HI Daniel,

On 15 May 2018 at 21:56, Daniel Lezcano [off-list ref] wrote:

On Mon, May 14, 2018 at 04:55:26PM +0800, Baolin Wang wrote:

quoted

Hi,

We will meet below issues when compensating the suspend time for the timekeeping.

1. We have too many different ways of dealing with persistent timekeeping
across architectures, so it is hard for one driver to compatable with different
architectures.

2. On some platforms (such as Spreadtrum platform), we registered the high
resolution timer as one clocksource to update the OS time, but the high
resolution timer will be stopped in suspend state. So we use another one
always-on timer (but low resolution) to calculate the suspend time to
compensate the OS time. Though we can register the always-on timer as one
clocksource, we need re-calculate the mult/shift with one larger conversion
range to calculate the suspend time and need update the clock in case of
running over the always-on timer.

More duplicate code will be added if other platforms meet this case.

3. Now we have 3 sources that could be used to compensate the OS time:
Nonstop clocksource during suspend, persistent clock and rtc device,
which is complicated. Another hand is that the nonstop clocksource can
risk wrapping if the suspend time is too long, so we need one mechanism
to wake up the system before the nonstop clocksource wrapping.

According to above issues, we can introduce one common persistent clock
framework to compatable with different architectures, in future we will
remove the persistent clock implementation for each architecture. Also
this framework will implement common code to help drivers to register easily.
Moreover if we converted all SUSPEND_NONSTOP clocksource to register to
be one persistent clock, we can remove the SUSPEND_NONSTOP clocksource
accounting in timekeeping, which means we can only compensate the OS time
from persistent clock and RTC.

Will be appreciated for any comments. Thank you all.

Why do we need another API ?

Why not remove the present persistent API and rely on the SUSPEND_NONSTOP flag
to do the right action at suspend and resume?

We register different clocksources, the rating does the selection.

When entering 'suspend', we check against the SUSPEND_NONSTOP flag and switch
to the first clocksource with the best rating and the flag set. When resuming,
we switch back to the highest rating.

I agree with John's view he posted before, he said:

"For context, these abstractions have grown out of the need for using
different hardware components for all of these. It was quite common
for x86 hardware to use the acpi_pm for clocksource, lapic/PIT for
clockevent, tsc for sched_clock and CMOS RTC for persistent clock.
While some of these could be backed by the same hardware, it wasn't
common. However, hardware with less restrictions have allowed in some
cases for these to be more unified, but I'm not sure if its particularly common.

Another part of the reason that we don't combine the above
abstractions, even when they are backed by the same hardware, is
because some of the fields used for freq conversion (mult/shift) have
different needs for the different types of accounting.

For instance, with a clocksource, we are very focused on avoiding
error to keep timekeeing accurate, thus we want to use as large a
shift (and thus mult) as possible (and do our shifting as late as
possible in our accounting). However, that then shrinks the amount of
time that can be accumulated in one go w/o causing an overflow.

Where as with sched_clock, we don't worry as much as about accuracy,
so we can use smaller shifts (and thus mults), and thus can go for
longer periods of time between accumulating without worrying.

Similarly for the persistent clock case we don't need need to worry as
much about accuracy, so we can can use smaller shifts, but we are not
in as much of a hot patch, so we can also"

-- 
Baolin.wang
Best Regards