[U-Boot] [PATCH v2] mmc: omap: timeout counter fix

[Reinhard Meyer <u-boot@emk-elektronik.de>]

Dear Wolfgang Denk,
> In message<4CC62B6C.30601@emk-elektronik.de>  you wrote:
>>
>> In such cases I prefer to use:
>>
>> 	uint64_t etime;
>> ...
>> 	etime = get_ticks() + get_tbclk(); /* 1 second */
>> 	do {
>> 		whatever;
>> 		udelay (xx);
>> 	} while (condition&&  get_ticks()<= etime);
>>
>> That is far more accurate than calling udelay() 100000 times.
>
> It may be more accuratre, but it may also be HORRIBLY WRONG!!
>
> Do NOT do that!! NEVER implement such a delay loop as
>
> 	end = time() + delay;
> 	while (time()<  end)
> 		...
>
> It fails in case the timer wraps around.
>
> Assume 32 bit counters, start time = 0xFFFFFFF0, delay = 0x20. It
> will compute end = 0x10, the while codition is immediately false, and
> you don't have any delay at all, which most probably generates a
> false error condition.

I used and assumed a 64 bit counter, that will not wrap around while
our civilization still exists...

If get_ticks() is only 32 bits worth, both methods will misbehave
at a 32 bit wrap over.

>
>
> Correct implementation of a timeout like that should always look like
> that:
>
> 	start = time();
> 	while ((time() - start)<  delay)
> 		...
>
> This works much better (assuming unsigned arithmetics).

True, provided the underlying timer is really 64 bits, otherwise
this fails, too...

Best would be to assign get_ticks() to a 32 bit unsigned and use
32 bit vars for start and delay as well.

The original udelay() implementation in AT91 would have failed
at a 32 bit wrap over, too (fixed in mainline). I hope other
implementations are done right, too...

Best regards,

Reinhard