On 7/20/2020 2:05 PM, Vladimir Oltean wrote: > 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. > Perhaps the issue is that the MAC driver using SKBTX_IN_PROGRESS as the mechanism for telling if it should deliver a timestamp. Shouldn't they be relying on SKBTX_HW_TSTAMP for the "please timestamp" notification, and then using their own mechanism for forwarding that timestamp once it's complete? I see a handful of drivers do rely on checking this, but I think that's the real bug here. > -Vladimir >
