On Tue, Apr 18, 2023 at 02:14:52PM +0300, Vladimir Oltean wrote: > 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@xxxxxxx> Reviewed-by: Simon Horman <simon.horman@xxxxxxxxxxxx>
