linux-kernel - Re: [PATCH 1/3] iio: light: Add driver for ap3216c

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Message-ID: <CAGngYiV_6MzKOSomAWs1znp8ThPRAL-e2CVZfn+ZHZOq8DiD0A@mail.gmail.com>
Date:   Mon, 11 Feb 2019 17:30:18 -0500
From:   Sven Van Asbroeck <thesven73@...il.com>
To:     Jonathan Cameron <jic23@...nel.org>
Cc:     Robert Eshleman <bobbyeshleman@...il.com>,
        Hartmut Knaack <knaack.h@....de>,
        Lars-Peter Clausen <lars@...afoo.de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-iio@...r.kernel.org
Subject: Re: [PATCH 1/3] iio: light: Add driver for ap3216c

On Mon, Feb 11, 2019 at 4:27 PM Jonathan Cameron <jic23@...nel.org> wrote:
>
> Agreed.  Or potentially just use regmap_bulk_read and rely on
> the regmap internal locking to do it for you.

Neat solution. But it may only work correctly iff regmap_bulk_read()
reads the low
address first. I'm not sure if this function has that guarantee. If
somebody changes
the read order, the driver will break. But I think I'm being overly
paranoid here :)

> So yes, it's more than possible that userspace won't get the same number
> of events as samples taken over the limit, but I don't know why we care.
> We can about missing a threshold being passed entirely, not about knowing
> how many samples we were above it for.

I suspect that we run a small risk of losing an event, like so:

PS (12.5 ms)
--> interrupt -> iio event
ALS (100 ms)
--> interrupt -> iio event
PS (12.5 ms)
--> interrupt ========= no iio event generated
ALS (100 ms)
--> interrupt -> iio event

To see why, imagine that the scheduler decides to move away from the
threaded interrupt
handler right before ap3216c_clear_int(). Say 20ms, which I know is a
loooong time,
but bear with me, the point is that it _could_ happen as we're not a RTOS.

static irqreturn_t ap3216c_event_handler(int irq, void *p)
{
        /* imagine ALS interrupt came in, INT_STATUS is 0b01 */
        regmap_read(data->regmap, AP3216C_INT_STATUS, &status);
        if (status & mask1) iio_push_event(PROX);
        if (status & mask2) iio_push_event(LIGHT);

        /* imagine schedule happens here */
        msleep(20);
        /* while we were not running, PS interrupt came in
           INT_STATUS is now 0b11
           yet no new interrupt is generated, as we are ONESHOT
        */
        ap3216c_clear_int(data);
        /* clears both bits, interrupt line goes low.
            knowledge that the PS interrupt came in is now lost */
}

Not sure if that's acceptable driver behaviour. In real life it
probably wouldn't matter much,
except for occasional added latency maybe ?