[U-Boot] [RFC] Review of U-Boot timer API

[Albert ARIBAUD <albert.u.boot@aribaud.net>]

Hi all,

Sorry, could not follow the discussion although I find it very 
interesting, so I will handle the task of coming in late and asking the 
silly questions.

Le 23/05/2011 07:25, Graeme Russ a ?crit :

> On Mon, May 23, 2011 at 3:02 PM, J. William Campbell
> <jwilliamcampbell@comcast.net>  wrote:
>> On 5/22/2011 6:42 PM, Graeme Russ wrote:
>>>
>>> OK, so in summary, we can (in theory) have:
>>>   - A timer API in /lib/ with a single u32 get_timer(u32 base) function
>>>   - A HAL with two functions
>>>     - u32 get_raw_ticks()
>>>     - u32 get_raw_tick_rate() which returns the tick rate in kHz (which
>>>       max's out at just after 4GHz)
>>>   - A helper function in /lib/ u32 get_raw_ms() which uses get_raw_ticks()
>>>     and get_tick_rate() to correctly maintain the ms counter used by
>>>     get_timer() - This function can be weak (so next point)
>>>   - If the hardware supports a native 32-bit ms counter, get_raw_ms()
>>>     can be overridden to simply return the hardware counter. In this case,
>>>     get_raw_ticks() would return 1

Are you sure you did not mean 'get_raw_ticks_rate' here? Besides, I'd 
like the name to specify the units used: 'get_raw_ticks_rate_in_khz' (or 
conversively 'get_raw_ticks_per_ms', depending on which is simpler to 
implement and use).

>>>   - Calling of get_raw_ticks() regularly in the main loop (how ofter will
>>>     depend on the raw tick rate, but I image it will never be necessary
>>>     to call more often than once every few minutes)

That's to keep track of get_raw_ticks() rollovers, right? And then the 
get_timer function (which, again, I would prefer to have '... in ms' 
expressed in its name) would call get_raw_ticks() in confidence that at 
most one rollover may have occurred since the last time the helper 
function was called, so a simple difference of the current vs last tick 
value will always be correct.

>>>   - If the hardware implements a native 32-bit 1ms counter, no call in
>>>     the main loop is required
>>>   - An optional HAL function u32 get_raw_us() which can be used for
>>>     performance profiling (must wrap correctly)
>>
>> Hi All,
>>       Graeme, I think you have stated exactly what is the "best" approach to
>> this problem.  I will provide a version of "get_raw_ms" that is  initialized
>> using get_raw_tick_rate that will work for all "reasonable" values of
>> raw_tick_rate. This will be the "generic" solution. Both the initialization
>> function and the get_raw_ms function can be overridden if there is reason to
>> do so, like "exact" clock rates. I will do the same with get_raw_us. This
>> will be posted sometime on Monday for people to review, and to make sure I
>> didn't get too far off base. Thank you to both Graeme and Reinhard for
>> looking at/working on this.. Hopefully, this solution will put this timing
>> issue to rest for all future ports as well as the presently existing ones.

In Greame's description, I did not see a get_raw_ms, only a get_raw_us. 
Was this last one a typo or is that a third HAL function?

> Great - I am in the middle of cleaning up the current usages of the timer
> API, reducing it all down to get_timer() - I will then 'libify'
> get_timer() and setup the hooks into the HAL get_raw_ticks() and
> get_raw_tick_rate() API
>
> I think there will need to be a lot of cleanup, especially in arch/arm to
> get this to all fit
>
> Regards,
>
> Graeme

Amicalement,
-- 
Albert.