Re: [PATCH v3 3/4] iio: health: max30102: do not use internal iio_dev lock

From: "Nuno Sá" <noname.nuno@gmail.com>
To: "Miquel Raynal" <miquel.raynal@bootlin.com>,
	"Nuno Sá" <nuno.sa@analog.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-iio@vger.kernel.org,
	 linux-amlogic@lists.infradead.org,
	linux-rockchip@lists.infradead.org,  linux-imx@nxp.com,
	Chen-Yu Tsai <wens@csie.org>,
	Andriy Tryshnivskyy <andriy.tryshnivskyy@opensynergy.com>,
	Ciprian Regus <ciprian.regus@analog.com>,
	 Vladimir Zapolskiy <vz@mleia.com>,
	Cixi Geng <cixi.geng1@unisoc.com>,
	Neil Armstrong <narmstrong@baylibre.com>,
	Sascha Hauer <s.hauer@pengutronix.de>,
	Heiko Stuebner <heiko@sntech.de>,
	Fabio Estevam <festevam@gmail.com>,
	Jerome Brunet <jbrunet@baylibre.com>,
	 Martin Blumenstingl <martin.blumenstingl@googlemail.com>,
	Pengutronix Kernel Team <kernel@pengutronix.de>,
	Baolin Wang <baolin.wang@linux.alibaba.com>,
	Hans de Goede <hdegoede@redhat.com>,
	 Alexandru Ardelean <aardelean@deviqon.com>,
	Michael Hennerich <Michael.Hennerich@analog.com>,
	Haibo Chen <haibo.chen@nxp.com>,
	Lars-Peter Clausen <lars@metafoo.de>,
	Jyoti Bhayana <jbhayana@google.com>,
	Jonathan Cameron <jic23@kernel.org>,
	Andy Shevchenko <andy.shevchenko@gmail.com>,
	Florian Boor <florian.boor@kernelconcepts.de>,
	 Chunyan Zhang <zhang.lyra@gmail.com>,
	Orson Zhai <orsonzhai@gmail.com>, Shawn Guo <shawnguo@kernel.org>,
	 Kevin Hilman <khilman@baylibre.com>
Subject: Re: [PATCH v3 3/4] iio: health: max30102: do not use internal iio_dev lock
Date: Fri, 14 Oct 2022 09:25:59 +0200	[thread overview]
Message-ID: <fccf3f41b32e881eb79ee78852fa40a62b134c00.camel@gmail.com> (raw)
In-Reply-To: <20221012204556.7648df2e@xps-13>

On Wed, 2022-10-12 at 20:45 +0200, Miquel Raynal wrote:
> Hi Nuno,
> 
> nuno.sa@analog.com wrote on Wed, 12 Oct 2022 17:16:19 +0200:
> 
> > The pattern used in this device does not quite fit in the
> > iio_device_claim_direct_mode() typical usage. In this case, we want
> > to
> > know if we are in buffered mode or not to know if the device is
> > powered
> > (buffer mode) or not. And depending on that max30102_get_temp()
> > will
> > power on the device if needed. Hence, in order to keep the same
> > functionality, we try to:
> > 
> > 1. Claim Buffered mode;
> > 2: If 1) succeeds call max30102_get_temp() without powering on the
> >    device;
> > 3: Release Buffered mode;
> > 4: If 1) fails, Claim Direct mode;
> > 5: If 4) succeeds call max30102_get_temp() with powering on the
> > device;
> > 6: Release Direct mode;
> > 7: If 4) fails, goto to 1) and try again.
> > 
> > This dance between buffered and direct mode is not particularly
> > pretty
> > (as well as the loop introduced by the goto statement) but it does
> > allow
> > us to get rid of the mlock usage while keeping the same behavior.
> 
> What about adding a TODO comment saying something like: "this comes
> from static analysis and helped dropping mlock access, but someone
> with
> the device needs to figure out if we can simplify this dance"?
> Because
> the reason behind all this is that we don't want to risk breaking the
> driver, but perhaps a simpler approach would work, right?
> 

Hi Miquel,

AFAIU, either the device is powered (when buffer mode enabled) and we
can do the reading or it's not and we need to power it on/off
"manually" while making sure we don't race against enable/disabling
buffers. This "dance" is needed mainly to make sure that we grab
'mlock' one way or another... The other way would be to use some
specific device lock together with a flag (as discussed) but as
discussed with Jonathan we decided to go down this road... So,
honestly, I don't really see the necessity of "marking" this code with
a TODO but of course if someone comes in with something simpler, great
:).

Anyways, as I said, I'm not really keen in spinning a new version to
add this comment so I will defer the decision to Jonathan :)  

Thanks for the help!
- Nuno Sá

_______________________________________________
Linux-rockchip mailing list
Linux-rockchip@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-rockchip