[PATCH] time: Fix get_ticks being non-monotonic

[Simon Glass]

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

>
> (Does get_count even need to have a return value? I think it's
> reasonable to always expect the timer to return a value.)

I saw your patch, seems OK.

Regards,
Simon

>
> --Sean
>
> [1] https://patchwork.ozlabs.org/project/uboot/list/?series=198797