On Mon, Jul 20, 2020 at 11:54:30AM -0700, Jacob Keller wrote: > On 7/18/2020 4:35 AM, Vladimir Oltean wrote: > > On Fri, Jul 17, 2020 at 04:12:07PM -0700, Jacob Keller wrote: > >> On 7/17/2020 9:10 AM, Vladimir Oltean wrote: > >>> +When the interface they represent offers both ``SOF_TIMESTAMPING_TX_HARDWARE`` > >>> +and ``SOF_TIMESTAMPING_TX_SOFTWARE``. > >>> +Originally, the network stack could deliver either a hardware or a software > >>> +time stamp, but not both. This flag prevents software timestamp delivery. > >>> +This restriction was eventually lifted via the ``SOF_TIMESTAMPING_OPT_TX_SWHW`` > >>> +option, but still the original behavior is preserved as the default. > >>> + > >> > >> So, this implies that we set this only if both are supported? I thought > >> the intention was to set this flag whenever we start a HW timestamp. > >> > > > > It's only _required_ when SOF_TIMESTAMPING_TX_SOFTWARE is used, it > > seems. I had also thought of setting 'SKBTX_IN_PROGRESS' as good > > practice, but there are many situations where it can do more harm than > > good. > > > > I guess I've only ever implemented a driver with software timestamping > enabled as an option. What sort of issues arise when you have this set? > I'm guessing that it's some configuration of stacked devices as in the > other cases? If the issue can't be fixed I'd at least like more > explanation here, since the prevailing convention is that we set this > flag, so understanding when and why it's problematic would be useful. > > Thanks, > Jake Yes, the problematic cases have to do with stacked PHCs (DSA, PHY). The pattern is that: - DSA sets SKBTX_IN_PROGRESS - calls dev_queue_xmit towards the MAC driver - MAC driver sees SKBTX_IN_PROGRESS, thinks it's the one who set it - MAC driver delivers TX timestamp - DSA ends poll or receives TX interrupt, collects its timestamp, and delivers a second TX timestamp In fact this is explained in a bit more detail in the current timestamping.rst file. Not only are there existing in-tree drivers that do that (and various subtle variations of it), but new code also has this tendency to take shortcuts and interpret any SKBTX_IN_PROGRESS flag set as being set locally. Good thing it's caught during review most of the time these days. It's an error-prone design. On the DSA front, 1 driver sets this flag (sja1105) and 3 don't (felix, mv88e6xxx, hellcreek). The driver who had trouble because of this flag? sja1105. On the PHY front, 2 drivers set this flag (mscc_phy, dp83640) and 1 doesn't (ptp_ines). The driver who had trouble? dp83640. So it's very far from obvious that setting this flag is 'the prevailing convention'. For a MAC driver, that might well be, but for DSA/PHY, there seem to be risks associated with doing that, and driver writers should know what they're signing up for. -Vladimir
