[PATCH] time: Fix get_ticks being non-monotonic

[Sean Anderson <seanga2@gmail.com>]

On 9/8/20 7:56 PM, Simon Glass wrote:
> Hi Sean,
> 
> On Mon, 7 Sep 2020 at 09:51, Sean Anderson <seanga2@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@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@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@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?

--Sean