Re: [PATCH] iio: sx9310: Prefer async probe

From: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
To: Stephen Boyd <swboyd@chromium.org>
Cc: Doug Anderson <dianders@chromium.org>,
	Jonathan Cameron <jic23@kernel.org>,
	Alexandru Ardelean <alexandru.ardelean@analog.com>,
	Daniel Campello <campello@chromium.org>,
	Enrico Granata <egranata@chromium.org>,
	Hartmut Knaack <knaack.h@gmx.de>,
	"Lars-Peter Clausen" <lars@metafoo.de>,
	Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
	linux-iio <linux-iio@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Dmitry Torokhov <dtor@chromium.org>
Subject: Re: [PATCH] iio: sx9310: Prefer async probe
Date: Tue, 1 Sep 2020 14:45:20 +0100	[thread overview]
Message-ID: <20200901144520.0000576c@Huawei.com> (raw)
In-Reply-To: <159894383586.334488.2938301456606601581@swboyd.mtv.corp.google.com>

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

> Quoting Doug Anderson (2020-08-31 11:59:37)
> > On Sat, Aug 29, 2020 at 10:18 AM Jonathan Cameron <jic23@kernel.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