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

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Date:   Mon, 7 Mar 2022 16:01:22 -0800
From:   Bjorn Andersson <bjorn.andersson@...aro.org>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Daniel Scally <djrscally@...il.com>,
        Heikki Krogerus <heikki.krogerus@...ux.intel.com>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        linux-usb@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-acpi@...r.kernel.org,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>
Subject: Re: [PATCH v4 7/7] usb: typec: mux: Add On Semi fsa4480 driver

On Mon 07 Mar 14:13 PST 2022, Andy Shevchenko wrote:

> 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".
> 

Right, we usually see a much more narrow range, as you say 2x or perhaps
10x, and this why I said I was glad you asked. I have been wondering
about this in a few different cases...

Thanks,
Bjorn