linux-kernel - Re: [PATCH] iio: sx9310: Prefer async probe

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Date:   Tue, 1 Sep 2020 14:45:20 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Stephen Boyd <swboyd@...omium.org>
CC:     Doug Anderson <dianders@...omium.org>,
        Jonathan Cameron <jic23@...nel.org>,
        Alexandru Ardelean <alexandru.ardelean@...log.com>,
        Daniel Campello <campello@...omium.org>,
        Enrico Granata <egranata@...omium.org>,
        Hartmut Knaack <knaack.h@....de>,
        "Lars-Peter Clausen" <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        linux-iio <linux-iio@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Dmitry Torokhov <dtor@...omium.org>
Subject: Re: [PATCH] iio: sx9310: Prefer async probe

On Tue, 1 Sep 2020 00:03:55 -0700
Stephen Boyd <swboyd@...omium.org> wrote:

> Quoting Doug Anderson (2020-08-31 11:59:37)
> > On Sat, Aug 29, 2020 at 10:18 AM Jonathan Cameron <jic23@...nel.org> wrote:  
> > > Agreed. However, I'd like a comment next to the place we turn it on
> > > saying what delays we are trying to mitigate by enabling it in this
> > > driver.  
> > 
> > OK, I used function graph to get a trace.  Some of the time is simply
> > i2c transfers, which we do A LOT of during initialization (and i2c
> > transfers aren't known for being instant).  However, that's not the
> > big problem.  Check out the regmap_read_poll_timeout() in
> > sx9310_init_compensation().  20 ms per sleep with a 2 second timeout.
> > This seems to eat over around 220 ms of the time on my system.
> > 
> > So how do you want to do this?  Do you want me to re-post the patch
> > and mention the regmap_read_poll_timeout() in the commit message?  Do
> > you want to just add that to the commit message yourself?
> >   
> 
> I think the suggestion is to put a comment next to the
> PROBE_PREFER_ASYNCHRONOUS flag indicating that
> sx9310_init_compensation() can take a while. 

Yup. That's what I meant. Sorry for lack of clarity.
A comment in the code stops others copying it without making sure they
understand whether it is relevant in their case.  No one is going
to look in commit messages.

> Should the compensation be
> delayed even further though to some time when userspace opens the device
> node?

It's tricky because we may hit it on an individual sysfs read.
We could spin off a thread to do it I suppose, though that would be
more or less the same as allowing async probe.

Jonathan