Re: [PATCH v4 7/7] usb: typec: mux: Add On Semi fsa4480 driver

Andy Shevchenko
2022-03-07T22:14:03Z

On Mon, Mar 07, 2022 at 01:04:50PM -0800, Bjorn Andersson wrote:
> On Mon 07 Mar 08:13 PST 2022, Andy Shevchenko wrote:
> > On Mon, Mar 07, 2022 at 06:48:25AM -0800, Bjorn Andersson wrote:
> > > On Mon 07 Mar 02:16 PST 2022, Andy Shevchenko wrote:
> > > > On Sun, Mar 06, 2022 at 07:40:40PM -0800, Bjorn Andersson wrote:

...

> > > > > + /* 15us to allow the SBU switch to turn off */
> > > > > + usleep_range(15, 1000);
> > > >
> > > > This is quite unusual range.
> > > >
> > > > If you are fine with the long delay, why to stress the system on it?
> > > > Otherwise the use of 1000 is unclear.
> > > >
> > > > That said, I would expect one of the below:
> > > >
> > > > usleep_range(15, 30);
> > > > usleep_range(500, 1000);
> > >
> > > Glad you asked about that, as you say the typical form is to keep the
> > > range within 2x of the lower value, or perhaps lower + 5.
> > >
> > > But if the purpose is to specify a minimum time and then give a max to
> > > give the system some flexibility in it's decision of when to wake up.
> > > And in situations such as this, we're talking about someone connecting a
> > > cable, so we're in "no rush" and I picked the completely arbitrary 1ms
> > > as the max.
> > >
> > > Do you see any drawback of this much higher number? (Other than it
> > > looking "wrong")
> >
> > I see the drawback of low number.
>
> 15us is based on the data sheet and if the kernel is ready to serve us
> after 15us then let's do that.
>
> > The 1000 makes not much sense to me with the minimum 66x times less.
> > If there is no rush, use some reasonable values,
> > what about
> >
> > usleep_range(100, 1000);
> >
> > ? 10x is way better than 66x.
>
> I don't agree, and in particular putting 100 here because it's 1/10 of
> the number I just made up doesn't sounds like a good reason. The
> datasheet says 15us, so that is at least based on something real.
>
> In https://www.kernel.org/doc/Documentation/timers/timers-howto.txt
> I find the following:
>
> With the introduction of a range, the scheduler is
> free to coalesce your wakeup with any other wakeup
> that may have happened for other reasons, or at the
> worst case, fire an interrupt for your upper bound.
>
> The larger a range you supply, the greater a chance
> that you will not trigger an interrupt; this should
> be balanced with what is an acceptable upper bound on
> delay / performance for your specific code path. Exact
> tolerances here are very situation specific, thus it
> is left to the caller to determine a reasonable range.
>
> Which to me says that the wider range is perfectly reasonable. In
> particular 15, 30 (which seems to be quite common) makes the available
> range to the scheduler unnecessarily narrow.
>
> And it's clear that whatever the upper bound it's going to be some
> arbitrary number, but 1ms should ensure that there are other hrtimer
> interrupts to piggy back on.

Okay, I have grepped for usleep_range(x[x], yyyy) and there are 9 modules
use it. A few commit messages call 1000 as "reasonable upper limit".

--
With Best Regards,
Andy Shevchenko