[PATCH 0/4] Support UART over MicroSD slot on Transformer T20

[Svyatoslav Ryhel]

пн, 24 бер. 2025 р. о 22:01 Artur Kowalski <arturkow2000 at gmail.com> пише:
>
> Hello
>
> Thanks for quick response. I know UARTD is the dedicated UART for
> debugging on TF101, but exposing UART over microSD
> is a handy trick useful for development. As I'm doing development on

It is a handy trick indeed, but it is still just a trick and not smth
that fits mainline. Maximum I can propose is picking funcmux change
along with uart over sd section in transformer t20 documentation.

> this tablet (bringing U-Boot's UEFI support and latest
> Linux), and also still using the device on old OS I'd like to avoid
> disassembling and reassembling it over and over again.

I am not sure what are you doing but TF101 has full support of U-Boot
as primary bootloader along with full mainline support till 6.14
including. It uses extlinux boot method and needs no intermediates or
chainloading.

If you wish to use U-Boot for tethered booting that is perfectly fine as well.

> Others too may find the feature useful e. g. to avoid having to solder
> which requires specialized equipment, skills and
> is potentially dangerous for the board. Also, there already seems to be
> support for that on Sunxi platforms through
> CONFIG_UART0_PORT_F.
>
> Apart from pinmux changes, would it be acceptable to include an option
> to allow switching UARTA mux to
> SDB/SDD (currently this has to be selected from board's Kconfig file).
> Doing so would allow to enable UART
> over microSD without patching U-Boot's source without changing any
> existing configuration. Then I would
> keep DTS overlays and config changes local.
>

I am fine with any changes which may simplify debugging for you as
long as they do not invade into completed device configuration without
a valid reason. Device must expose a dedicated UART for debug, not a
mux which breaks some device features just because you do not want to
contain them locally.

For example, patches which add UART-E support along with funcmux
change which exposes uart-e over usd were accepted, but patch similar
to yours which would reconfigure usd to uart for surface rt or p880
would be rejected.

> Also, today, after running U-Boot with debug enabled, I discovered some
> failed assertions in pinmux code probably
> caused by my changes, I will get that fixed before sending v2.
>
> Best regards
> Artur Kowalski
>
> W dniu 18.03.2025 o 09:54, Svyatoslav Ryhel pisze:
> > вт, 18 бер. 2025 р. о 10:31 Artur Kowalski <arturkow2000 at gmail.com> пише:
> >>
> >> The first patch in the series brings general pinmux support for exposing
> >> UART over SDB/SDD pins on T20. The following patches bring support
> >> Transformer T20 allowing to enable UART over uSD by setting
> >>
> >> CONFIG_TEGRA_ENABLE_UARTA=y
> >> CONFIG_DEVICE_TREE_INCLUDES="tegra20-asus-transformer-uart-usd.dtsi"
> >>
> >> Similarily, support for UART over uSD could be brought to Ventana which
> >> has uSD on the same pins as TF101, and other Ventana-based boards.
> >>
> > Hey there, thank you for looking into U-Boot for Tegra. If something I
> > will say hurts or offends you, I am sorry it was not intentional.
> >
> > Anyway, apart funcmux changes, all other changes you propose are
> > redundant. Asus dedicated uart for logging and console is UARTD any
> > other uart configuration must not be added since it will disrupt
> > existing configuration. If you need uart and cannot resolve an issue
> > without it you can always solder to uart-d or locally mux sd to uart
> > console fix your issue and remove it. Uart is not intended way to
> > interact with Asus TF101 and doesn't need to be exposed, panel and
> > fastboot console are and can be used to resolve most of issues.
> >
> > Best regards,
> > Svyatoslav R.
> >
> >> Artur Kowalski (4):
> >>    tegra: add funcmux for exposing UART over uSD slot on Tegra 20
> >>    tegra: respect CONFIG_TEGRA_ENABLE_UART* options on Transformer T20
> >>    ARM: tegra: expose UART-A over uSD on Transformer T20
> >>    board: asus: transformer: add DTSI file for enabling UART over uSD
> >>      slot
> >>
> >>   .../tegra20-asus-transformer-uart-usd.dtsi    | 31 +++++++++++++++++++
> >>   arch/arm/include/asm/arch-tegra20/funcmux.h   |  1 +
> >>   arch/arm/mach-tegra/board.c                   |  4 ++-
> >>   arch/arm/mach-tegra/tegra20/Kconfig           |  4 +++
> >>   drivers/pinctrl/tegra/funcmux-tegra20.c       |  9 +++++-
> >>   include/configs/transformer-t20.h             | 10 ++++++
> >>   6 files changed, 57 insertions(+), 2 deletions(-)
> >>   create mode 100644 arch/arm/dts/tegra20-asus-transformer-uart-usd.dtsi
> >>
> >> --
> >> 2.48.1
> >>