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doc: add timestamp upper limitation in mlx5 guide

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Add description about Tx scheduling timestamp upper limit.
If timestamp exceeds the value, it is marked by PMD as being
into "too-distant-future" and not scheduled at all
(is being sent without any wait).

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
This commit is contained in:
Viacheslav Ovsiienko 2020-07-27 15:51:04 +00:00 committed by Thomas Monjalon
parent 38a5704629
commit b5f615612a
1 changed files with 8 additions and 0 deletions
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8
doc/guides/nics/mlx5.rst
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@ -253,6 +253,14 @@ Limitations
reported via device xstats to assist applications to detect the
time-related problems.
The timestamp upper "too-distant-future" limit
at the moment of invoking the Tx burst routine
can be estimated as ``tx_pp`` option (in nanoseconds) multiplied by 2^23.
Please note, for the testpmd txonly mode,
the limit is deduced from the expression::
(n_tx_descriptors / burst_size + 1) * inter_burst_gap
There is no any packet reordering according timestamps is supposed,
neither within packet burst, nor between packets, it is an entirely
application responsibility to generate packets and its timestamps