linux-kernel - Re: [PATCH 0/2] gpio: 74x164: use a compatible fallback and don't extend the driver

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Message-ID: <CAMuHMdWhEZ0No8mXdymE8O8+rMCkD2SXAifZwReb1BbfYASOeQ@mail.gmail.com>
Date: Fri, 10 Jan 2025 15:09:49 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Bartosz Golaszewski <brgl@...ev.pl>
Cc: Csókás Bence <csokas.bence@...lan.hu>, 
	Linus Walleij <linus.walleij@...aro.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, 
	Maxime Ripard <mripard@...nel.org>, J . Neuschäfer <j.ne@...teo.net>, 
	linux-gpio@...r.kernel.org, devicetree@...r.kernel.org, 
	linux-kernel@...r.kernel.org, 
	Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
Subject: Re: [PATCH 0/2] gpio: 74x164: use a compatible fallback and don't
 extend the driver

Hi Bartosz,

On Fri, Jan 10, 2025 at 2:38 PM Bartosz Golaszewski <brgl@...ev.pl> wrote:
> On Fri, Jan 10, 2025 at 2:32 PM Csókás Bence <csokas.bence@...lan.hu> wrote:
> > On 2025. 01. 10. 14:00, Bartosz Golaszewski wrote:
> > > From: Bartosz Golaszewski <bartosz.golaszewski@...aro.org>
> > >
> > > There were other suggested solutions (for instance: just use the
> > > existing compatible for the On Semi variant) but I figured the most
> > > common approach is to use a fallback value for 100% compatible models
> > > and this is what Rob suggested as well.
> > >
> > > This reverts the driver change and makes the "onnn,74hc595a" compatible
> > > use "fairchild,74hc595" as fallback.
> >
> > Is there any reason to introduce a new compatible name at all? Does some
> > pre-existing, widely-used DT blob use it in the wild already? If not,
> > then I don't think it's necessary; for any new boards, their DT's
> > authors should just use the pre-existing names.
>
> I don't have a strong opinion on this and will defer to DT maintainers
> but a similar case I'm familiar with is the at24 EEPROM driver where
> we've got lots of 1:1 compatible chips and we tend to add new
> compatibles to DT bindings (with fallbacks to associated atmel models)
> just for the sake of correct HW description in DTS.

At24 EEPROMs differ from '595 shift registers in that they provide an
API with multiple commands, and some commands or parameter bits may
differ among different implementations (but usually these differences
are called quirks).

All '595 (I'm deliberately writing it like that) shift registers
should be 100% compatible, modulo some electrical specifications
(voltage levels, maximum speed, power consumption, ...).

Interestingly, the driver is called gpio-74x164.c, while no '164
compatible value is present. Most important difference is that the
'164 lacks the output latch, which is used as chip-select with SPI[1].

> > I'm especially against introducing a new, vendor-specific (On Semi, in
> > this case) name; if we really want to introduce a new compatible, at
> > least make it as generic as possible, i.e. `generic,74x595`, or even
> > `generic,spi-shift-register-output`.
>
> If anything, that would have to be the fallback that the driver knows.
> The first string in the compatible property has to have an actual
> vendor (I think, I'll let DT maintainers correct me).

For the inverse operation (parallel in, serial out), there's just
"pisosr-gpio".

[1] https://www.ovaga.com/blog/transistor/74hc164-vs-74hc595#simple-list-item-2

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds