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

[Linus Walleij]

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