[PATCH v4 1/2] iopoll: Introduce memory-mapped IO polling macros

[Mitchel Humpherys]

On Wed, Oct 08 2014 at 06:40:46 AM, Arnd Bergmann <arnd at arndb.de> wrote:
> On Tuesday 07 October 2014 18:47:59 Mitchel Humpherys wrote:
>> On Wed, Oct 01 2014 at 01:25:33 AM, Arnd Bergmann <arnd at arndb.de> wrote:
>> > On Tuesday 30 September 2014 18:28:12 Mitchel Humpherys wrote:
>> >> + */
>> >> +#define readl_poll_timeout(addr, val, cond, sleep_us, timeout_us) \
>> >> +({ \
>> >> +       ktime_t timeout = ktime_add_us(ktime_get(), timeout_us); \
>> >> +       might_sleep_if(timeout_us); \
>> >
>> > Does it make sense to call this with timeout_us = 0?
>> 
>> Yes, the idea there being to "never timeout".  That mode should, of
>> course, be used with extreme caution since never timing out is not
>> really "playing nice" with the system.
>
> But then you certainly still 'might_sleep' here. The
> might_sleep_if(timeout_us) line suggests that it won't sleep, but
> that isn't the case.

Yes looks like that was actually a bug.  Should have been
might_sleep_if()'ing on sleep_us.  This is fixed in the v5 I just sent
out.


[...]

>> Regarding the division, for the overwhelmingly common case where the
>> user of the API passes in a constant for sleep_us the compiler optimizes
>> out this calculation altogether and just sticks the final result in (I
>> verified this with gcc 4.9 and the kernel build system's built-in
>> support for generating .s files).  Conveying semantic meaning by using
>> `DIV_ROUND_UP' is nice but if you feel strongly about it we can make
>> this a shift instead.
>
> The more important question is probably if you want to keep the _ROUND_UP
> part. If that's not significant, I think a shift would be better.

If we drop the _ROUND_UP then passing a sleep_us <= 4 would result in a
minimum sleep time of 0, so we'd be polling a lot faster than the user
had expected.



-Mitch

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