On 3/14/14, 9:33 AM, "Miroslav Lichvar" <[email protected]> wrote:
>Does anyone know what is the typical accuracy we can expect in time >stepping of a PHC via ADJ_SETOFFSET? I briefly looked in the kernel >code and it seems most (all?) drivers implement it as a read and write >to a register without any compensation for how long the operation >takes, which causes a significant error. > >With an i210 I see: > >port 1: LISTENING to UNCALIBRATED on RS_SLAVE >master offset 415 s0 freq -22825 path delay 622 >master offset 423 s0 freq -22825 path delay 622 >master offset 426 s0 freq -22825 path delay 621 >master offset 427 s1 freq -22820 path delay 621 >master offset -5174 s2 freq -27997 path delay 621 >port 1: UNCALIBRATED to SLAVE on MASTER_CLOCK_SELECTED >master offset 10 s2 freq -25399 path delay 620 >master offset 2729 s2 freq -22674 path delay 493 >master offset 2764 s2 freq -19982 path delay 328 >master offset -59 s2 freq -22719 path delay 328 > >The initial 400ns offset is corrected by stepping, but it results in >ten times larger offset and the servo needs more time to settle down >from that error. I think we should increase the default first step >threshold so it's used only when the offset is much larger than the >stepping error, but I'm not sure what would be a good value for most >PTP HW. > >-- >Miroslav Lichvar Determining how long the PCIe read/write will take, for devices that support time adjustment in HW and aren't using a timecounter, is tricky. It depends on the state of the system at that point, so finding a constant (or even rough approximation) for the delay would be problematic. One thing we could look at is adding a write flush to the drivers after writing their registers (for example, the SYSTIM registers on i210) to force the writes through quicker, but on a quiet network I wouldn't expect to see a massive improvement. Would you like me to get a patch together for you to try out on i210 and see if it improves the initial jump? Cheers, Matthew Matthew Vick Linux Development Networking Division Intel Corporation ------------------------------------------------------------------------------ Learn Graph Databases - Download FREE O'Reilly Book "Graph Databases" is the definitive new guide to graph databases and their applications. Written by three acclaimed leaders in the field, this first edition is now available. Download your free book today! http://p.sf.net/sfu/13534_NeoTech _______________________________________________ Linuxptp-devel mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/linuxptp-devel
