linux-kernel - Re: [PATCH 2/2] DTS: ARM: gta04: introduce legacy spi-cs-high to make display work again

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Date:   Mon, 5 Aug 2019 12:29:19 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Rob Herring <robh@...nel.org>
Cc:     "H. Nikolaus Schaller" <hns@...delico.com>,
        Mark Brown <broonie@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Benoît Cousson <bcousson@...libre.com>,
        Tony Lindgren <tony@...mide.com>,
        Discussions about the Letux Kernel 
        <letux-kernel@...nphoenux.org>,
        linux-spi <linux-spi@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-OMAP <linux-omap@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH 2/2] DTS: ARM: gta04: introduce legacy spi-cs-high to make
 display work again

On Fri, Jul 26, 2019 at 12:43 AM Rob Herring <robh@...nel.org> wrote:
> On Thu, Jul 25, 2019 at 12:23 AM H. Nikolaus Schaller <hns@...delico.com> wrote:

> > I tried to convince Linus that this is the right way but he convinced
> > me that a fix that handles all cases does not exist.
> >
> > There seem to be embedded devices with older DTB (potentially in ROM)
> > which provide a plain 0 value for a gpios definition. And either with
> > or without spi-cs-high.
> >
> > Since "0" is the same as "GPIO_ACTIVE_HIGH", the absence of
> > spi-cs-high was and must be interpreted as active low for these
> > devices. This leads to the inversion logic in code.
> >
> > AFAIR it boils down to the question if gpiolib and the bindings
> > should still support such legacy devices with out-of tree DTB,
> > but force in-tree DTS to add the legacy spi-cs-high property.
> >
> > Or if we should fix the 2 or 3 cases of in-tree legacy cases
> > and potentially break out-of tree DTBs.
>
> If it is small number of platforms, then the kernel could handle those
> cases explicitly as needed.
>
> > IMHO it is more general to keep the out-of-tree DTBs working
> > and "fix" what we can control (in-tree DTS).
>
> If we do this, then we need to not call spi-cs-high legacy because
> we're stuck with it forever.

I agree. The background on it is here:
https://lkml.org/lkml/2019/4/2/4

Not using the negatively defined (i.e. if it is no there, the line is
by default active low) spi-cs-high would break
PowerPC, who were AFAICT using this to ship devices.

Yours,
Linus Walleij