Re: [PATCH 3/4] rtc: rtc-hid-sensor-time: add option hctosys to settime at boot

From: Alexander Holler
Date: Tue May 21 2013 - 19:16:19 EST

Am 22.05.2013 00:02, schrieb John Stultz:
On 05/05/2013 04:21 AM, Alexander Holler wrote:
drivers/rtc/hctosys (CONFIG_RTC_HCTOSYS) doesn't work for
rtc-hid-sensor-time because it will be called in late_init, and thus
rtc-hid-sensor-time gets loaded. To set the time through
rtc-hid-sensor-time at startup, the module now checks by default if the
system time is before 1970-01-02 and sets the system time (once) if
this is
the case.

To disable this behaviour, set the module option hctosys to zero, e.g. by
using rtc-hid-sensor-time.hctosys=0 at the kernel command line if the
driver is statically linked into the kernel.

Sorry I missed this earlier, it fell into my spam box for some reason.

I've recently heard that gmail seems to move my mails to a SPAM folder (too). As I don't know why, I can't help with that and I have to live with the fact that I seem to get censored by Google (if it isn't a problem of my mail setup, but I'm not aware of such). Anyway, I'm sure some people like that I'm silenced this way. ;)

Like Andrew, I think this feels particularly hacky.

Why exactly is late_init too early? (I'm unfamiliar with the
rtc-hid-sensor-time driver)

Currently it can be an USB device (and maybe Bluetooth or even i2c in the future, depends on hid-sensor-hub). That has some implications:

(1) Initialization might need longer (or happens later) than late_init, even if everything is linked into the kernel (same problem as with a boot from USB-storage)
(2) It might not even be available at boot, but it should work if a user plugs it in afterwards.
(3) To accomplish (2) it should set the system time (by default) IFF nothing else did set the time.

That "nothing else" in (3) is for security reasons, because no plugable HID device should be able to change the system time by default.

The check if something else did set the system time can't be accomplished only by the RTC subsystem because userspace, network or whatever else is able to set the system time most likely doesn't use the RTC subsystem (or hctosys).

E.g. one of those setups could be:

hctosys (fails because of no RTC)
ntpdate/rdate/date < whatever
load modules (rtc-hid-sensor-time)

If we would use a flag in the hctosys module then rtc-hid-sensor-time would be able to change the time (in the setup above).

Using a module option which is by default off doesn't help too. Users (or even distros) which would turn it on, might forget it and systems would be at risk if no HID clock will be found at boot (but later plugged in by some blackhat).

A flag in the time subsystem itself would do the trick. Such a flag might help with the problem if the RTC subsystem or the persistent clock code did set the time too. You've mentioned in another thread that you had to solve such a problem, but I'm not aware how you did that.

Implementation could be as easy as a bool "time_set_at_least_once" in the timer subsystem itself (e.g. in do_settimeofday() and whatever similiar is available).

> If this is a hotplug rtc device (why I'm guessing its not available at
> late_init), would it not be better to leave the setting of time using
> hwclock --hctosys via a udev rule or something?

I want to set the time with millisecond precision (if the HID clock offers that), which currently isn't available through the RTC subsystem.

But even if milliseconds would be available through /dev/rtcN, the problem if something else did set the time still would be the same, just that an udev-rule now would have that problem.


Alexander Holler
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