mbuf: introduce accurate packet Tx scheduling

mbuf: introduce accurate packet Tx scheduling

There is the requirement on some networks for precise traffic timing
management. The ability to send (and, generally speaking, receive)
the packets at the very precisely specified moment of time provides
the opportunity to support the connections with Time Division
Multiplexing using the contemporary general purpose NIC without involving
an auxiliary hardware. For example, the supporting of O-RAN Fronthaul
interface is one of the promising features for potentially usage of the
precise time management for the egress packets.

The main objective of this patchset is to specify the way how applications
can provide the moment of time at what the packet transmission must be
started and to describe in preliminary the supporting this feature
from mlx5 PMD side [1].

The new dynamic timestamp field is proposed, it provides some timing
information, the units and time references (initial phase) are not
explicitly defined but are maintained always the same for a given port.
Some devices allow to query rte_eth_read_clock() that will return
the current device timestamp. The dynamic timestamp flag tells whether
the field contains actual timestamp value. For the packets being sent
this value can be used by PMD to schedule packet sending.

The device clock is opaque entity, the units and frequency are
vendor specific and might depend on hardware capabilities and
configurations. If might (or not) be synchronized with real time
via PTP, might (or not) be synchronous with CPU clock (for example
if NIC and CPU share the same clock source there might be no
any drift between the NIC and CPU clocks), etc.

After PKT_RX_TIMESTAMP flag and fixed timestamp field supposed
deprecation and obsoleting, these dynamic flag and field might be
used to manage the timestamps on receiving datapath as well. Having
the dedicated flags for Rx/Tx timestamps allows applications not
to perform explicit flags reset on forwarding and not to promote
received timestamps to the transmitting datapath by default.
The static PKT_RX_TIMESTAMP is considered as candidate to become
the dynamic flag and this move should be discussed.

When PMD sees the "rte_dynfield_timestamp" set on the packet being sent
it tries to synchronize the time of packet appearing on the wire with
the specified packet timestamp. If the specified one is in the past it
should be ignored, if one is in the distant future it should be capped
with some reasonable value (in range of seconds). These specific cases
("too late" and "distant future") can be optionally reported via
device xstats to assist applications to detect the time-related
problems.

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
in desired order. The timestamps can be put only in the first packet
in the burst providing the entire burst scheduling.

PMD reports the ability to synchronize packet sending on timestamp
with new offload flag:

This is palliative and might be replaced with new eth_dev API
about reporting/managing the supported dynamic flags and its related
features. This API would break ABI compatibility and can't be introduced
at the moment, so is postponed to 20.11.

For testing purposes it is proposed to update testpmd "txonly"
forwarding mode routine. With this update testpmd application generates
the packets and sets the dynamic timestamps according to specified time
pattern if it sees the "rte_dynfield_timestamp" is registered.

The new testpmd command is proposed to configure sending pattern:

set tx_times <burst_gap>,<intra_gap>

<intra_gap> - the delay between the packets within the burst
              specified in the device clock units. The number
              of packets in the burst is defined by txburst parameter

<burst_gap> - the delay between the bursts in the device clock units

As the result the bursts of packet will be transmitted with specific
delays between the packets within the burst and specific delay between
the bursts. The rte_eth_read_clock is supposed to be engaged to get the
current device clock value and provide the reference for the timestamps.

[1] http://patches.dpdk.org/patch/73714/

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>