Date:	Thu, 21 Jun 2012 09:10:54 +0200
From:	David Jander <david.jander@...tonic.nl>
To:	Jean-François Dagenais 
	<jeff.dagenais@...il.com>
Cc:	Linus Walleij <linus.walleij@...aro.org>,
	Grant Likely <grant.likely@...retlab.ca>,
	open list <linux-kernel@...r.kernel.org>,
	torvalds@...ux-foundation.org
Subject: Re: gpio: pca953x: interrupt feature unreliable


Dear Jean-François,

On Wed, 20 Jun 2012 15:28:05 -0400
Jean-François Dagenais <jeff.dagenais@...il.com> wrote:

> (putting Linus in CC 'cause I hear he enjoys interaction with hardware. As do I,
> and this is a funny hard/soft timing issue, insignificant maybe in the large
> scale of the kernel, but an interesting puzzle. Sorry if doing so is out-of-line.

Oh dear. Well, if you really need to do that.... ;-)
I hope we are not boring the hell out of a lot of people with this silly
discussion. My excuses to the rest of the CC if we are...

> It seems a few answers missed the lists, the interesting bits are quoted, and
> should give a flavour of the discussion.)
> 
> On 2012-06-20, at 11:01 AM, David Jander wrote:
> > 
> > First of all: As a hardware designer, if you intended to connect this chip's
> > nINT output to a PCA953x input, I assume you know what you are doing, and not
> > just trust the features a particular driver advertises.
> 
> Of course, I found out about this while "proof of concept"ing chaining the
> AD714X INT to a PCA9555. The product this goes in is not in production yet.

Phew! Then what's the problem? You saved yourself right on time ;-)

> > Second: What I try to explain as a possible workaround for your case, is to
> > move the complete ISR callback functionality into a thread. But since I don't
> > know the details of your driver, I don't know if that is possible.
> I trie not to modify the kernel wherever I can. For the "purist" part of me,
> hacking a chip driver here to compensate the flaws of another there is not the
> best way to do it. I would prefer fixing a chip's shortcomings in it's own
> driver.

I agree, but I thought you had a major problem so I made a suggestion about how
you could work around it.... not to suggest how to write an acceptable driver.

> >>> It still is an enormous advantage to have this functionality, because polling
> >>> an I2C peripheral just to wait for input state can be a terrible waste of
> >>> CPU-time ;-)
> >> 
> >> I totally respect this. However, it may be OK for a student project to have a
> >> flaky interrupt controller, but for industrial or medical applications, the
> >> interrupt controller must be reliable.
> > 
> > We use it in industrial applications, and we have no problems at all ;-)
> 
> If you examine the ASCII art of the timing I showed in the original message, you
> will see that my particular timing is not the only problematic one... Can you
> tell me how your 953x recovers from the state established at step 5? i.e. when
> you clear the slave's INT through whatever mechanism, the 953x re-assert's it's
> INT signal.

Easy: Just release the button and push it again ;-)
(I mentioned our application was a gpio_keys keyboard in my previous e-mail.)
That's the kind of applications you can use the PCA953x as interrupt
controller for. My patches incorporated support for device-tree bindings and a
few fixes for both gpio-pca953x.c and gpio_keys.c, so it became possible to do
just this in a device-tree:

	i2c@...0 {
		[...]
		interrupts = <0x9 0x8>;
		[...]
		gpio0: gpio@74 {
			compatible = "nxp,pca9539";
			reg = <0x74>;
			interrupts = <0x11 0x8>;
			#gpio-cells = <2>;
			gpio-controller;
		}
	}
	[...]
	gpio_keys {
		compatible = "gpio-keys";
		#address-cells = <1>;
		#size-cells = <0>;
		autorepeat;
		button@20 {
			label = "GPIO Key ESC";
			linux,code = <1>;
			gpios = <&gpio0 0 1>;
		};
		button@21 {
			label = "GPIO Key UP";
			linux,code = <103>;
			gpios = <&gpio0 1 1>;
		};
		...

Get the idea? There is no other code involved, and it "just works"! I am
absolutely amazed by this marvel ;-)

> If I can attempt an answer: since the 953x has requested it's own ISR using
> level-low, when the 953x's ISR thread is done, since the INT is still low, it
> re-enters it's threaded ISR, and read it's input register again, which clears
> 953x's INT. (And this is key to the problem here, I hope you can see it, or I'm
> mistaken here, but the ISR thread of the 953x runs a SECOND TIME) On this second
> run, 953x's ISR examines the bits that changed since the last read, in this
> case, two things can happen: (assuming your slave is trigger_falling) 1. Things

I perfectly understand your reasoning. But it is not the case in our
application, since the user will not keep his finger on the key forever if the
key doesn't react ;-)
Also, in our case, pushing and releasing a button is such a slow process, the
the likelihood this will ever occur is negligible.

> are quiet, the last read had your slave chip's INT low (and client's the nested
> ISR was called (falling edge)), but this current read now says high. This is
> considered like a rising and doesn't trigger calling the nested ISR again. All
> is well then.  2. if for whatever reason (fast slave OR some delay scheduling
> the 953x's ISR thread a second time) the second ISR run comes after the client's
> chip re-asserting it's INT. This event makes the 953x's input the same as when
> the input register was read on the first ISR thread run, so by definition, this
> de-asserts the 953x's INT. When 953x finally reads the input register moments
> later, there is no difference between old_stat and cur_stat and you are
> essentially locked for this slave until you reset the whole thing.
> 
> Since the timings of the slave and the OS are factors here, it's quite hard to
> reproduce. Even with the best intention and diligence at design, a race is
> possible.

Yes, of course. But that is a shortcoming of the extremely simple design of
PCA953x gpio expanders. There is not much we can do about this in the driver.
Maybe we could place a warning in the documentation about the shortcomings of
these chips...?

> > Really, you must know what you are doing if you want to use an I2C I/O
> > expander as interrupt chain controller!
> 
> After a good read of the specs and code, I had the idea to use our existing 9555
> to separate the interrupts of two i2c slaves. I put this together on a bread
> board and put the idea to the test. So I am in agreement that you must know what
> you are doing when doing this. I had to know for sure before telling the
> hardware guy to put this on real hardware.

Never mind. I am a hardware designer myself and we all make such mistakes.
Just try not to blame someone or something else for it publicly ;-)

> >> For example, we are most likely going to
> >> replace the 9555 in our next hardware rev because of this problem.
> > 
> > Then most probably you started your design committing a mistake. You should
> > just assume your hardware design is flawed and not try to blame some driver
> > for it.
> 
> Again, bread board. Plus, I hope you see that since we will most likely not be
> using this combination of i2c devices, I no longer have any personal interest of
> my own in this, other than my wish to improve the kernel in general.

Feel free to improve the driver... but please do not remove features that work
for others.

> >> Remember though that I was proposing a fix instead of a complete removal (I
> >> admit that suggesting removing the feature was a way to attract attention to the
> >> problem ;) The only problem is that since we are most likely not keeping this
> >> part in our design, putting effort into a fix of the pca953x.c driver is much
> >> lower now in my priority list.
> > 
> > Ok, then what about just leaving it as it is now?
> 
> If you are comfortable with the scenario I demonstrated originally, plus the one
> I mention in this message, or you can explain to me (and others) why I am wrong,
> then by all means keep the driver as it is! I just thought, first the world should
> know about this, and second, get feedback before attempting modifications, if I
> ever do get there.

I hope my explanation was clear enough this time. As I said, we may put some
warnings in the documentation of the driver.

Best regards,

-- 
David Jander
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