[U-Boot] Timer implementations

[Reinhard Meyer]

Dear J. William Campbell,
> Hi All,
>
> I am pretty sure the migration to 64 bits was caused by 1) people not understanding that the timer operating on time DIFFERENCES would work fine even if the underlying timer wrapped around (most probable problem) and possibly 2) broken timer functions causing bogus timeouts, improperly "fixed" by switching to 64 bits.
>
> I think u-boot could get along just fine with only 2 time related functions, uint_32 get_timer(uint_32 base) and udelay(uint 32 delay). udelay will only work on "small" values of delay, on the order of milliseconds. It is to be used when a "short" but "precise" delay in microsecond resolution is required. Users of get_timer must understand that it is only valid if it is called "often enough", i.e. at least once per period of the underlying timer. This is required because u-boot does not want to rely on interrupts as a timer update method. Therefore, all uses of get_timer must 1) call it once initially to get a start value, and 2) call get_timer at least once per period of the underlying hardware counter. This underlying period is guaranteed to be at least 4.29 seconds (32 bit counter at 4 GHz). Note that this does NOT mean that the total wait must be less than 4.29 seconds, only that the rate at which the elapsed time is TESTED must be adequate.
>
> In order to implement this functionality, at least one hardware timer of some kind is required. An additional software "timer" in 1 ms resolution may be useful in maintaining the software time. If the hardware timer update rate is programmable, u-boot MAY set the update rate on initialization On initialization, u-boot MAY reset the hardware timer and MAY reset any associated software timer. The hardware timer MAY be started on initialization. On each call to get_timer(), u-boot MUST start the hardware timer if it was not started already. On calls to get_timer, u-boot MUST NOT reset the hardware timer if it was already started. The software timer MAY be reset if u-boot can unambiguously determine that more than 4.29 seconds has elapsed since the last call to get_timer.
>
> The simplest case for implementing this scheme is if two programmable timers exist that can be set to 1ms and 1us. The timers are initialized at start-up, get_timer just returns the 32 bit 1 ms timer and udelay just waits for the number of ticks required on the second timer to elapse. The most common harder case is where there is only one timer available, it is running at 1 us per tick or faster, and we cannot control the rate. udelay is still easy, because we can convert the (small) delay in us to a delay in ticks by a 32 bit multiply that will not overflow 32 bits even if we have quite a few fractional bits in the tics per microsecond value. The elapsed ticks required is the (delay in us * us/per tick) >> # fractional bits in us/per tick. If that is not close enough for you, you can do it as (delay in us * (integer part of us/tick)) + ((delay in us * (fractional part)us/tick) >> # fraction bits). For "nice" numbers, like any integral number of MHz, there is no fractional

> part. Only numbers like 66 MHz, or 1.666 GHz require messing with the fractional part.
> For get_timer, it is a bit harder. The program must keep two 32 bit global variables, the timer reading "last time" and the software timer in 1 ms resolution. Whenever get_timer is called, it must increase the software timer by the number of ms that have elapsed since the previous update and record the corresponding timer reading as the new "last time". Note that if the number of ms elapsed is not an integer (a common case), the value recorded as the "last time" must be decreased by the number of ticks not included in the 1 ms resolution software timer. There are many different ways to accomplish update, depending on what hardware math capabilities are available, and whether one thinks efficiency is important here. Conceptually, you convert the elapsed time in ticks into an equivalent number of ms, add that number to the software timer, store the current value of the hardware timer in last time, and subtract any "remainder" ticks from that value. If the elapsed time is les
s
> that one ms, do no update of "last hardware time" and return the current software counter. If the elapsed time is greater than 4.29 seconds, reset the software counter to 0, record the current hardware counter time and return the current software counter. In between, do the math, which will fit into 32 bits.
>
> If this idea seems like a good one, I can provide more detail on the conversions for various hardware capabilities is people want. Comments welcome.

To get the timer mess cleaned up three things have to happen:

1. A consensus and documentation how it MUST be handled

2. Fix all timer implementations to adhere to that

3. Fix all timer uses to adhere to that

To start, RFC:

a) udelay(u32) MUST NOT interfere with other timer functions
b) u32 get_timer(u32 base) MUST return (current_time - base) using
millisecond units
c) get_timer() MUST exhaust the full 32 bit range before wrapping
d) to ensure the function of internal high frequency wrapping
processes, get_timer() MUST be called at least once a second while
a timeout loop is run (this will typically be the case anyway)
e) reset_timer() is not needed, potentially harmful if used and
shall be removed. This will also allow for nested timeouts, e.g.
inner timeout for accessing a hardware and outer timeout for having
the hardware perform a function
f) get_ticks() and get_tbclk() SHALL NOT be used to check for timeouts
except for cases where the better than ms resolution is really needed
g) get_ticks() MUST return true 64 bit time
h) all uses of get_ticks() and get_timer() shall be fixed to use
get_timer() as follows:

u32 start_ms;

start_ms = get_timer(0);

any_type_of_loop {
	...
	/* check for timeout */
	if (get_timer(start_ms) >= TIMEOUT_IN_MS)
		/* handle timeout */
	...
}
Alternative, but in contrast to current get_timer() implementation
is to drop the get_timer parameter and do the subtraction in the if.

--> get_timer(base) is a bit of a misnomer, it should be better named
something like get_ms_since(base).

_Observation:_

It seems possible to move udelay() and get_timer() and static helper
functions into a common, ARCH and SoC independent file, provided that
the ARCH/SoC/board dependant implementations of get_ticks() runs
at >= 1 MHz and is true 64 bits. IF above d) is met get_ticks might be
replaced by a u32 function to be used by the common udelay() and
get_timer() implementation. That would allow all timer code to use 32
bit arithmetic only.

Best Regards,
Reinhard