Re: [PATCH RFC v2 5/7] arm64: dts: imx: fix touchscreen reset GPIO polarity

[Robin Murphy <robin.murphy@arm.com>]

On 2022-11-22 09:58, Quentin Schulz wrote:
> Hi David,
> 
> Thanks Fabio for the Cc.
> 
> On 11/22/22 08:18, David Jander wrote:
>> On Mon, 21 Nov 2022 15:18:32 -0300
>> Fabio Estevam <festevam@gmail.com> wrote:
>>
>>> [Adding Angus and David]
>>
>> Thanks. This was apparently necessary ;-)
>>
>>> On Mon, Nov 21, 2022 at 3:12 PM Quentin Schulz 
>>> <foss+kernel@0leil.net> wrote:
>>>>
>>>> From: Quentin Schulz <quentin.schulz@theobroma-systems.com>
>>>>
>>>> The reset line is active low for the Goodix touchscreen controller so
>>>> let's fix the polarity in the Device Tree node.
>>>>
>>>> Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
>>>> ---
>>>>   arch/arm64/boot/dts/freescale/imx8mm-prt8mm.dts         | 2 +-
>>>>   arch/arm64/boot/dts/freescale/imx8mq-librem5-devkit.dts | 2 +-
>>>>   2 files changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/boot/dts/freescale/imx8mm-prt8mm.dts 
>>>> b/arch/arm64/boot/dts/freescale/imx8mm-prt8mm.dts
>>>> index 9fbbbb556c0b3..df7e5ae9698e1 100644
>>>> --- a/arch/arm64/boot/dts/freescale/imx8mm-prt8mm.dts
>>>> +++ b/arch/arm64/boot/dts/freescale/imx8mm-prt8mm.dts
>>>> @@ -107,7 +107,7 @@ touchscreeen@5d {
>>>>                  interrupt-parent = <&gpio1>;
>>>>                  interrupts = <8 IRQ_TYPE_NONE>;
>>>>                  irq-gpios = <&gpio1 8 GPIO_ACTIVE_HIGH>;
>>>> -               reset-gpios = <&gpio1 9 GPIO_ACTIVE_HIGH>;
>>>> +               reset-gpios = <&gpio1 9 GPIO_ACTIVE_LOW>;
>>
>> NACK!
>>
>> The PRT8MM has an inverter in the reset line. The reason for that is 
>> that the
>> reset line needs to be inactive when the driving side is unpowered.
>> The DT was correct, this change will break it.
>>
> 
> The DT was correct. The implementation in the driver is changed (the 
> polarity is swapped) in this patch series, therefore the DT isn't 
> correct anymore, hence this patch.

I'm not sure it's quite that simple... FWIW I'm using an add-on LCD 
module with a GT9271[1] (and I won't be the only one - Raspberry Pi and 
other SBC users using DT overlays or custom-built DTBs are a whole other 
can of worms here), where GPIO_ACTIVE_LOW is correctly specified per the 
schematics, thus "wrong" for the current driver behaviour, yet it *is* 
working OK as-is. I guess that's because /RSTB ends up driven low for 
long enough between the current "deassertion" by 
gpiod_direction_output(1) and gpiod_direction_input() allowing the 
external pull-up to take it high again.

Robin.

[1] 
https://www.friendlyelec.com/index.php?route=product/product&path=81&product_id=230

> 
> See 
> https://lore.kernel.org/linux-input/20221103-upstream-goodix-reset-v2-0-2c38fb03a300@theobroma-systems.com/ for the whole patch series.
> 
> This DT patch alone is obviously incorrect, but the context around it 
> matters. I could/should have made it all into one big patch, the 
> question is then how this big tree-crossing patch would be merged into 
> Linux (if there's consensus). We're not there yet.
> 
> For some additional background on the discussion that was had in the v1:
> https://lore.kernel.org/all/267de96a-0129-a97d-9bf6-e1001b422a1a@theobroma-systems.com/
> I messed up the Cc list in the v1, apologies for the missing context in 
> the archived mails, I think one should be able to understand the 
> important bits by reading the answers in-mail. There, Dmitry, Hans and I 
> discussed the meaning of the active level of GPIOs/reset lines and I 
> expressed the reasons for such a change (which are also listed in the 
> cover letter of this patch series).
> 
> As stated in v1 cover letter, no implementation will satisfy every one. 
> We either make the DT binding implementation specific (which is what it 
> shouldn't be), or we swap the polarity in the Linux implementation and 
> thus the DT but then break DT backward compatibility.
> 
> Cheers,
> Quentin
> 
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