Re: [PATCH v2 3/3] tegra: allow to expose UART on microSD slot

[Artur Kowalski <arturkow2000@gmail.com>]

W dniu 31.03.2025 o 07:51, Svyatoslav Ryhel pisze:
> нд, 30 бер. 2025 р. о 22:27 Artur Kowalski <arturkow2000@gmail.com> пише:
>> Tegra20 has mux allowing to repurpose microSD as UART on boards which
>> use sdmmc3 controller for microSD. Add TEGRA_UARTA_MICROSD to enable
>> this feature. It is currently supported on Transformer T20, support for
>> other boards may require Device Tree changes.
>>
>> Update Transformer T20 DTS to select correct pinmux and stdout-path
>> based on TEGRA_UARTA_MICROSD.
>>
>> Signed-off-by: Artur Kowalski <arturkow2000@gmail.com>
>> ---
>>
>> Changes in v2:
>> - added TEGRA_UARTA_MICROSD Kconfig option
>> - removed .dtsi for enabling UART over microSD
>> - modified transformer DT to use UARTA and correct pinmux when
>>    TEGRA_UARTA_MICROSD is enabled; it is enough to flip a single
>>    switch to enable/disable UART over microSD
>>
>>   arch/arm/dts/tegra20-asus-transformer.dtsi | 24 ++++++++++++++++++++++
>>   arch/arm/mach-tegra/tegra20/Kconfig        | 15 ++++++++++++++
>>   2 files changed, 39 insertions(+)
>>
>> diff --git a/arch/arm/dts/tegra20-asus-transformer.dtsi b/arch/arm/dts/tegra20-asus-transformer.dtsi
>> index 49efabbfd92..9ea323c81f3 100644
>> --- a/arch/arm/dts/tegra20-asus-transformer.dtsi
>> +++ b/arch/arm/dts/tegra20-asus-transformer.dtsi
>> @@ -5,7 +5,13 @@
>>
>>   / {
>>          chosen {
>> +#if CONFIG_IS_ENABLED(TEGRA_ENABLE_UARTA)
>> +               stdout-path = &uarta;
>> +#elif CONFIG_IS_ENABLED(TEGRA_ENABLE_UARTD)
>>                  stdout-path = &uartd;
>> +#else
>> +#error Unsupported UART
>> +#endif
>>          };
>>
>>          aliases {
>> @@ -195,10 +201,17 @@
>>                                  nvidia,function = "i2c1";
>>                          };
>>
>> +#if CONFIG_IS_ENABLED(TEGRA_UARTA_SDB_SDD)
>> +                       dbguart {
>> +                               nvidia,pins = "sdb", "sdd";
>> +                               nvidia,function = "uarta";
>> +                       };
>> +#else
>>                          sdb {
>>                                  nvidia,pins = "sdb", "sdc", "sdd", "slxc", "slxk";
>>                                  nvidia,function = "sdio3";
>>                          };
>> +#endif
>>
>>                          sdio1 {
>>                                  nvidia,pins = "sdio1";
>> @@ -376,7 +389,13 @@
>>                  };
>>          };
>>
>> +#if CONFIG_IS_ENABLED(TEGRA_ENABLE_UARTA)
>> +       uarta: serial@70006000 {
>> +#elif CONFIG_IS_ENABLED(TEGRA_ENABLE_UARTD)
>>          uartd: serial@70006300 {
>> +#else
>> +#error Unsupported UART
>> +#endif
>>                  status = "okay";
>>                  clock-frequency = <216000000>;
>>          };
>> @@ -434,7 +453,12 @@
>>          };
>>
>>          sdmmc3: sdhci@c8000400 {
>> +#if CONFIG_IS_ENABLED(TEGRA_UARTA_MICROSD)
>> +               /* Can't use both at the same time */
>> +               status = "disabled";
>> +#else
>>                  status = "okay";
>> +#endif
>>                  bus-width = <4>;
>>
>>                  cd-gpios = <&gpio TEGRA_GPIO(I, 5) GPIO_ACTIVE_LOW>;
>> diff --git a/arch/arm/mach-tegra/tegra20/Kconfig b/arch/arm/mach-tegra/tegra20/Kconfig
>> index 3349f1e2786..c2c51660216 100644
>> --- a/arch/arm/mach-tegra/tegra20/Kconfig
>> +++ b/arch/arm/mach-tegra/tegra20/Kconfig
>> @@ -77,6 +77,21 @@ endchoice
>>   config SYS_SOC
>>          default "tegra20"
>>
>> +config TEGRA_UARTA_MICROSD
>> +       bool "UARTA on MicroSD breakout board"
>> +       default n
>> +       depends on TEGRA_ENABLE_UARTA && TARGET_TRANSFORMER_T20
>> +       select TEGRA_UARTA_SDB_SDD
>> +       help
>> +         Repurpose the SD card slot for getting access to the UARTA serial
>> +         console. Primarily useful only for low level u-boot debugging on
>> +         tablets, where normal UARTD is difficult to access and requires
>> +         device disassembly and/or soldering. Enabling this option causes
>> +         UARTA TX to become exposed on SDD and RX on SDB pins which correspond
>> +         to microSD CLK and CMD pins respectively. Currently this is supported
>> +         only on Transformer T20, support for other platforms may require DT
>> +         and configs (from include/configs/*) updates.
>> +
>>   source "board/nvidia/harmony/Kconfig"
>>   source "board/avionic-design/medcom-wide/Kconfig"
>>   source "board/compal/paz00/Kconfig"
>> --
>> 2.48.1
>>
> This is not applicable since this is a hack by nature, BUT if you
> contain this in the transformer t20 board and Kconfig (no dts
> changes), this may be applied.
Ok, then I'll drop DTS changes. I will add few words about this option 
in Transformer documentation
and a note in Kconfig help to not surprise users with not booting U-Boot 
as enabling this option without
changing DTS causes U-Boot to hang.