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

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Date:   Wed, 24 Jul 2019 13:42:59 -0600
From:   Rob Herring <robh@...nel.org>
To:     "H. Nikolaus Schaller" <hns@...delico.com>
Cc:     Mark Brown <broonie@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Benoît Cousson <bcousson@...libre.com>,
        Tony Lindgren <tony@...mide.com>, letux-kernel@...nphoenux.org,
        linux-spi@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-omap@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH 2/2] DTS: ARM: gta04: introduce legacy spi-cs-high to
 make display work again

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.

> This was not a problem until gpiolib changed the interpretation of
> GPIO_ACTIVE_HIGH and missing spi-cs-high.

Then we should fix gpiolib...

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