[PATCH] time: Fix get_ticks being non-monotonic

[Simon Glass]

Hi Sean,

On Tue, 8 Sep 2020 at 17:59, Sean Anderson <seanga2 at gmail.com> wrote:
>
> On 9/8/20 7:56 PM, Simon Glass wrote:
> > Hi Sean,
> >
> > On Mon, 7 Sep 2020 at 09:51, Sean Anderson <seanga2 at gmail.com> wrote:
> >>
> >> On 9/7/20 9:57 AM, Simon Glass wrote:
> >>> Hi Sean,
> >>>
> >>> On Sun, 6 Sep 2020 at 20:02, Sean Anderson <seanga2 at gmail.com> wrote:
> >>>>
> >>>> On 9/6/20 9:43 PM, Simon Glass wrote:
> >>>>> Hi Sean,
> >>>>>
> >>>>> On Tue, 1 Sep 2020 at 13:56, Sean Anderson <seanga2 at gmail.com> wrote:
> >>>>>>
> >>>>>> get_ticks does not always succeed. Sometimes it can be called before the
> >>>>>> timer has been initialized. If it does, it returns a negative errno.
> >>>>>> This causes the timer to appear non-monotonic, because the value will
> >>>>>> become much smaller after the timer is initialized.
> >>>>>>
> >>>>>> No users of get_ticks which I checked handle errors of this kind. Further,
> >>>>>> functions like tick_to_time mangle the result of get_ticks, making it very
> >>>>>> unlikely that one could check for an error without suggesting a patch such
> >>>>>> as this one.
> >>>>>>
> >>>>>> This patch changes get_ticks to always return 0 when there is an error.
> >>>>>> 0 is the least unsigned integer, ensuring get_ticks appears monotonic. This
> >>>>>> has the side effect of time apparently not passing until the timer is
> >>>>>> initialized. However, without this patch, time does not pass anyway,
> >>>>>> because the error value is likely to be the same.
> >>>>>>
> >>>>>> Fixes: c8a7ba9e6a5
> >>>>>> Signed-off-by: Sean Anderson <seanga2 at gmail.com>
> >>>>>> ---
> >>>>>>
> >>>>>>  lib/time.c | 4 ++--
> >>>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>>>>
> >>>>> Would it be better to panic so people can fix the bug?
> >>>>
> >>>> I thought this was expected behavior. It's only a bug if you do
> >>>> something like udelay before any timers are created. We just can't
> >>>> report errors through get_ticks, because its users assume that it always
> >>>> returns a time of some kind.
> >>>
> >>> I think it indicates a bug. If you use a device before it is ready you
> >>> don't really know what it will do. I worry that this patch is just
> >>> going to cause confusion, since the behaviour depends on when you call
> >>> it. If we panic, people can figure out why the timer is being inited
> >>> too late, or being used too early.
> >>
> >> Hm, maybe. I don't think it's as clear cut as "us[ing] a device before
> >> it is ready," because get_ticks tries to initialize the timer if it
> >> isn't already initialized. Unless someone else does it first, the first
> >> call to get_ticks will always be before the timer is initialized.
> >>
> >> The specific problem I ran into was that after relocation, the watchdog
> >> may be initialized before the timer. This occurs on RISC-V because
> >> without [1] a timer only exists after arch_early_init_r. So, for the
> >> first few calls to watchdog_reset there is no timer.
> >>
> >> The second return could probably be turned into a panic. I checked, and
> >> all current timer drivers always succeed in getting the time (except for
> >> the RISC-V timer, which is fixed in [1]), so the only way for
> >> timer_get_count to fail is if timer_ops.get_count doesn't exist. That is
> >> almost certainly an error on the driver author's part, so I think
> >> panicking there is the only reasonable option.
> >
> > OK good, let's do that and update docs in timer.h
>
> That being to panic both times, or just panic the second time?

Well I like a panic if the call is invalid, ie. in both cases.

Regards,
Simon