Re: [PATCH 1/2] dt-bindings: GPIO: Add generic serializer binding

[Andrew F. Davis]

On 12/14/2015 04:36 PM, Rob Herring wrote:
> On Mon, Dec 14, 2015 at 10:41 AM, Andrew F. Davis <afd@xxxxxx> wrote:
> > On 12/11/2015 03:48 PM, Linus Walleij wrote:
> > > On Fri, Dec 11, 2015 at 8:46 PM, Andrew F. Davis <afd@xxxxxx> wrote:
> > >
> > > > Add binding for generic parallel-in/serial-out shift register devices
> > > > used as GPIO.
> > > >
> > > > Signed-off-by: Andrew F. Davis <afd@xxxxxx>
> > >
> > >
> > > > +Generic Parallel-in/Serial-out Shift Register GPIO Driver
> > > > +
> > > > +This binding describes generic parallel-in/serial-out shift register
> > > > +devices that can be used for GPI (General Purpose Input). This includes
> > > > +SN74165 serial-out shift registers and the SN65HVS88x series of
> > > > +industrial serializers.
> > > > +
> > > > +Required properties:
> > > > + - compatible          : Should be "pisosr-gpio".
> > >
> > >
> > > I think it should also define compatible strings on the "vendor,device"
> > > format apart from the generic compatible. Sooner or later we may need
> > > to differentiate them and then that comes in handy.
> >
> > Would it be better to wait until/if this issue arises? This driver
> > targets the generic features, as these parts are very generic and
> > have been produced by many companies since the 70s I'm not sure
> > if privileging any of them makes much sense.
> >
> > What I'm worried about looks to have happened with the gpio-74x164
> > driver, this is kind of the companion device to mine (74164 / 74165)
> > and should work with any 74164 compatible shift register (possibly 100s
> > of versions of them), but the compatible string that was added is
> > "fairchild,74hc595", a relatively new device by a single manufacturer.
> > The problem this has is then that boards will use this compatible string
> > even if the parts are not actually the Fairchild version, just to get
> > the match, when they should be using a generic string.
>
> I agree the generic version is fine (or find who made the first part
> ;)). What "pisosr" is is not very obvious though. Having 74165 in the
> compatible would make it somewhat more obvious it is a standard logic
> part.

A quick search shows shift-registers being made from vacuum tubes for
the Colossus! Those might work with this driver if you could match the
voltage to an SPI bus... :)

I agree about the name not being very good, but I'm not sure about
74165 ether as it is also just a single part number. The idea was to
have a non-part number compatible string for any shift-register you
can hook to the SPI line. That way when we have boards with a sn65x882
or something we wont have to call it a 74165. But I guess that's why
it's a "compatible:" string, and not "is-a:" string.

> > > > +Optional properties:
> > > > + - ngpios              : Number of GPIO lines, default is 8.
> > >
> > >
> > > If you didn't do "pisosr-gpio" but instead "foo,sn74165", maybe you
> > > don't need to have this in the device tree but instead it can be
> > > determined from the compatible string?
> > >
> > > In that case do that.
> >
> > These devices can be daisy-chained together, so three 8bit registers
> > look exactly like one 24bit register. The only way to know this is
> > from the physical wiring of the board, not from the part number.
>
> Then you should say it must be multiple of 8 (or are there other lengths?).

Some are 4bit, you can even just hook a single flip-flop to the SPI bus for
a single bit (or multiples) of input.

Andrew

> Rob
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