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

[Russell King - ARM Linux]

On Mon, Nov 20, 2017 at 09:38:46AM -0800, Doug Anderson wrote:
> Hi,
> 
> On Thu, Nov 16, 2017 at 3:22 PM, Russell King - ARM Linux
> <linux at armlinux.org.uk> wrote:
> > On Thu, Nov 16, 2017 at 02:15:02PM -0800, Doug Anderson wrote:
> >> 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 盜 and spinning only 25 盜 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.
> >
> > I look forward to reviewing your implementation.
> 
> It's unlikely I'll post a patch in the near term since this isn't
> presenting me with a big problem right now.  Mostly I saw Marc's patch
> and thought it would be a good patch to land and I knew this type of
> thing had bitten me in the past.
> 
> One happy result of this whole discussion, though, is that you now
> sound as if you'll be happy the next time someone brings this up since
> you're looking forward to reviewing an implementation.  That's a nice
> change from the original statement questioning why someone was asking
> about this again.  :)

What I'd be happy with, and what I've always been happy with is what I've
stated: either we fix _all_ implementations or none of them.  We can't
have the situation where some implementations give one expectation and
others give something completely different.

That's always been my argument against _just_ fixing the timer-based
delays and ignoring the rest of the problem.

Nothing has changed about my position.

-- 
RMK's Patch system: http://www.armlinux.org.uk/developer/patches/
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