Re: [patch 2.6.31-rc7] gpiolib: allow poll() on value

From: Andrew Morton
Date: Tue Sep 01 2009 - 16:43:52 EST

On Thu, 27 Aug 2009 16:50:57 -0700
David Brownell <david-b@xxxxxxxxxxx> wrote:

> From: Daniel Gl__ckner <dg@xxxxxxxxx>
> Subject: [PATCH v3] gpiolib: allow poll(2) on gpio value
> Many gpio chips allow to generate interrupts when the value of a pin
> changes. This patch gives usermode application the opportunity to make
> use of this feature by calling poll(2) on the /sys/class/gpio/gpioN/value
> sysfs file. The edge to trigger can be set in the edge file in the same
> directory. Possible values are "none", "rising", "falling", and "both".
> Using level triggers is not possible with current sysfs since nothing
> changes the GPIO value (and the IRQ keeps triggering). Edge triggering
> will "just work". Note that if there was an event between read() and
> poll(), the poll() returns immediately.
> Also note that this version only supports true GPIO interrupts. Some
> later patch might be able to synthesize this behavior by timer-driven
> polling; some systems seem to need that.
> ...
> +static struct idr pdesc_idr;

There's no locking to protect this tree. If that isn't a bug then I'd
suggest that a comment be added here explaining why.

> +static irqreturn_t gpio_sysfs_irq(int irq, void *priv)
> +{
> + struct work_struct *work = priv;
> +
> + schedule_work(work);
> + return IRQ_HANDLED;
> +}

The only place where we explicitly cancel the pending work is in
gpio_setup_irq(). Is that sufficient? Is there any way in which the
work callback can occur after things have been

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