Date:   Tue, 23 Mar 2021 11:06:39 +0100
From:   Thierry Reding <thierry.reding@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Dipen Patel <dipenp@...dia.com>,
        Kent Gibson <warthog618@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jon Hunter <jonathanh@...dia.com>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        linux-tegra <linux-tegra@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>,
        Richard Cochran <richardcochran@...il.com>,
        Marc Zyngier <maz@...nel.org>
Subject: Re: GTE - The hardware timestamping engine

On Tue, Mar 23, 2021 at 10:08:00AM +0100, Linus Walleij wrote:
> On Mon, Mar 22, 2021 at 9:17 PM Dipen Patel <dipenp@...dia.com> wrote:
> 
> > My follow-up concerns on both Linus's and Kent's feedback:
> >
> > 1.  Please correct me if I am wrong, lineevent in the gpiolib* is only
> >     serves the userspace clients.
> > 1.a What about kernel drivers wanting to use this feature for monitoring its
> >     GPIO lines, see gyroscope example somewhere below. In that regards,
> >     lineevent implementation is not sufficient.
> > 1.b Are you also implying to extend lineevent implementation to kernel
> >     drivers?
> 
> I was talking about lineevent because you mentioned things like
> motors and robotics, and those things are traditionally not run in
> kernelspace because they are not generic hardware that fit in the
> kernel subsystems.
> 
> Normally industrial automatic control tasks are run in a userspace
> thread with some realtime priority.
> 
> As Kent says, in-kernel events are exclusively using IRQ as
> mechanism, and should be modeled as IRQs. Then the question
> is how you join the timestamp with the IRQ. GPIO chips are
> just some kind of irqchip in this regard, we reuse the irqchip
> infrastructure in the kernel for all GPIO drivers that generate
> "events" in response to state transitions on digital lines.

One potential problem I see with this is that Kent's proposal, if I
understand correctly, would supplant the original IRQ of a device with
the GTE IRQ for the corresponding event. I'm not sure that's desirable
because that would require modifying the device tree and would no longer
accurately represent the hardware. Timestamping also sounds like
something that drivers would want to opt into, and requiring people to
update the device tree to achieve this just doesn't seem reasonable.

This proposal would also only work if there's a 1:1 correspondence
between hardware IRQ and GTE IRQ. However, as Dipen mentioned, the GTE
events can be configured with a threshold, so a GTE IRQ might only
trigger every, say, 5th hardware IRQ. I'm not sure if those are common
use-cases, though.

Obviously if we don't integrate this with IRQs directly, it becomes a
bit more difficult to relate the captured timestamps to the events
across subsystem boundaries. I'm not sure how this would be solved
properly. If the events are sufficiently rare, and it's certain that
none will be missed, then it should be possible to just pull a timestamp
from the timestamp FIFO for each event.

All of that said, I wonder if perhaps hierarchical IRQ domains can
somehow be used for this. We did something similar on Tegra not too long
ago for wake events, which are basically IRQs exposed by a parent IRQ
chip that allows waking up from system sleep. There are some
similarities between that and GTE in that the wake events also map to a
subset of GPIOs and IRQs and provide additional functionalities on top.

I managed to mess up the implementation and Marc stepped in to clean
things up, so Cc'ing him since he's clearly more familiar with the topic
than I am.

Thierry

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