Re: [PATCH/RFC] ARM: dts: armadillo800eva: Split LCD mux and gpio

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Laurent,

On Wed, May 24, 2017 at 2:57 PM, Laurent Pinchart
<laurent.pinchart@ideasonboard.com> wrote:
> On Wednesday 24 May 2017 13:43:34 Geert Uytterhoeven wrote:
>> Configuration of the lcd0 pinmux group and GPIO hog for the external
>> GPIO mux are done using a single device node, causing the "output-high"
>> property to be applied to both.  This will fail for the pinmux group,
>> but doesn't cause any harm, as the failure is ignored silently.
>>
>> However, after "pinctrl: sh-pfc: propagate errors on group config", the
>> failure will become fatal, leading to a broken display:
>>
>>     sh-pfc e6050000.pin-controller: pin_config_group_set op failed for group
>> 102 sh-pfc e6050000.pin-controller: Error applying setting, reverse things
>> back sh-pfc e6050000.pin-controller: failed to select default state
>>
>> Split the configuration in two subnodes, and replace the GPIO hog by a
>> pull-up pin bias to fix this.
>>
>> Fixes: ffd2f9a5afb730b9 ("ARM: shmobile: armadillo800eva dts: Add pinctrl
>> and gpio-hog for lcdc0") Signed-off-by: Geert Uytterhoeven
>> <geert+renesas@glider.be>
>> ---
>> This is a hard dependency for "pinctrl: sh-pfc: propagate errors on
>> group config" (https://patchwork.kernel.org/patch/9186713/).
>>
>> Notes:
>>   1. Just keeping the GPIO hog doesn't work, as the sh-pfc driver
>>      requires each pinctrl (sub)node to have at least one "pins" or
>>      "groups" property,
>>   2. Keeping "output-high" doesn't work, as the sh-pfc driver does not
>>      implement PIN_CONFIG_OUTPUT.
>>
>> Should I fix 1 and/or 2 instead?
>> ---
>>  arch/arm/boot/dts/r8a7740-armadillo800eva.dts | 15 +++++++++------
>>  1 file changed, 9 insertions(+), 6 deletions(-)
>>
>> diff --git a/arch/arm/boot/dts/r8a7740-armadillo800eva.dts
>> b/arch/arm/boot/dts/r8a7740-armadillo800eva.dts index
>> 44b335b9f035436d..7feedc86f6915654 100644
>> --- a/arch/arm/boot/dts/r8a7740-armadillo800eva.dts
>> +++ b/arch/arm/boot/dts/r8a7740-armadillo800eva.dts
>> @@ -267,13 +267,16 @@
>>       };
>>
>>       lcd0_pins: lcd0 {
>> -             groups = "lcd0_data24_0", "lcd0_lclk_1", "lcd0_sync";
>> -             function = "lcd0";
>> +             mux {
>> +                     groups = "lcd0_data24_0", "lcd0_lclk_1", "lcd0_sync";
>> +                     function = "lcd0";
>> +             };
>>
>> -             /* DBGMD/LCDC0/FSIA MUX */
>> -             gpio-hog;
>> -             gpios = <176 0>;
>> -             output-high;
>> +             gpio {
>> +                     /* DBGMD/LCDC0/FSIA MUX */
>> +                     pins = "PORT176";
>> +                     bias-pull-up;
>
> The signal is pulled low through a 10kΩ resistor, so a pull-up is a bit weak.

According to the schematics, the resistor is not mounted.

> You'll end up with an intermediate voltage level that could result in meta-
> stability and/or latch-up. I think we need to really drive the output to 1.

Agreed. Better safe than sorry.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds