[dpdk-dev] [PATCH v2 5/6] doc: clarification of usage of HARQ in bbdev doc

[Nicolas Chautru]

New paragraph detailing typical VRAN usecase and mapping
to bbdev API usage.

Signed-off-by: Nicolas Chautru <nicolas.chautru at intel.com>
---
 doc/guides/prog_guide/bbdev.rst | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/doc/guides/prog_guide/bbdev.rst b/doc/guides/prog_guide/bbdev.rst
index 8bd7cba..f39b62f 100644
--- a/doc/guides/prog_guide/bbdev.rst
+++ b/doc/guides/prog_guide/bbdev.rst
@@ -1054,6 +1054,29 @@ capability RTE_BBDEV_LDPC_INTERNAL_HARQ_MEMORY_OUT_ENABLE is set
 then the HARQ is stored in memory internal to the device and not visible
 to BBDEV.
 
+.. note::
+    More explicitly for a typical usage of HARQ retransmission in a VRAN
+    application using a HW PMD, there will be 2 cases.
+
+    For 1st transmission, only the HARQ output is enabled :
+
+    - the harq_combined_output.offset is provided to a given address. ie. typically an integer index * 32K, where the index is tracked by the application based on code block index for a given UE and HARQ process.
+
+    - the related operation flag would notably include RTE_BBDEV_LDPC_HQ_COMBINE_OUT_ENABLE and RTE_BBDEV_LDPC_HARQ_6BIT_COMPRESSION.
+
+    - note that not explicit flush or reset of the memory is required.
+
+    For 2nd transmission, an input is also required to benefit from HARQ combination gain:
+
+    - the changes mentioned above are the same (note that rvIndex may be adjusted).
+
+    - the operation flag would additionally include the LDPC_HQ_COMBINE_IN_ENABLE flag.
+
+    - the harq_combined_input.offset must set to the address of the related code block (ie. same as the harq_combine_output index above for the same code block, HARQ process, UE).
+
+    - the harq_combined_input.length must be set to the length which was provided back in the related harq_combined_output.length when it has processed and dequeued (previous HARQ iteration).
+
+
 The output mbuf data structures are expected to be allocated by the
 application with enough room for the output data.
 
-- 
1.8.3.1