From: Sven Van Asbroeck <thesven73@gmail.com>
To: Jonathan Cameron <jonathan.cameron@huawei.com>
Cc: Jonathan Cameron <jic23@kernel.org>,
Robert Eshleman <bobbyeshleman@gmail.com>,
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
Subject: Re: [PATCH 1/3] iio: light: Add driver for ap3216c
Date: Mon, 18 Feb 2019 14:35:51 -0500 [thread overview]
Message-ID: <CAGngYiVQLwFxNJcCCH3byL=_p=L_BjMHJ1kCbjR8XkOF2mN3Ew@mail.gmail.com> (raw)
In-Reply-To: <20190218151606.00000d10@huawei.com>
Hi Jonathan,
Thanks again for your clear and extensive feedback !
On Mon, Feb 18, 2019 at 10:16 AM Jonathan Cameron
<jonathan.cameron@huawei.com> wrote:
>
> I suspect that would break lots of devices if it happened, but
> fair enough that explicit might be good. One option would be
> to document clearly in regmap the requirement that bulk read is ordered.
>
Yes, it would be interesting to hear the regmap people's opinion on ordering.
In the mean time, we can make this explicit.
Re-reading the thread, I can also see that Peter Meerwald-Stadler was first
to spot this race condition.
> What we need to guarantee is:
>
> 1) If the sensor reads on an occasion where the threshold is passed, we do not miss the event
> The event is the threshold being passed, not the existence of the reading, or how many
> readings etc.
>
> 2) A data read will result in a value. There is no guarantee that it will match with the
> event. All manner of delays could result in new data having occurred before that read.
>
My feedback was based on two incorrect assumptions:
a. the interrupt fires whenever new PS/ALS values become available (wrong)
b. there are strict consistency guarantees between the THRESH event, and what
userspace will read out (also wrong)
Taking that into account, I am 100% in agreement with your other comments.
Thank you so much for the explanation!
There is one exception, though:
> > +static int ap3216c_write_event_config(struct iio_dev *indio_dev,
> > + const struct iio_chan_spec *chan,
> > + enum iio_event_type type,
> > + enum iio_event_direction dir, int state)
> > +{
> > + struct ap3216c_data *data = iio_priv(indio_dev);
> > +
> > + switch (chan->type) {
> > + case IIO_LIGHT:
> > + data->als_thresh_en = state;
> > + return 0;
> > +
> > + case IIO_PROXIMITY:
> > + data->prox_thresh_en = state;
> > + return 0;
> > +
> > + default:
> > + return -EINVAL;
> > + }
> > +static irqreturn_t ap3216c_event_handler(int irq, void *p)
> > +{
> > + if ((status & AP3216C_INT_STATUS_PS_MASK) && data->prox_thresh_en)
> > + iio_push_event(...);
> > +
> >
> > I think this may not work as intended. One thread (userspace) writes
> > a variable, another thread (threaded irq handler) checks it. but there
> > is no explicit or implicit memory barrier. So when userspace activates
> > thresholding, it may take a long time for the handler to 'see' it !
>
> Yes. But if userspace took a while to get around to writing this value,
> it would also take longer... It's not time critical exactly when you
> enable the event. One can create cases where someone might
> care, but they are pretty obscure.
>
Are you sure? I suspect that it's perfectly possible for the threaded irq
handler not to 'see' the store to (als|prox)_thresh_en for a _very_ long time.
AFAIK only a memory barrier will guarantee that the handler 'sees' the store
right away. A lock will do - it issues an implicit memory barrier.
Most drivers use a lock to guarantee visibility. There are a few drivers that
resort to explicit barriers to make a flag visible from one thread to another.
E.g. search for mb() or wmb() in:
drivers/input/keyboard/matrix_keypad.c
drivers/input/misc/cm109.c
drivers/input/misc/yealink.c
next prev parent reply other threads:[~2019-02-18 19:36 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-10 20:36 [PATCH 1/3] iio: light: Add driver for ap3216c Robert Eshleman
2019-02-10 20:39 ` [PATCH 3/3] dt-bindings: iio: light: Add ap3216c Robert Eshleman
2019-02-11 14:58 ` [PATCH 1/3] iio: light: Add driver for ap3216c Peter Meerwald-Stadler
2019-02-13 16:33 ` Robert Eshleman
2019-02-11 19:09 ` Sven Van Asbroeck
2019-02-11 21:27 ` Jonathan Cameron
2019-02-11 22:30 ` Sven Van Asbroeck
2019-02-12 20:47 ` Jonathan Cameron
2019-02-13 4:40 ` Sven Van Asbroeck
2019-02-13 4:56 ` Sven Van Asbroeck
2019-02-18 15:16 ` Jonathan Cameron
2019-02-18 19:35 ` Sven Van Asbroeck [this message]
2019-02-20 12:32 ` Jonathan Cameron
2019-02-20 15:09 ` Sven Van Asbroeck
2019-02-13 16:18 ` Robert Eshleman
2019-02-11 19:29 ` Sven Van Asbroeck
2019-02-13 2:17 ` Robert Eshleman
2019-02-13 3:25 ` Sven Van Asbroeck
2019-02-18 15:22 ` Jonathan Cameron
2019-02-18 17:13 ` Sven Van Asbroeck
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