Writing clocksource driver, tips?

Writing clocksource driver, tips?

[Oleksij Rempel]

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Hello all,

currently i work on porting linux to Aplscale ASM9260,
most basic parts are done, now is the time for clocksource driver. I
never did it before, so i need some beginner tips.
My current driver clocksource provide periodic events at 100Hz rate. But
this hardvare can do more then it.
This SoC has 4 32bit-timer controller attached to 100MHz source. Each
controller has 4 separate timer register each of them can be configure
separately for scale, match and interrupt. 3 controllers can choice the
tick source.
This controller looks similar to one found on NXP LPC1311, but looks
like no similar hardware in drivers/clocksource.

For now i have fallowing questions:
- 1 timer can be used for clocksource other as  clock_event_device, how
can i use other 2 timers on same controller. and how can i use other 3
controllers?
- theoretically i can use clocksource_mmio_init to read directly
timer_counter_register. Which scale should be used on it. 100MHz is
probably too match.
- should clocksource provide interrupt on overflow or some kind of limit?

-- 
Regards,
Oleksij


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Re: Writing clocksource driver, tips?

[Thomas Gleixner]

On Sun, 31 Aug 2014, Oleksij Rempel wrote:
> - 1 timer can be used for clocksource other as  clock_event_device, how
> can i use other 2 timers on same controller. and how can i use other 3
> controllers?

All you need is two.

One for the clocksource (free running counter):

    100MHz is fine, it gives the timekeeping code nice
    granularity. There is no need for a wrap around interrupt. The
    core code knows how to deal with it.

The second one is for the clock event device

    Avoid a match timer if possible, it's a pain in the neck. Either a
    autoreloading timer for the periodic mode or a simple downcounting
    one for one shot mode is good enough.

The other timers are not of any value for Linux. Just leave them
alone.

Thanks,

	tglx