linux-kernel - Re: [PATCH] iio: adc: exynos_adc: Replace indio

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Message-ID: <CA+U=Dsoo6YABe5ODLp+eFNPGFDjk5ZeQEceGkqjxXcVEhLWubw@mail.gmail.com>
Date:   Thu, 27 Aug 2020 11:53:44 +0300
From:   Alexandru Ardelean <ardeleanalex@...il.com>
To:     Krzysztof Kozlowski <krzk@...nel.org>
Cc:     Alexandru Ardelean <alexandru.ardelean@...log.com>,
        linux-iio <linux-iio@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jonathan Cameron <jic23@...nel.org>, kgene@...nel.org,
        Sergiu Cuciurean <sergiu.cuciurean@...log.com>
Subject: Re: [PATCH] iio: adc: exynos_adc: Replace indio_dev->mlock with own
 device lock

On Thu, Aug 27, 2020 at 9:57 AM Krzysztof Kozlowski <krzk@...nel.org> wrote:
>
> On Wed, Aug 26, 2020 at 04:22:03PM +0300, Alexandru Ardelean wrote:
> > From: Sergiu Cuciurean <sergiu.cuciurean@...log.com>
> >
> > As part of the general cleanup of indio_dev->mlock, this change replaces
> > it with a local lock, to protect potential concurrent access to the
> > completion callback during a conversion.
>
> I don't know the bigger picture (and no links here for general cleanup)
> but I assume it is part of wider work and that mlock is unwanted. In
> such case:
>
> Reviewed-by: Krzysztof Kozlowski <krzk@...nel.org>
>
> If it is part of some bigger work, please put a link to lore.kernel.org
> under separators ---, so everyone can get the context.

Will keep that in mind.
I am not sure if there is a lore.kernel.org link that's easy to find
for a discussion on this topic, maybe I can describe it here and use
the link [from this later].

This was something that popped up during reviews we got from Jonathan
[or others], saying "please don't use indio_dev->mlock, that is an IIO
framework lock, and an IIO driver should not use it".
Reasons include [and some may be repeated a bit]:
- this could cause a deadlock if the IIO framework holds this lock and
an IIO driver also tries to get a hold of this lock
- similar to the previous point, this mlock is taken by
iio_device_claim_direct_mode() and released by
iio_device_release_direct_mode() ; which means that mlock aims to
become more of an IIO framework lock, than a general usage lock;
- this wasn't policed/reviewed intensely in the older driver [a few
years ago], but has become a point in recent reviews;
- if we want to develop/enhance the IIO framework, some elements like
this need to be taken care of, as more drivers get added and more
complexity gets added;
- there is an element of fairness [obviously], where someone writing a
new IIO driver, takes an older one as example, and gets hit on the
review; the person feels they did a good job in mimicking the old
driver; their feeling is correct; the IIO framework should provide
good references and/or cleanup existing drivers;
- same as the previous point, we don't want to keep telling people
writing new IIO drivers [and starting out with IIO] to "not use mlock
[because it was copied from an old driver]"; it's more/needless review
work


>
>
> Best regards,
> Krzysztof
>
> >
> > Signed-off-by: Sergiu Cuciurean <sergiu.cuciurean@...log.com>
> > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.com>
> > ---
> >  drivers/iio/adc/exynos_adc.c | 12 ++++++++----
> >  1 file changed, 8 insertions(+), 4 deletions(-)
> >