netdev - Unit of measurement for clock

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Message-ID: <af63bcaf-f00d-c6be-9aca-e7403c3b9c94@gmail.com>
Date:   Sat, 27 Apr 2019 15:55:15 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     stephen@...workplumber.org
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Unit of measurement for clock_t in the bridge driver

Hi Stephen,

"man ip-link" mentions for ageing_time, forward_delay, hello_time and so 
on, that they specify a number of seconds.

The bridge core takes the value from userspace unfiltered and passes it 
through the clock_t_to_jiffies function. This begs the questions "what 
is clock_t measured in"?

One would think it does this since patch "[BRIDGE]: Make use of 
jiffies_to_clock." [1], but actually it appears to have done that even 
before (this patch is only a non-functional API change).

Actually the clock_t_to_jiffies function (added in patch [2] with rather 
minimal introduction) should arguably be called user_ticks_to_jiffies, 
and there aren't a lot of users apart from the bridge driver.

I'm finding that in order for me to convert the value emitted by the 
bridge driver into anything with physical significance, I have to 
multiply the ageing_time with the cosmological constant of USER_HZ (100 
per popular vote).

# ip link add name br0 type bridge stp_state 1 ageing_time 1
[ 2368.667020] br0: br_set_ageing_time: ageing_time (clock_t) 1 t 
(jiffies) 1
[ 2368.673891] br0: __set_ageing_time: t (jiffies) 1 ageing_time (clock_t) 1

It is quite apparent that clock_t_to_jiffies doesn't convert from 
seconds to jiffies. By the way I am quite puzzled as to how downstream 
users of this function, such as dsa_port_ageing_time which further 
convert using jiffies_to_msecs, have ever worked.

Am I missing something obvious?

Thanks,
-Vladimir

[1] 
https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=ef443d0f02bb66d8ed4976e459ddd11b8b1407b7

[2] 
https://git.kernel.org/pub/scm/linux/kernel/git/tglx/history.git/commit/?id=220443f8fa6562345e6848fa9f3c956343daf0d6