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

From: Rob Herring
Date: Thu Jul 25 2019 - 18:43:08 EST

On Thu, Jul 25, 2019 at 12:23 AM H. Nikolaus Schaller <hns@xxxxxxxxxxxxx> wrote:
> Hi Rob,
> > Am 24.07.2019 um 21:42 schrieb Rob Herring <robh@xxxxxxxxxx>:
> >
> > On Mon, Jul 08, 2019 at 04:46:05PM +0200, H. Nikolaus Schaller wrote:
> >> commit 6953c57ab172 "gpio: of: Handle SPI chipselect legacy bindings"
> >>
> >> did introduce logic to centrally handle the legacy spi-cs-high property
> >> in combination with cs-gpios. This assumes that the polarity
> >> of the CS has to be inverted if spi-cs-high is missing, even
> >> and especially if non-legacy GPIO_ACTIVE_HIGH is specified.
> >>
> >> The DTS for the GTA04 was orginally introduced under the assumption
> >> that there is no need for spi-cs-high if the gpio is defined with
> >> proper polarity GPIO_ACTIVE_HIGH.
> >
> > Given that spi-cs-high is called legacy, that would imply that DT's
> > should not have to use spi-cs-high.
> Yes.
> >
> >> This was not a problem until gpiolib changed the interpretation of
> >> GPIO_ACTIVE_HIGH and missing spi-cs-high.
> >
> > Then we should fix gpiolib...
> 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.