Hi!
My broad goal is to transmit the current system time somewhat
accurately to devices (namely fullspeed microcontrollers).
I'd like to be able correlate some instant of a USB frame (i.e. a point
in time with a known offset to the SOF) with the host's system time.
The host PC can then create a snapshot containing the frame number, a
timestamp (corrected by the offset to the frame's SOF if applicable)
and send this in a normal data packet to the device(s). On the devices
I can use SOF interrupts to take note of a local clock and compare that
to the measurements of the host to derive real time.
These are the notes of my research in respect of standards:
- UHCI does not define any interrupts that are related to the SOF/frame
borders.
- OHCI defines a SOF interrupt (7.1.4 HcInterruptStatus Register).
- EHCI does only define a Frame List Rollover Interrupt (2.3.2 USBSTS),
but that should suffice too for my purpose.
- USB 3.0 is not my main concern, but there are some interesting bits:
* MFINDEX Wrap Events seem to be similar to the rollover interrupts.
* there exist Isochronous Timestamp Packets (8.7) to compensate for
the missing SOF packets.
* a special field in the hub descriptors (wHubDelay) allows to take
delays due to different hierarchies into account of synchronization
schemes.
* "Set Isochronous Delay" requests informs devices about transmission
delays.
see also http://www.usb.org/developers/presentations/SuperSpeed_USB_DevCon_Isochronous_Froelich.pdf
Regarding existing code i have seen a few references to the features above.
EHCI code seems to just ignore/mask away the rollover interrupt
STS_FLR (apart from debug prints).
OHCI code uses the SOF interrupt (OHCI_INTR_SF) just for quirks and
queue management(?).
I have not really looked at XHCI code.
(TBH i have looked at 3.2 code only yet, please flame me, if there have
been any major related changes since ;)
>From a user's perspective there is AFAIK only
usb_get_current_frame_number() which is not very useful for my use case
due to its polling nature. libusb does not even expose that:
http://marc.info/?t=128477632000002&r=1&w=4
What do you think would be a reasonable approach to make userspace
aware of SOF-related events/timestamps and/or achieve my goal? Do you
think this might be useful/generic enough to be merged upstream? Do
think i should instead just implement something vaguely similar to
NTP/PTP on top of normal USB pipes? I am a complete linux kernel newbie
(apart from applying existing patches, staring a bit at foreign code and
compiling it), but I have always wanted to start hacking it... :)
--
Kind regards/Mit freundlichen Grüßen, Stefan Tauner
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