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

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Date:	Mon, 14 Dec 2015 17:19:55 -0600
From:	"Andrew F. Davis" <afd@...com>
To:	Rob Herring <robh+dt@...nel.org>
CC:	Linus Walleij <linus.walleij@...aro.org>,
	"linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
	Mark Brown <broonie@...nel.org>,
	Alexandre Courbot <gnurou@...il.com>,
	Pawel Moll <pawel.moll@....com>,
	Mark Rutland <mark.rutland@....com>,
	Ian Campbell <ijc+devicetree@...lion.org.uk>,
	Kumar Gala <galak@...eaurora.org>,
	"linux-gpio@...r.kernel.org" <linux-gpio@...r.kernel.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] dt-bindings: GPIO: Add generic serializer binding

On 12/14/2015 04:36 PM, Rob Herring wrote:
> On Mon, Dec 14, 2015 at 10:41 AM, Andrew F. Davis <afd@...com> wrote:
>> On 12/11/2015 03:48 PM, Linus Walleij wrote:
>>>
>>> On Fri, Dec 11, 2015 at 8:46 PM, Andrew F. Davis <afd@...com> wrote:
>>>
>>>> Add binding for generic parallel-in/serial-out shift register devices
>>>> used as GPIO.
>>>>
>>>> Signed-off-by: Andrew F. Davis <afd@...com>
>>>
>>>
>>>> +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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