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From:   Sven Van Asbroeck <thesven73@gmail.com>
Date:   Tue, 12 Feb 2019 22:25:39 -0500
Message-ID: <CAGngYiUvEpSgESzFGJOtYjLi+_xmZ18GwiSTxaaMjEmjk2pu4Q@mail.gmail.com>
Subject: Re: [PATCH 1/3] iio: light: Add driver for ap3216c
To:     Robert Eshleman <bobbyeshleman@gmail.com>
Cc:     Jonathan Cameron <jic23@kernel.org>,
        Hartmut Knaack <knaack.h@gmx.de>,
        Lars-Peter Clausen <lars@metafoo.de>,
        Peter Meerwald-Stadler <pmeerw@pmeerw.net>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-iio@vger.kernel.org
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Hi Bobby,

On Tue, Feb 12, 2019 at 9:17 PM Robert Eshleman <bobbyeshleman@gmail.com> wrote:
>
> First, thank you for the feedback.

First of all, thank _you_ for doing the hard work on this driver !
I very much respect what you've done here.

>
> I had initially went with a similar design, but there is
> the case in which the interrupt fires and then before the status
> register is read by the handler a user process reads the data and
> clears the interrupt.  When the handler continues execution it will
> read a zero status and return IRQ_NONE.  My understanding of how
> Linux handles IRQ_NONE is pretty poor, but I felt that this behavior
> is incorrect even if inconsequential.  This could be avoided by
> doing a status register read with every data read, and buffering
> that as well, but then we lose the benefit altogether by increasing
> I2C reads.
>
> In the approach you describe here, it seems like that would
> work if this driver wasn't supporting shared interrupts.  In the
> case that a user-space read happens to clear the interrupt before
> the handler reads the status register, I think we would end up
> falsely returning IRQ_NONE.
>
> Is my understanding of this correct?  It's very possible I'm
> misunderstanding IRQ_NONE and shared interrupts.
>

Yes, I can see how one can run into those issues.

I believe that this whole class of problems goes away if PS/ALS
are _exclusively_ read inside the interrupt, and cached.

Then, whenever a user process wants to read the data, the function
does not touch the h/w, but simply return the cached value.

But hang on, I will have more to say on this when replying to Jonathan's
feedback.