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

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Message-ID: <CAL_JsqJbg-VsZjg6QuN5SUvDXGR1DnShmhLvp3b_bipVn1q74A@mail.gmail.com>
Date:	Wed, 16 Dec 2015 10:29:37 -0600
From:	Rob Herring <robh+dt@...nel.org>
To:	"Andrew F. Davis" <afd@...com>
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 Mon, Dec 14, 2015 at 5:19 PM, Andrew F. Davis <afd@...com> wrote:
> 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:

[...]

>>>>> + - 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.

If there are a couple then I think it is okay. If there are 10s then
maybe not. Perhaps logic-pisosr or discrete-pisosr?

>>>>> +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.

One would hope you would just connect the CS signal directly to that
input for 1-bit... I guess you could be out of GPIOs and only have a
free output only CS signal.

Rob
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