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.
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