Re: [PATCH v1 6/7] iio: st_sensors: Add lsm9ds0 IMU support

From: Jonathan Cameron <jic23@kernel.org>
To: Andy Shevchenko <andy.shevchenko@gmail.com>
Cc: "Andy Shevchenko" <andriy.shevchenko@linux.intel.com>,
	"Jonathan Cameron" <Jonathan.Cameron@huawei.com>,
	"Gaëtan André" <rvlander@gaetanandre.eu>,
	"Nuno Sá" <nuno.sa@analog.com>,
	"Denis Ciocca" <denis.ciocca@st.com>,
	linux-iio <linux-iio@vger.kernel.org>,
	devicetree <devicetree@vger.kernel.org>,
	"Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
	"Lars-Peter Clausen" <lars@metafoo.de>,
	"Peter Meerwald-Stadler" <pmeerw@pmeerw.net>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Matija Podravec" <matija_podravec@fastmail.fm>,
	"Sergey Borishchenko" <borischenko.sergey@gmail.com>
Subject: Re: [PATCH v1 6/7] iio: st_sensors: Add lsm9ds0 IMU support
Date: Sun, 18 Apr 2021 17:45:10 +0100	[thread overview]
Message-ID: <20210418174510.64df5344@jic23-huawei> (raw)
In-Reply-To: <CAHp75VeRgGcat18p+dN+pbHEYqm+YLGB_06kFEjFsahB2EW9Fw@mail.gmail.com>

On Sun, 18 Apr 2021 16:59:02 +0300
Andy Shevchenko <andy.shevchenko@gmail.com> wrote:

> On Sun, Apr 18, 2021 at 4:49 PM Andy Shevchenko
> <andy.shevchenko@gmail.com> wrote:
> >
> > On Sun, Apr 18, 2021 at 2:07 PM Jonathan Cameron <jic23@kernel.org> wrote:
> >
> > Thanks for review, my answers below.
> >  
> > > On Wed, 14 Apr 2021 22:54:53 +0300
> > > Andy Shevchenko <andriy.shevchenko@linux.intel.com> wrote:
> > >  
> > > > We can utilize separate drivers for accelerometer and magnetometer,
> > > > so here is the glue driver to enable LSM9DS0 IMU support.
> > > >
> > > > The idea was suggested by Crestez Dan Leonard in [1]. The proposed change
> > > > was sent as RFC due to race condition concerns, which are indeed possible.  
> > >
> > > If you are going to mention races, good to give some flavour in here!  
> >
> > I meant that the initial idea is racy due to different devices
> > communicating to the same i2c address.
> > So, any sequence of transfers are not serialized and you may end up with
> >
> > drv1 -> i2c
> > drv2 -> i2c
> > drv1 <- i2c # garbage
> >  
> > > This driver makes me very nervous indeed.  
> >
> > Why?! This one is race free as far as I can see. Or maybe I interpret
> > this wrongly and you are talking about initial RFC?
> >  
> > >  I haven't 'found' any places
> > > where the fact we'll write the same registers from each of the drivers
> > > causes problems (e.g. int pin setup etc) but perhaps I'm missing something.
> > >
> > > Shall we say that makes me rather keener to get eyes (and thought) on this
> > > patch than normal :)  
> >
> > How should I amend the commit message to state:
> > 1. First idea (RFC by the link) *is* racy AFAIU
> > 2. This one *is not* racy.  

Great.  I read it as meaning they were both potentially racey!
This is less worrying.

> 
> I re-read this and now understand better what you meant.
> So, it may be that the initial proposal may work without any
> amendment, but since I haven't investigated much, I should rather use
> the phrase "potentially racy". In my variant it's using one regmap for
> both drivers (not two), which makes the register state consistent. Am
> I wrong?

I think this approach is fine.  I'd be more worried about the two 'sub' drivers
not necessarily being happy that someone else touches state they care about.
There are places where I think we write the same value to the same register
twice during setup with this model, but that shouldn't matter.   I'm not 100%
sure that there aren't other cases though I think there aren't.

So what you have is probably fine, but more eyes would make me happier ;)

Lots of people care about this particular driver so hopefully we'll get
them.

> Do we have some places where we may write to the same register concurrently?
> 
Only ones I can find are the setup ones where it writes the same value twice
I think.  So *crosses fingers* :)

Given timing (missed merge window) we have masses of time to let this sit
on list a while and see if anyone can spot issues neither of us have found.

Jonathan