[U-Boot] core ticks/timer code

Dirk Behme
2009-03-28T07:07:45Z

Mike Frysinger wrote:
> the Blackfin core tick/timer code has been around since the start of the
> original port, but i'm not sure it's entirely correct. some common code that
> uses timers seems to be misbehaving in that the timeout is pretty much
> immediate. makes me think that we've spent time on making udelay() work, but
> then just glossed over the rest.
>
> unfortunately, there doesnt seem to be any docs on what exactly these
> functions do so it's hard for me to verify/change any of it.

Yes, you are right. Some days ago I found that no proper documentation
for the ticks/timer related code is there, too.

From looking at the existing code for some ARM boards (OSK, DaVinci,
OMAP3) my idea of this code is:

> my understanding is that:
> - get_ticks - return some notion of "cpu ticks"

Yes. Returns the number of cpu ticks since power up. I.e. with 1000
ticks per second (CONFIG_SYS_HZ) the number of elapsed ms since power up.

> - get_tbclk - return number of "cpu ticks" that elapse in one second

Yes. Some implementations just return CONFIG_SYS_HZ which is supposed
to be 1000.

> - timer_init - setup a core timer

Some implementations do this in interrupt_init(), too.

> - get_timer(x) - not really sure what this is supposed to represent, or how
> "x" is used

This is similar to get_ticks(), i.e. returns the number of ticks since
power up. Parameter x is used as a base which is subtracted from
ticks. With x == 0 this function is the same as get_ticks(). If x != 0
the value returned is

number of ticks since power up - x

> - reset_timer - reset core timer to 0

Some implementations don't reset the timer (hw device) itself. They
just reset the timestamp counter (sw variable) which counts the ticks
since power up (i.e. that is returned by get_ticks()).

> - CONFIG_SYS_HZ - no idea how this relates to ticks/timer in U-Boot as in the
> Linux world, this is the core timer (scheduler) frequency (how many times to
> execute per second)

CONFIG_SYS_HZ is supposed to be 1000

Does this make sense?

Best regards

Dirk