Re: [PATCH 1/3] arm64: dts: mediatek: Introduce MT8195 LAPTOP and IOT's USB configurations

[AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>]

Il 05/01/23 10:28, Macpaul Lin ha scritto:
> Introduce the split MT8195 laptop and iot USB configurations.
> The hardware specifications for LAPTOP devices is different from IOT
> devices. The major differences include some hardware constrains for
> dual-role switch for USB controllers in different configurations,
> especially for power management and other control flows as well.
> 
> Here are some hardware specifiction differences listed:
>    1. LAPTOP (Cherry Tomato boards) don't support USB gadget (device mode).
>    2. IOT devices must support multiple gadget devices and host mode.
>    3. Dual-role switch is not fully supported. Only USB PORT0 support
>       dual-role switch.
>    4. Power management is designed in primary and secondary dominator.
>       For a dual-role port, the device controller is the primary controller
>       for power management; while the host controller is the secondary.
>       LAPTOP devices should remove device nodes for avoiding abnormal
>       behavior.
> 
> This modifcation is to add USB configurations "mt8195-laptop-usb.dtsi"
> for LAPTOP devices, and add "mt8195-iot-usb.dtsi" for IOT devices.
> 
> To remove common USB configurations for mt8195.dtsi and switch includes
> dtsi these new files for the boards will come in next patch.
> 
> Signed-off-by: Macpaul Lin <macpaul.lin@mediatek.com>

I'm mostly sure that there's no reason to split the two configurations.

I agree in that Tomato doesn't support gadget mode on the Type-A port and I
honestly don't currently know (and I'll test that later!) if it would be possible
to act as gadget on any of the two Type-C ports.
Of course I agree on the fact that a laptop acting as a gadget may not be useful,
but that's not something that I want to judge, as someone may find a usecase.

In any case, even if Tomato does *not* support gadget mode on *any* port at all,
I wonder why we wouldn't be able to probe MTU3 (and correctly describe the SoC)
on Chromebooks but only on MT8195-based IoT boards...
...and in case there's any real issue, we can always force host mode (with a
generic  devicetree property!) on the MTU3 on Tomato.

Finally, if we're able to add MTU3 to Tomato boards, this means that we won't be
seeing these two DTSI files and that USB nodes are still going to all lie in the
main `mt8195.dtsi` file, without all this duplication that I'm seeing here.

What do you think?

Regards,
Angelo