linux-kernel - Re: [PATCH 1/1] gpio: core: Decouple open drain/source flag with active low/high

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Message-ID: <20170719131640.GA2533@localhost>
Date:   Wed, 19 Jul 2017 15:25:33 +0200
From:   Johan Hovold <johan@...nel.org>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Laxman Dewangan <ldewangan@...dia.com>,
        Alexandre Courbot <gnurou@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Frank Rowand <frowand.list@...il.com>,
        "linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>
Subject: Re: [PATCH 1/1] gpio: core: Decouple open drain/source flag with
 active low/high

On Fri, Apr 07, 2017 at 12:25:49PM +0200, Linus Walleij wrote:
> On Thu, Apr 6, 2017 at 3:35 PM, Laxman Dewangan <ldewangan@...dia.com> wrote:
> 
> > Currently, the GPIO interface is said to Open Drain if it is Single
> > Ended and active LOW. Similarly, it is said as Open Source if it is
> > Single Ended and active HIGH.
> >
> > The active HIGH/LOW is used in the interface for setting the pin
> > state to HIGH or LOW when enabling/disabling the interface.
> >
> > In Open Drain interface, pin is set to HIGH by putting pin in
> > high impedance and LOW by driving to the LOW.
> >
> > In Open Source interface, pin is set to HIGH by driving pin to
> > HIGH and set to LOW by putting pin in high impedance.
> >
> > With above, the Open Drain/Source is unrelated to the active LOW/HIGH
> > in interface. There is interface where the enable/disable of interface
> > is ether active LOW or HIGH but it is Open Drain type.
> >
> > Hence decouple the Open Drain with Single Ended + Active LOW and
> > Open Source with Single Ended + Active HIGH.
> >
> > Adding different flag for the Open Drain/Open Source which is valid
> > only when Single ended flag is enabled.
> >
> > Signed-off-by: Laxman Dewangan <ldewangan@...dia.com>
> 
> Patch applied.
> 
> Good that you found this and fixed it before someone git hurt.

Well, while decoupling single-endedness from polarity was the right
thing to do, this change did actually break the DT binary interface.

If you have an old compiled dtb whose source used GPIO_OPEN_DRAIN, you
now instead get *open-source* behaviour on 4.12:

	GPIO_OPEN_DRAIN = GPIO_SINGLE_ENDED | GPIO_ACTIVE_LOW
	
		=> active-low, but *open source*

while if you recompile that source against 4.12 you do get the expected
open-drain behaviour, but now with inverted polarity:

	GPIO_OPEN_DRAIN = GPIO_SINGLE_ENDED | GPIO_LINE_OPEN_DRAIN
	
		=> open drain, but *active high*

requiring the device tree to be updated by specifying

	(GPIO_OPEN_DRAIN | GPIO_ACTIVE_LOW)

I guess the latter is fine, even if it is likely to amount to a fair bit
of debugging world wide.

Perhaps all this can still be avoided by adding further flags and
deprecating others before people start migrating to 4.12 (after all,
GPIO_OPEN_DRAIN has been around since 4.4 even if there are no in-kernel
users).

Or we accept the binary interface breakage -- it probably is pretty rare
that people update the kernel without updating the dtb. I can just
update the dts on the system that broke for me, and hopefully anyone
debugging this issue while updating to 4.12 will find this mail quickly.

Johan