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Date:   Tue, 23 Feb 2021 18:45:16 +0100
From:   Oleksij Rempel <o.rempel@pengutronix.de>
To:     William Breathitt Gray <vilhelm.gray@gmail.com>
Cc:     Rob Herring <robh+dt@kernel.org>,
        Ahmad Fatoum <a.fatoum@pengutronix.de>,
        devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        David Jander <david@protonic.nl>,
        Robin van der Gracht <robin@protonic.nl>,
        linux-iio@vger.kernel.org,
        Linus Walleij <linus.walleij@linaro.org>,
        Jonathan Cameron <jic23@kernel.org>
Subject: Re: [PATCH v5 2/2] counter: add IRQ or GPIO based event counter
Message-ID: <20210223174516.wjlh7hnrd5qe5s6w@pengutronix.de>
References: <20210208135347.18494-1-o.rempel@pengutronix.de>
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Hello William,

Here is cooled down technical answer. Excuse me for over reacting.

On Tue, Feb 23, 2021 at 11:06:56AM +0100, Oleksij Rempel wrote:
> On Mon, Feb 22, 2021 at 10:43:00AM +0900, William Breathitt Gray wrote:
> > On Mon, Feb 15, 2021 at 10:17:37AM +0100, Oleksij Rempel wrote:
> > > > > +static irqreturn_t event_cnt_isr(int irq, void *dev_id)
> > > > > +{
> > > > > +	struct event_cnt_priv *priv = dev_id;
> > > > > +
> > > > > +	atomic_inc(&priv->count);
> > > > 
> > > > This is just used to count the number of interrupts right? I wonder if
> > > > we can do this smarter. For example, the kernel already keeps track of
> > > > number of interrupts that has occurred for any particular IRQ line on a
> > > > CPU (see the 'kstat_irqs' member of struct irq_desc, and the
> > > > show_interrupts() function in kernel/irq/proc.c). Would it make sense to
> > > > simply store the initial interrupt count on driver load or enablement,
> > > > and then return the difference during a count_read() callback?
> > > 
> > > This driver do not makes a lot of sense without your chardev patches. As
> > > soon as this patches go mainline, this driver will be able to send
> > > event with a timestamp and counter state to the user space.
> > > 
> > > With other words, we will need an irq handler anyway. In this case we
> > > can't save more RAM or CPU cycles by using system irq counters.
> > 
> > It's true that this driver will need an IRQ handler when the timestamp
> > functionality is added, but deriving the count value is different matter
> > regardless. There's already code in the kernel to retrieve the number of
> > interrupts, so it makes sense that we use that rather than rolling our
> > own -- at the very least to ensure the value we provide to users is
> > consistent with the ones already provided by other areas of the kernel.

The value provided by the driver is consistent only if it is not
overwritten by user. The driver provides an interface to reset/overwrite it.
At least after this step the value is not consistent.

> We are talking about one or two code lines. If we will take some
> duplication search engine, it will find that major part of the kernel
> is matching against it.
> 
> Newer the less, this driver provides a way to reset the counter. Why
> should we drop this functionality no advantage?
> 
> > To that end, I'd like to see your cnt_isr() function removed for this
> > patchset (you can bring it back once timestamp support is added).

It make no sense to request an interrupt without interrupt service
routine.

https://elixir.bootlin.com/linux/latest/source/kernel/irq/manage.c#L2072if
	if (!handler) {
		if (!thread_fn)
			return -EINVAL;

As you can see, requesting an irq need at least handler or thread_fn.

enable_irq: this will explode at least here:
https://elixir.bootlin.com/linux/latest/source/kernel/irq/manage.c#L778

If he have no IRQ handler and some how was able to enable it, at
some point this IRQ will be disabled by this code:
https://elixir.bootlin.com/linux/latest/source/kernel/irq/spurious.c#L410
	if (unlikely(desc->irqs_unhandled > 99900)) {
		/*
		 * The interrupt is stuck
		 */
		__report_bad_irq(desc, action_ret);
		/*
		 * Now kill the IRQ
		 */
		printk(KERN_EMERG "Disabling IRQ #%d\n", irq);
		desc->istate |= IRQS_SPURIOUS_DISABLED;
		desc->depth++;
		irq_disable(desc);

With current code, we can't request or enable IRQ without cnt_isr(). Not
that it is not possible, but it make no sense to me.

> Are you suggesting to enable IRQ without interrupt handler? May be i'm
> missing some thing.. I do not understand it.
> 
> > Reimplement your cnt_read/cnt_write() functions to instead use
> > kstat_irqs_usr() from <linux/kernel_stat.h> to get the current number of
> > interrupts the IRQ line and use it to derive your count value for this
> > driver.

irq descriptor has 3 counters:
- irq_count: this value can be reset any time by the kernel at least by
  the note_interrupt()
- irqs_unhandled: this value is increased in case of missing irq
  handler. Or if handler has returns IRQ_NONE.
- tot_count: this value should not be reset.

Non of this values is suitable for cnt_read() and cnt_write(). Only
tot_count would be suitable if cnt_write() is removed. I do not see it
as acceptable option.

For this driver, we still need extra counter, where only this driver is
responsible for writing to it.

> I can follow the counter read way, but overwriting system wide counter
> for local use is bad idea.
> 
> > > > > +static struct counter_signal event_cnt_signals[] = {
> > > > > +	{
> > > > > +		.id = 0,
> > > > > +		.name = "Channel 0 signal",
> > > > 
> > > > You should choose a more description name for this Signal;
> > > > "Channel 0 signal" isn't very useful information for the user. Is this
> > > > signal the respective GPIO line state?
> > > 
> > > Sounds plausible. How about "Channel 0, GPIO line state"?
> > 
> > Ideally, this would match the GPIO name (or I suppose the IRQ number if
> > not a GPIO line). So in your probe() function you can do something like
> > this I believe:
> > 
> > 	cnt_signals[0].name = priv->gpio->name;
> 

> to make this possible, i would need hack gpiolib framework and add
> name/label exporter. But after endless rounds of pingponging me for
> renaming the driver and removing interrupt handler, i feel like we are
> not having serious discussion for mainlining this driver.

Probably for good reason, struct gpio_desc was made local and is located
in the drivers/gpio/gpiolib.h. It feels like additional hack to include
it. I assume, it should be done properly so there is a function to
provide gpio name or label.

@Linus Walleij are there any good way to get the GPIO name? And which
name will be actually used? A label provided over devicetree?

If I see it correctly, it would need more work to make the kernel infrastructure
suitable for this suggestions. Some of them are only needed before
chardev support will go mainline and , in long term, not worth to
spend time on it.

Probably I do not understand you and i missing some thing?

Regards,
Oleksij
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