Messages in this thread | | | From | "Henrik Rydberg" <> | Date | Tue, 13 Nov 2012 19:28:25 +0100 | Subject | Re: [PATCH v3 13/13] HID: hid-multitouch: forwards ABS_SCAN_TIME |
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> >> @@ -98,6 +99,9 @@ struct mt_device { > >> bool serial_maybe; /* need to check for serial protocol */ > >> bool curvalid; /* is the current contact valid? */ > >> unsigned mt_flags; /* flags to pass to input-mt */ > >> + __s32 dev_time; /* the scan time provided by the device */ > >> + unsigned long jiffies; /* the frame's jiffies */ > >> + unsigned timestamp; /* the timestamp to be sent */ > > > > Why not just dev_time here? > > because max dev_time is at least 65535 according to the norm, and the > win 8 device I have has his max value of 65535. > Which means that every 6 seconds and a half the counter resets, and > the value is not properly reset in the way I understand the > specification. The device just forwards an internal clock that is > never reset.
Ok, I though it was a 32-bit value, and that it would wrap with a longer period. It does not change the essence of the definition, though. If we say "seconds" instead of "hours", we should still be fine, no?
> So if you wait 653500 us + 8ms, everything happens as if the device > sent this frame right after the previous one (you get the same value).
Yes, but we have this effect on a 32-bit counter as well.
> I think that it's the job of the kernel to provide clean and coherent > values through evdev, which won't be the case if the jiffies thing is > not in place: every devices will have a different behavior, leading to > complicate things in the user-space.
The whole point is to provide the device clock to userland when it exists, isn't it? Thus, the jiffies would never be used. If a future device needs additions to work conformly, we just have to deal with it at that point in time.
To conclude, we obviously have devices with a rather short wrap-around time. However, since the normal inter-frame time is in the millisecond range, it should not be overly restrictive to change the definition of the minimum wraparound time from hours to seconds.
Thanks, Henrik
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