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The IEEE1588 Standard specifies that the timestamps of Packets must be captured when the PTP message timestamp point (leading edge of first octet after the start of frame delimiter) crosses the boundary between the node and the network. As the MAC latches the timestamp at an internal point, the captured timestamp must be corrected for the additional data transmission latency, as described in the publicly available datasheet [1]. This patch only corrects for the MAC-Internal delay, which can be read out from the MAC_Ingress_Timestamp_Latency register on DWMAC version 5, since the Phy framework currently does not support querying the Phy ingress and egress latency. The Closs Domain Crossing Circuits errors as indicated in [1] are already being accounted in the stmmac_get_tx_hwtstamp() function and are not corrected here. As the Latency varies for different link speeds and MII modes of operation, the correction value needs to be updated on each link state change. As the delay also causes a phase shift in the timestamp counter compared to the rest of the network, this correction will also reduce phase error when generating PPS outputs from the timestamp counter. Since the correction registers may be unavailable on some hardware and no feature bits are documented for dynamically detection of the MAC propagation delay readout, introduce a feature bit to explicitely enable MAC delay Correction in the gluecode driver. [1] i.MX8MP Reference Manual, rev.1 Section 11.7.2.5.3 "Timestamp correction" Signed-off-by: Johannes Zink <j.zink@pengutronix.de> Link: https://lore.kernel.org/r/20230719-stmmac_correct_mac_delay-v2-1-3366f38ee9a6@pengutronix.de Link: https://lore.kernel.org/r/20230719-stmmac_correct_mac_delay-v3-1-61e63427735e@pengutronix.de Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.