linux-kernel - Re: [PATCH 1/2] gpio: gpio-74x164: add support for CDx4HC4094

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Date:   Tue, 26 Jul 2022 09:30:38 +0200
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     Marcus Folkesson <marcus.folkesson@...il.com>,
        Bartosz Golaszewski <brgl@...ev.pl>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Maxime Ripard <mripard@...nel.org>,
        "open list:GPIO SUBSYSTEM" <linux-gpio@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] gpio: gpio-74x164: add support for CDx4HC4094

On Tue, Jul 26, 2022 at 9:23 AM Linus Walleij <linus.walleij@...aro.org> wrote:
> On Tue, Jul 26, 2022 at 8:55 AM Andy Shevchenko
> <andy.shevchenko@...il.com> wrote:
> > On Tue, Jul 26, 2022 at 12:22 AM Linus Walleij <linus.walleij@...aro.org> wrote:
>
> > > Well it will work, because all values come out, however *during*
> > > the transfer, not *after* the transfer as intended with the
> > > STR signal.
> >
> > No. I think you missed one little but crucial detail. The strobe is
> > edge, not level, while you consider CS as a level signal (only).
>
> Aha! Then it works :)
>
> > See Figure 7 and Table 9 in the datasheet you linked to. When STR is
> > low, the data is not changed. Maybe it's not clear in this datasheet,
> > I found somewhere
> > in a more clear table.
> >
> > > At least how I see it ...?
> >
> > If what you are saying is true, the hardware is totally broken and I
> > wouldn't use it. I.o.w. if the strobe passes all data when it's at low
> > level and latches at low-to-high, it's broken by definition in my
> > point of view.
>
> I have seen strobes that are level-active in my life, but if you
> say this one isn't then all is fine!

It's rather my common sense, because we have a clock here to shift a
register. It's kinda obvious *to me* that when we have a latch, that
latch shouldn't pass data on a level. Yes, there are buffers with
level strobe, but it's different. So, as I said, the oscilloscope can
finish our discussion one way or the other, but I believe it will be
my way ;) I will very (badly) surprised if it becomes your way. It
will ruin my beliefs in humanity. Not less.

-- 
With Best Regards,
Andy Shevchenko