linux-kernel - Re: [PATCH v2 0/3] arm64: boot: dts: ti: k3-am62l: allow WKUP UART wakeup from LPM

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Message-ID: <afac4fe3-1dbc-4c8a-a0f1-6e25666eabb4@ti.com>
Date: Thu, 8 Jan 2026 11:52:32 -0600
From: Kendall Willis <k-willis@...com>
To: Bryan Brattlof <bb@...com>
CC: Nishanth Menon <nm@...com>, Vignesh Raghavendra <vigneshr@...com>, "Tero
 Kristo" <kristo@...nel.org>, Rob Herring <robh@...nel.org>, "Krzysztof
 Kozlowski" <krzk+dt@...nel.org>, Conor Dooley <conor+dt@...nel.org>, "Dhruva
 Gole" <d-gole@...com>, <vishalm@...com>, <sebin.francis@...com>,
	<msp@...libre.com>, <khilman@...libre.com>, <a-kaur@...com>,
	<s-kochidanadu@...com>, <linux-arm-kernel@...ts.infradead.org>,
	<devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 0/3] arm64: boot: dts: ti: k3-am62l: allow WKUP UART
 wakeup from LPM

On 1/8/26 11:05, Bryan Brattlof wrote:
> On January  7, 2026 thus sayeth Kendall Willis:
>> On 1/7/26 10:45, Bryan Brattlof wrote:
>>> On January  6, 2026 thus sayeth Kendall Willis:
>>>> K3 TI AM62L SoC supports wakeup from WKUP UART when the SoC is in the
>>>> DeepSleep low power mode. To allow wakeup from WKUP UART the target-module
>>>> device tree node is enabled. The ti-sysc interconnect target module driver
>>>> is used to configure the the SYSCONFIG related registers. In this case,
>>>> the interconnect target module node configures the WKUP UART to be able to
>>>> wakeup from system suspend. The SYSC register is used to enable wakeup
>>>> from system suspend for the WKUP UART. Refer to 14.7.2.5 UART in the
>>>> AM62L Techincal Reference Manual for registers referenced [1].
>>>>
>>>> Previous TI SoCs configure the WKUP UART to wakeup from system suspend
>>>> using the ti-sysc interconnect target module driver. Refer to commit
>>>> ce27f7f9e328 ("arm64: dts: ti: k3-am62-wakeup: Configure ti-sysc for
>>>> wkup_uart0") for an example of this.
>>>
>>> I think I may be confused. What is setting the pinmux for the wkup_uart
>>> to allow us to trigger the wake event? It looks like they reset to GPIO
>>> pins if not set. Is firmware doing this?
>>
>> On AM62L, the WKUP UART pinmux is initially set by TFA so that it is set to
>> the UART pins, not GPIO. The target-module node sets the WKUP UART SYSC
>> register so that wakeup is enabled.
>>
> 
> Nice however should we mark the UART in the board file as reserved for
> TFA or is it free to be used by Linux and only during the low power
> modes after Linux is asleep will it be used by TFA?
> 
> ~Bryan

Sorry for the confusion, I said earlier that the WKUP UART could be used 
by firmware, but that is not to say it is specifically reserved by the 
firmware in general. TFA sets the WKUP UART pins, but it does not use 
the WKUP UART for anything else. The only exception for TFA using the 
WKUP UART is if the user chooses to use the WKUP UART for TFA debugging [1].

All of this to say that the WKUP UART *can* be used by Linux, it does 
not need to be reserved for firmware.

[1] 
https://software-dl.ti.com/processor-sdk-linux/esd/AM62LX/latest/exports/docs/linux/Foundational_Components/Power_Management/pm_am62lx_debug.html

Best,
Kendall