On 05/21/2013 04:15 PM, Alexander Holler wrote:
Am 22.05.2013 00:02, schrieb John Stultz:
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:
boot
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 it were to be done this way, it would be good to have the RTC layer
check when RTC devices are registered and call the internal hctosys
functionality then (rather then just at late_init). Do you want to try
to rework the patch in this way?
I'm not totally sure I'd agree that it would be better over leaving it
to userspace, but if we're going to go with an in-kernel policy for
this, then it seems like a better approach then the current patch.
>
> 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.
True. This is one area I'd like to see addressed at some point. A few
RTC devices have sub-second granularity and we're not exposing it via
the RTC subsystem to either kernel or userland consumers.
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.
Though a udev hack to check if its 1970 would be fairly simple. And
pushes the policy of setting or not setting clearly off to userland
(which allows for less compile-time or boot option tweaks to manipulate).
thanks
-john
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