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The MMCSR register contains 2 fields with overlapping meaning: - LPA (Local preemption active): This read-only status bit indicates whether preemption is active for this port. This bit will be set if preemption is both enabled and has completed the verification process. - TXSTS (Merge status): This read-only status field provides the state of the MAC Merge sublayer transmit status as defined in IEEE Std 802.3-2018 Clause 99. 00 Transmit preemption is inactive 01 Transmit preemption is active 10 Reserved 11 Reserved However none of these 2 fields offer reliable reporting to software. When connecting ENETC to a link partner which is not capable of Frame Preemption, the expectation is that ENETC's verification should fail (VSTS=4) and its MM TX direction should be inactive (LPA=0, TXSTS=00) even though the MM TX is enabled (ME=1). But surprise, the LPA bit of MMCSR stays set even if VSTS=4 and ME=1. OTOH, the TXSTS field has the opposite problem. I cannot get its value to change from 0, even when connecting to a link partner capable of frame preemption, which does respond to its verification frames (ME=1 and VSTS=3, "SUCCEEDED"). The only option with such buggy hardware seems to be to reimplement the formula for calculating tx-active in software, which is for tx-enabled to be true, and for the verify-status to be either SUCCEEDED, or DISABLED. Without reliable tx-active reporting, we have no good indication when to commit the preemptible traffic classes to hardware, which makes it possible (but not desirable) to send preemptible traffic to a link partner incapable of receiving it. However, currently we do not have the logic to wait for TX to be active yet, so the impact is limited. Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com> Reviewed-by: Simon Horman <simon.horman@corigine.com> 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.