Re: [Linuxptp-users] Overflow issue for 32bit machines: ptp_clock_info.max_adj versus timex.freq

[Janusz U. <j.u...@el...>]

> On Thu, Oct 06, 2022 at 12:11:39AM +0200, Janusz Użycki wrote:
>> Hi.
>>
>> A lot of PTP/PHC drivers set max_adj value quite big. Howover due to
>> kernel's API limit of 32bit long type value (freq) and POSIX frequency
>> conversion it should limited to 32767999 ppb.
>>
>> It concerns the frequency limit check both for kernel, testptp and ptp4linux
>> servos. Simply 65.536 * 32767999 is almost 2^31... It is also common for
>> adjfreq() vs newer adjfine().
>> Moreover such big frequency limits seem not practical (phase jump used
>> instead, 1000ppm is huge for any PTP oscillator application), even driver or
>> rather PHC hardware has ability to tune is such range.
> What issue are you seeing with linuxptp?
>
> In phc_max_adj() there is some code clamping the value on 32-bit
> systems, so the overflow should be avoided.

When I set testptp -f 32768000 or more PHC time starts to drift in 
opposite direction relative to synced REALTIME system clock, tested 
using testptp -k1.
First I noted the issue on much older kernel than latest mainline.


best regrds
Janusz