Hi Rob,

Am 24.07.2019 um 21:42 schrieb Rob Herring [off-list ref]:

On Mon, Jul 08, 2019 at 04:46:05PM +0200, H. Nikolaus Schaller wrote:

quoted

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.

quoted

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.

IMHO it is more general to keep the out-of-tree DTBs working
and "fix" what we can control (in-tree DTS).

quoted

The effect is that the missing spi-cs-high is now interpreted as CS being
low (despite GPIO_ACTIVE_HIGH) which turns off the SPI interface when the
panel is to be programmed by the panel driver.

Therefore, we have to add the redundant and legacy spi-cs-high property
to properly pass through the legacy handler.

Since this is nowhere documented in the bindings, we add some words of
WARNING.

Cc: stable@vger.kernel.org
Signed-off-by: H. Nikolaus Schaller <redacted>
---
Documentation/devicetree/bindings/spi/spi-bus.txt | 6 ++++++
arch/arm/boot/dts/omap3-gta04.dtsi                | 1 +
2 files changed, 7 insertions(+)

BR,
Nikolaus