coming. Since the bit is not implemented in X550, the irq cannot
+be handled correctly and cannot report the event fd to DPDK apps. Then apps
+cannot get interrupts and ``dmesg`` will show messages like ``irq #No.: ``
+``nobody cared.``
+
+Workaround
+^^^^^^^^^^
+Do not bind the ``uio_pci_generic`` module in X550 NICs.
+Do not bind ``igb_uio`` with legacy mode in X550 NICs.
+Before binding ``vfio`` with legacy mode in X550 NICs, use ``modprobe vfio ``
+``nointxmask=1`` to load ``vfio`` module if the intx is not shared with other
+devices.
 
 Inline crypto processing support
 --------------------------------
-- 
2.7.4