[RFC] Improving udelay/ndelay on platforms where that is possible

[Doug Anderson]

Hi,

On Thu, Nov 16, 2017 at 1:05 PM, Marc Gonzalez
<marc_gonzalez at sigmadesigns.com> wrote:
> On 16/11/2017 18:05, Russell King - ARM Linux wrote:
>
>> On Thu, Nov 16, 2017 at 05:42:36PM +0100, Marc Gonzalez wrote:
>>
>>> Requesting 100 µs and spinning only 25 µs is still a problem,
>>> don't you agree?
>>
>> Which is why, as I've said *many* times already, that drivers are written
>> with leaway on the delays.
>
> A delay 75% too short is possible. Roger that.
>
>> I get the impression that we're just going around in circles, and what
>> you're trying to do is to get me to agree with your point of view.
>> That's not going to happen, because I know the history over about the
>> last /24/ years of kernel development (which is how long I've been
>> involved with the kernel.)  That's almost a quarter of a century!
>>
>> I know how things were done years ago (which is relevant because we
>> still have support in the kernel for these systems), and I also know the
>> history of facilities like cpufreq - I was the one who took the work
>> that Erik Mouw and others involved with the LART project, and turned it
>> into something a little more generic.  The idea of dynamically scaling
>> the CPU frequency on ARM SoCs was something that the SoC manufacturer
>> had not even considered - it was innovative.
>>
>> I know that udelay() can return short delays when used in a kernel with
>> cpufreq enabled, and I also know that's almost an impossible problem to
>> solve without going to a timer-based delay.
>>
>> So, when you think that sending an email about a udelay() that can be
>> 10x shorter might be somehow new information, and might convince people
>> that there's a problem, I'm afraid that it isn't really new information.
>> The SA1110 cpufreq driver is dated 2001, and carries my copyright, and
>> has the ability to make udelay()s 4x shorter or 4x longer depending on
>> the direction of change.
>>
>> We've discussed solutions in the past (probably 10 years ago) about
>> this, and what can be done, and the conclusion to that was, as Nicolas
>> has said, to switch to using a timer-based delay mechanism where
>> possible.  Where this is not possible, the platform is stuck with the
>> loops based delays, and their inherent variability and inaccuracy.
>>
>> These platforms have been tested with such a setup over many years.
>> They work even with udelay() having this behaviour, because it's a
>> known issue and drivers cater for it in ways that I've already covered
>> in my many previous emails to you.
>>
>> These issues are known.  They've been known for the last 15 odd years.
>
> So you've known for umpteen years that fixing loop-based delays is
> intractable, yet you wrote:
>
>> udelay() needs to offer a consistent interface so that drivers know
>> what to expect no matter what the implementation is.  Making one
>> implementation conform to your ideas while leaving the other
>> implementations with other expectations is a recipe for bugs.
>>
>> If you really want to do this, fix the loops_per_jiffy implementation
>> as well so that the consistency is maintained.
>
> In other words, "I'll consider your patch as soon as Hell freezes over".
>
> Roger that. I'll drop the subject then.

Presumably, though, you could introduce a new API like:

  udelay_atleast()

That was guaranteed to delay at least the given number of
microseconds.  Unlike the current udelay(), the new udelay_atleast()
wouldn't really try that hard to get a delay that's approximately the
one requested, it would just guarantee not to ever delay _less_ than
the amount requested.  Thus there would be some reasons to use one API
or the other and it would be up to the driver to pick one.  You
wouldn't regress any performance on old code since the old API would
work the same as it always did.


You could presumably implement udelay_atleast() something like I did
in an ancient patch I was involved in at http://crosreview.com/189885.
In this case you wouldn't modify the normal udelay() but you'd
actually add a new API.  (for the curious, we later picked back a
proper timer-based delay in http://crosreview.com/218961 so it's not
like we were stuck with the crappy delay for long)


Once you added udelay_atleast() you'd have a good reason to fixup the
timer-based udelay() since udelay_atleast() could just be a wrapper of
that function and you'd have the requested consistency.


-Doug