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The MAC Merge layer (IEEE 802.3-2018 clause 99) does all the heavy lifting for Frame Preemption (IEEE 802.1Q-2018 clause 6.7.2), a TSN feature for minimizing latency. Preemptible traffic is different on the wire from normal traffic in incompatible ways. If we send a preemptible packet and the link partner doesn't support preemption, it will drop it as an error frame and we will never know. The MAC Merge layer has a control plane of its own, which can be manipulated (using ethtool) in order to negotiate this capability with the link partner (through LLDP). Actually the TLV format for LLDP solves this problem only partly, because both partners only advertise: - if they support preemption (RX and TX) - if they have enabled preemption (TX) so we cannot tell the link partner what to do - we cannot force it to enable reception of our preemptible packets. That is fully solved by the verification feature, where the local device generates some small probe frames which look like preemptible frames with no useful content, and the link partner is obliged to respond to them if it supports the standard. If the verification times out, we know that preemption isn't active in our TX direction on the link. Having clarified the definition, this selftest exercises the manual (ethtool) configuration path of 2 link partners (with and without verification), and the LLDP code path, using the openlldp project. The test also verifies the TX activity of the MAC Merge layer by sending traffic through a traffic class configured as preemptible (using mqprio). There isn't a good way to make this really portable (user space cannot find out how many traffic classes there are for a device), but I chose num_tc 4 here, that should work reasonably well. I also know that some devices (stmmac) only permit TXQ0 to be preemptible, so this is why PREEMPTIBLE_PRIO was strategically chosen as 0. Even if other hardware is more configurable, this test should cover the baseline. This is not really a "forwarding" selftest, but I put it near the other "ethtool" selftests. $ ./ethtool_mm.sh eno0 swp0 TEST: Manual configuration with verification: eno0 to swp0 [ OK ] TEST: Manual configuration with verification: swp0 to eno0 [ OK ] TEST: Manual configuration without verification: eno0 to swp0 [ OK ] TEST: Manual configuration without verification: swp0 to eno0 [ OK ] TEST: Manual configuration with failed verification: eno0 to swp0 [ OK ] TEST: Manual configuration with failed verification: swp0 to eno0 [ OK ] TEST: LLDP [ OK ] Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com> Signed-off-by: Jakub Kicinski <kuba@kernel.org> |
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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.