linux-kernel - Re: [PATCH v3 5/9] ARM: dts: stm32: add Hardware debug port (HDP) on stm32mp13

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Message-ID: <782763e2-99d6-4533-b0db-79b618577586@foss.st.com>
Date: Thu, 12 Jun 2025 15:02:13 +0200
From: Clement LE GOFFIC <clement.legoffic@...s.st.com>
To: Krzysztof Kozlowski <krzk@...nel.org>,
        Linus Walleij
	<linus.walleij@...aro.org>,
        Rob Herring <robh@...nel.org>,
        Krzysztof
 Kozlowski <krzk+dt@...nel.org>,
        Conor Dooley <conor+dt@...nel.org>,
        Maxime
 Coquelin <mcoquelin.stm32@...il.com>,
        Alexandre Torgue
	<alexandre.torgue@...s.st.com>,
        Bartosz Golaszewski <brgl@...ev.pl>
CC: <linux-kernel@...r.kernel.org>, <linux-gpio@...r.kernel.org>,
        <devicetree@...r.kernel.org>,
        <linux-stm32@...md-mailman.stormreply.com>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 5/9] ARM: dts: stm32: add Hardware debug port (HDP) on
 stm32mp13

On 6/12/25 13:05, Krzysztof Kozlowski wrote:
> On 12/06/2025 11:31, Clement LE GOFFIC wrote:
>> On 6/11/25 17:48, Krzysztof Kozlowski wrote:
>>> On 11/06/2025 16:08, Clement LE GOFFIC wrote:
>>>> On 6/11/25 08:35, Krzysztof Kozlowski wrote:
>>>>> On 10/06/2025 15:33, Clement LE GOFFIC wrote:
>>>>>> On 6/10/25 14:38, Krzysztof Kozlowski wrote:
>>>>>>> On 10/06/2025 14:02, Clement LE GOFFIC wrote:
>>>>>>>> On 5/29/25 11:01, Krzysztof Kozlowski wrote:
>>>>>>>>> On 28/05/2025 14:14, Clement LE GOFFIC wrote:
>>>>>>>>>>>
>>>>>>>>>>>> +		};
>>>>>>>>>>>> +
>>>>>>>>>>>> +		hdp: pinctrl@...2a000 {
>>>>>>>>>>>> +			compatible = "st,stm32mp131-hdp";
>>>>>>>>>>>> +			reg = <0x5002a000 0x400>;
>>>>>>>>>>>> +			clocks = <&rcc HDP>;
>>>>>>>>>>>>        			status = "disabled";
>>>>>>>>>>>
>>>>>>>>>>> Why are you disabling it? What is missing?
>>>>>>>>>>
>>>>>>>>>> Nothing is missing just disabled by default.
>>>>>>>>>> The node is then enabled when needed in board's dts file.
>>>>>>>>> Nodes should not be disabled by default if they are complete. That's why
>>>>>>>>> I asked what is missing. Drop.
>>>>>>>>
>>>>>>>> Hi Krzysztof, OK I better understand now.
>>>>>>>> So yes the 'pinctrl-*' properties which are board dependent are lacking.
>>>>>>>
>>>>>>> These are not properties of this node.
>>>>>>
>>>>>> Does this mean I should add 'pinctrl-*' properties in bindings yaml file ?
>>>>>> I don't get it..
>>>>>
>>>>> These properties have no meaning here, so the hardware description is
>>>>> complete. You claim that you miss them thus device is incomplete is just
>>>>> not correct: these properties do not belong here! They belong to the
>>>>> board but even there they are totally optional. Why would they be a
>>>>> required resource?
>>>>>
>>>>> To remind: we talk here ONLY about required resources.
>>>>
>>>> Yes, 'pinctrl-*' properties belongs to the board and are not required.
>>>> So nothing is missing.
>>>>
>>>> This hdp node in the SoC dtsi file can be enabled by default.
>>>> But the hdp driver will probe and do nothing because without the
>>>> 'pinctrl-*' properties from the board files it would not be able to
>>>> access to any pin.
>>>
>>>
>>> Pinctrl has other features in general, including interfaces to userspace
>>> (as pretty often combined with gpio, although not sure if relevant here).
>>
>> You're right. Also HDP pinctrl has a GPO feature accessible from userspace.
>> But by default the HDP is not connected to any pad; it needs the board
> 
> OK, then that was the answer to my first question - what is missing.
> However aren't these pads connected internally also to other parts of
> the SoC (like in most other vendors)?

No, HDP "output pads" are only connected to SoC pinctrl to route outside 
the internal SoC signals for debug purpose.

>> 'pinctrl-*' properties to configure the SoC pinctrl and expose HDP on
>> the SoC pads.
>>
>> That's why for me the enabling of the driver should be in the board file
>> together with the SoC pinctrl configuration.
> 
> And what are the default pad settings configured by HPD driver in
> bootloader (and by bootloader I mean one of few bootloaders this is
> going to be used on like U-Boot)

The default is to use the GPIO of the SoC pinctrl. The HDP is not routed 
outside.
  >>
>> The userland cannot change the pinctrl alternate function mux, this is
>> statically defined by the devicetree.
> 
> If you expose GPIO then userland needs this regardless of alternate mux.
> IOW, board level could not configure mux because it should be always
> configured to safe GPIO input.

For userland sight view, SoC GPIO are preferred instead of HDP.
HDP is only GPO not GPIO. 'pinctrl-*' properties configure at the same 
time the SoC pinctrl mux to HDP and the HDP pinctrl mux to one of the 
HDP functions (e.g. GPO).

Best regards,
Clément