[U-Boot] [PATCH 4/4] ARM: bcm283x: Switch to generic timer

[Stephen Warren]

On 05/06/2015 12:13 PM, Marek Vasut wrote:
> On Wednesday, May 06, 2015 at 05:52:37 PM, Stephen Warren wrote:
> [...]
>>>>> So, if now is close to 0x7fffffff (which it can), then if endtime is
>>>>> big-ish, diff will become negative and this udelay() will not perform
>>>>> the correct delay, right ?
>>>>
>>>> I don't believe so, no.
>>>>
>>>> endtime and now are both unsigned. My (admittedly intuitive rather than
>>>> well-researched) understanding of C math promotion rules means that
>>>> "endtime - now" will be calculated as an unsigned value, then converted
>>>> into a signed value to be stored in the signed diff. As such, I would
>>>> expect the value of diff to be a small value in this case. I wrote a
>>>> test program to validate this; endtime = 0x80000002, now = 0x7ffffffe,
>>>> yields diff=4 as expected.
>>>>
>>>> Perhaps you meant a much larger endtime value than 0x80000002; perhaps
>>>> 0xffffffff? This doesn't cause issues either. All that's relevant is the
>>>> difference between endtime and now, not their absolute values, and not
>>>> whether endtime has wrapped but now has or hasn't. For example, endtime
>>>> = 0x00000002, now = 0xfffffff0 yields diff=18 as expected.
>>>
>>> So what if the difference is bigger than 1 << 31 ?
>>
>> As I said, I don't believe that case is relevant; it can only happen if
>> passing ridiculously large delay values into __udelay() (i.e. greater
>> than the 1<<31value you mention), and I don't believe there's any need
>> to support that.
>
> So what you say is that it's OK to have a function which is buggy in
> corner cases ?

A corner case (something that's within spec but perhaps hard/unusual) 
should not be buggy.

The behaviour of something outside spec isn't relevant; it's actively 
not specified.

I suppose there is no specification of what range of values this 
function is supposed to accept. I'd argue we should create one, and that 
spec should likely limit the range to much less than the 32-bit 
parameter can actually hold, since some HW timer implementations may 
have well less than 32-bits of range.