[PATCH] time: Fix get_ticks being non-monotonic

[Sean Anderson <seanga2@gmail.com>]

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.

--Sean