Re: [PATCH] drm/i915: use udelay for very small delays

[Jani Nikula <jani.nikula@linux.intel.com>]

On Thu, 15 Dec 2016, Nicholas Mc Guire <der.herr@hofr.at> wrote:
> On Thu, Dec 15, 2016 at 10:47:57AM +0200, Jani Nikula wrote:
>> On Thu, 15 Dec 2016, Nicholas Mc Guire <hofrat@osadl.org> wrote:
>> > usleep_range() is intended for delays in the 10us to 10ms range that need
>> > good precision. a useleep_range(1, will effectively be no more than an
>> > imprecise udelay with some added cache disruption as it will fire more or
>> > less immediately - use udelay() here.
>> >
>> > Fixes: commit be4fc046bed3 ("drm/i915: add VLV DSI PLL Calculations")
>> > Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
>> > ---
>> >
>> > Problem located by coccinelle
>> >
>> > The requirement of waiting at least 0.5 us is assured with the udelay(1)
>> > here which should be more effective than a usleep_range() - would 
>> > ndelay(500) make sense here ?
>> 
>> This is in the modeset path, i.e. pretty slow anyway. In this case, the
>> point is not to try hard to minimize the wait, the point is to guarantee
>> "at least 0.5 us" has passed. If the CPU can do something else,
>> including dozing off, in the mean time, great. I think we should stick
>> with usleep_range().
>
> well in that case maybe an acceptable solution would be to set it to 
> some suitable range 10,20 us ? or if not critical preferably even with a large
> upper limit.

I'd be fine with 10, 50 here. Please do send the patch, Cc: me.

>> 
>> I think the question is, how do we express this in code? IMO udelay() is
>> not the answer.
>
> if the delay need to be kept short then no - then its not the answer
> but usleep_ranges(1,2) I think is effectively just an inefficient version
> of udelay(1), by the time the timer is setup and the task gives
> up the cpu the timer would fire.
>
>> 
>> And why doesn't usleep_range() kernel-doc mention anything about the
>> ranges?
>> 
>
> interesting - that might be part of the reason there are many findings
> Documentation/timers/timers-howto.txt does
>
>         SLEEPING FOR ~USECS OR SMALL MSECS ( 10us - 20ms):
>                 * Use usleep_range

I'd appreciate short additions to the kernel-doc documentation of each
function to document the approximate range it's appropriate for. People
will know where to look if their use doesn't fall in that range.

Thanks,
Jani.

-- 
Jani Nikula, Intel Open Source Technology Center