Re: [RFC PATCH 0/6] leds: Fix pca955x GPIO pin mappings

From: Andy Shevchenko <andy.shevchenko@gmail.com>
To: Andrew Jeffery <andrew@aj.id.au>
Cc: "linux-leds@vger.kernel.org" <linux-leds@vger.kernel.org>,
	"linux-gpio@vger.kernel.org" <linux-gpio@vger.kernel.org>,
	"Cédric Le Goater" <clg@kaod.org>,
	"Rob Herring" <robh+dt@kernel.org>,
	"Joel Stanley" <joel@jms.id.au>, "Pavel Machek" <pavel@ucw.cz>,
	"Linus Walleij" <linus.walleij@linaro.org>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>,
	"linux-aspeed@lists.ozlabs.org" <linux-aspeed@lists.ozlabs.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 0/6] leds: Fix pca955x GPIO pin mappings
Date: Thu, 29 Jul 2021 10:40:09 +0300	[thread overview]
Message-ID: <CAHp75Vdx9QA7dmSWK8GHxBBxP0uYjrz=Gm=75yqaWbBX6k3v=w@mail.gmail.com> (raw)
In-Reply-To: <6cc64039-f82a-4c1e-ad2c-16fad7aa3178@www.fastmail.com>

On Thu, Jul 29, 2021 at 3:39 AM Andrew Jeffery <andrew@aj.id.au> wrote:
> On Wed, 28 Jul 2021, at 18:43, Andy Shevchenko wrote:
> > On Wed, Jul 28, 2021 at 8:43 AM Andrew Jeffery <andrew@aj.id.au> wrote:
> > > On Fri, 23 Jul 2021, at 17:45, Andy Shevchenko wrote:
> > >
> > > > I was briefly looking into patches 1-4 and suddenly
> > > > realized that the fix can be similar as in PCA9685 (PWM), I.e. we
> > > > always have chips for the entire pin space and one may map them
> > > > accordingly, requested in one realm (LED) in the other (GPIO)
> > > > automatically is BUSY. Or I missed the point?
> > >
> > > No, you haven't missed the point. I will look at the PCA9685 driver.
> > >
> > > That said, my goal was to implement the behaviour intended by the
> > > existing binding (i.e. fix a bug).
> >
> > Okay, so it implies that this used to work at some point.
>
> I don't think this is true. It only "works" if the lines specified as
> GPIO in the devicetree are contiguous from line 0. That way the pin and
> GPIO number spaces align. I suspect that's all that's been tested up
> until this point.
>
> We now have a board with a PCA9552 where the first 8 pins are LED and
> the last 8 pins are GPIO, and if you specify this in the devicetree
> according to the binding you hit the failure to map between the two
> number spaces.
>
> > What has
> > changed from that point? Why can't we simply fix the culprit commit?
>
> As such nothing has changed, I think it's always been broken, just we
> haven't had hardware configurations that demonstrated the failure.
>
> >
> > > However, userspace would never have
> > > got the results it expected with the existing driver implementation, so
> > > I guess you could argue that no such (useful) userspace exists. Given
> > > that, we could adopt the strategy of always defining a gpiochip
> > > covering the whole pin space, and parts of the devicetree binding just
> > > become redundant.
> >
> > I'm lost now. GPIO has its own userspace ABI, how does it work right
> > now in application to this chip?
>
> As above, it "works" if the GPIOs specified in the devicetree are
> contiguous from line 0. It's broken if they're not.

So, "it never works" means there is no bug. Now, what we need is to
keep the same enumeration scheme, but if you wish to be used half/half
(or any other ratio), the driver should do like the above mentioned
PWM, i.e. register entire space and depending on the requestor either
proceed with a line or mark it as BUSY.

Ideally, looking into what the chip can do, this should be indeed
converted to some like pin control + PWM + LED + GPIO drivers. Then
the function in pin mux configuration can show what exactly is enabled
on the certain line(s).

-- 
With Best Regards,
Andy Shevchenko

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