Re: [PATCH] time: Revert ALWAYS_USE_PERSISTENT_CLOCK compile timeoptimizaitons

From: John Stultz
Date: Wed Apr 24 2013 - 17:25:00 EST

On 04/24/2013 12:18 PM, Kay Sievers wrote:
On Wed, Apr 24, 2013 at 8:55 PM, John Stultz <john.stultz@xxxxxxxxxx> wrote:
On 04/24/2013 11:41 AM, Kay Sievers wrote:
On Wed, Apr 24, 2013 at 8:32 PM, John Stultz <john.stultz@xxxxxxxxxx>
Kay Sievers noted that the ALWAYS_USE_PERSISTENT_CLOCK config,
which enables some minor compile time optimization to avoid
uncessary code in mostly the suspend/resume path could cause
problems for userland.

In particular, the dependency for RTC_HCTOSYS on
!ALWAYS_USE_PERSISTENT_CLOCK, which avoids setting the time
twice and simplifies suspend/resume, has the side effect
of causing the /sys/class/rtc/rtcN/hctosys flag to always be
zero, and this flag is commonly used by udev to setup the
/dev/rtc symlink to /dev/rtcN, which can cause pain for
older applications.
FWIW, in the light of the original change, I've just removed the
/dev/rtc creation from the default udev rules now, so that thing will
be phased out in the future.
Is that actually wanted? What happens to applications that use /dev/rtc?

I think setting up the /dev/rtc link is important. Its just that setting it
up exclusively by the hctosys flag is maybe more fragile then we'd like.
Instead the hctosys flag maybe should only be used as a hint if there is
more then one RTC available.
So far we only created the symlink for an rtc with the hctosys flag
set. It was added as a workaround for ARM, which sometimes has
multiple RTCs. But there is now also logic in udev/systemd anyway to
search for the system's rtc, which does not rely on the symlink. Other
commonly used tools we checked just use /dev/rtc0 directly.

Yea, but there are likely a number of legacy applications that will still need the /dev/rtc link. I remember lots of pain when the /dev/rtcN devices showed up, and I suspect that's all been quiet due to the /dev/rtc links.

Dropping that link seems way too risky to me, thus why I'm submitting a revert. As I didn't realize the link was dependent on the hctosys flag.

As mentioned in the mail yesterday, I expected that
CONFIG_RTC_HCTOSYS=y was also needed on x86. The current udev logic
would need updating anyway, if we disable CONFIG_RTC_HCTOSYS=y now. So
let's just find out what's really needed here and add it back properly
if something really needs it.

I'm really fine with the revert, and would like to avoid any userland changes. There's more changes to come in that area of the kernel, so if there is any userland implications (which I'd actually prefer to avoid), lets make sure they are well thought out and not rushed.

While we are at it, the interface to read and set the persistent clock
from userspace, the clock the kernel internally uses, is still to just
use the /dev/rtc0 device with the ioctls? There is no other native
kernel timer interface or something else for that, right?

Right, the persistent_clock is only useful to the kernel over the RTC in the case where interrupts are disabled. Userland should stick to the RTC, and we shouldn't expose the persistent_clock out to userland. I'm hoping we'll be able to push the persistent_clock functionality into the RTC code (though I suspect the micro embedded folks will complain about having to compile the generic rtc core in as well - though we can always leave time initialization to userland for those crazier cases).

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