Message-ID: <2130b439-74d0-475d-8429-1a1b4d9738aa@linaro.org>
Date: Wed, 12 Feb 2025 20:35:08 +0100
From: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To: Markus Schneider-Pargmann <msp@...libre.com>
Cc: Andrew Davis <afd@...com>, Lee Jones <lee@...nel.org>,
 Rob Herring <robh@...nel.org>, Krzysztof Kozlowski <krzk+dt@...nel.org>,
 Conor Dooley <conor+dt@...nel.org>, Siddharth Vadapalli
 <s-vadapalli@...com>, Nishanth Menon <nm@...com>,
 Vignesh Raghavendra <vigneshr@...com>, Tero Kristo <kristo@...nel.org>,
 devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/5] dt-bindings: mfd: syscon: Add ti,am62-ddr-pmctrl

On 10/02/2025 11:35, Markus Schneider-Pargmann wrote:
> On Sun, Feb 09, 2025 at 01:21:27PM +0100, Krzysztof Kozlowski wrote:
>> On 07/02/2025 15:40, Markus Schneider-Pargmann wrote:
>>> Hi Krzysztof,
>>>
>>> On Mon, Jan 27, 2025 at 01:09:49PM +0100, Krzysztof Kozlowski wrote:
>>>> On 24/01/2025 23:35, Andrew Davis wrote:
>>>>> On 1/24/25 10:48 AM, Krzysztof Kozlowski wrote:
>>>>>> On 24/01/2025 17:05, Markus Schneider-Pargmann wrote:
>>>>>>> Hi Krzysztof,
>>>>>>>
>>>>>>> On Fri, Jan 24, 2025 at 09:22:54AM +0100, Krzysztof Kozlowski wrote:
>>>>>>>> On Fri, Jan 24, 2025 at 09:19:49AM +0100, Krzysztof Kozlowski wrote:
>>>>>>>>> On Wed, Jan 22, 2025 at 11:24:33AM +0100, Markus Schneider-Pargmann wrote:
>>>>>>>>>> Add compatible for ti,am62-ddr-pmctrl to the list. There is a DDR pmctrl
>>>>>>>>>> register in the wkup-conf register space of am62a and am62p. This
>>>>>>>>>> register controls DDR power management.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Markus Schneider-Pargmann <msp@...libre.com>
>>>>>>>>>> ---
>>>>>>>>>>   Documentation/devicetree/bindings/mfd/syscon.yaml | 2 ++
>>>>>>>>>>   1 file changed, 2 insertions(+)
>>>>>>>>>
>>>>>>>>> Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
>>>>>>>>
>>>>>>>> Un-acked, I missed the point that you really speak in commit msg about
>>>>>>>> register and you really treat one register is a device. I assumed you
>>>>>>>> only need that register from this device, but no. That obviously is not
>>>>>>>> what this device is. Device is not a single register among 10000 others.
>>>>>>>> IOW, You do not have 10000 devices there.
>>>>>>>
>>>>>>> Do I understand you correctly that the whole register range of the
>>>>>>> wkup_conf node as seen in arch/arm64/boot/dts/ti/k3-am62a-wakeup.dtsi
>>>>>>> should be considered a single syscon device?
>>>>>>
>>>>>> I don't have the datasheets (and not my task to actually check this),
>>>>>> but you should probably follow datasheet. I assume it describes what is
>>>>>> the device, more or less.
>>>>>>
>>>>>> I assume entire wkup_conf is considered a device.
>>>>>>
>>>>>>>
>>>>>>> Unfortunately wkup_conf is modeled as a simple-bus with currently 5
>>>>>>> subnodes defined of which 4 of them consist of a single register. Most
>>>>>>> of them are syscon as well. So I think I can't change the simple-bus
>>>>>>> back to syscon.
>>>>>>
>>>>>> Huh... Maybe TI folks will help us understand why such design was chosen.
>>>>>>
>>>>>
>>>>> Many of the devices inside the wkup_conf are already modeled as such.
>>>>> Clocks and muxes for instance already have drivers and bindings, this
>>>>> is nothing new to TI.
>>>>>
>>>>> If we just use a blank "syscon" over the entire region we would end up
>>>>> with drivers that use phandles to the top level wkup_conf node and
>>>>> poke directly the registers they need from that space.
>>>>>
>>>>> Would you rather have
>>>>>
>>>>> some-device {
>>>>> 	ti,epwm_tbclk = <&wkup_conf>;
>>>>> }
>>>>>
>>>>> or
>>>>>
>>>>> some-device {
>>>>> 	clocks = <&epwm_tbclk 0>;
>>>>> }
>>>>
>>>> How is this comparable? These are clocks. You would have clocks property
>>>> in both cases.
>>>>
>>>>
>>>>>
>>>>> with that epwm_tbclk being a proper clock node inside wkup_conf?
>>>>> I would much prefer the second, even though the clock node
>>>>> only uses a single register. And in the first case, we would need
>>>>> to have the offset into the wkup_conf space hard-coded in the
>>>>> driver for each new SoC. Eventually all that data would need to be
>>>>> put in tables and we end up back to machine board files..
>>>>>
>>>>> I'm not saying every magic number in all drivers should
>>>>> be offloaded into DT, but there is a line somewhere between
>>>>> that and having the DT simply contain the SoC's name compatible
>>>>
>>>> That's not the question here.
>>>>
>>>>> and all other data going into the kernel. That line might be a
>>>>> personal preference, so my question back is: what is wrong
>>>>> if we do want "1000 new syscons per each register" for our
>>>>> SoCs DT?
>>>>
>>>> Because it is false representation of hardware. You do not have 1000
>>>> devices. You have only one device.
>>>>
>>>>
>>>>>
>>>>> (and the number is not 1000, scanning the kernel I can see
>>>>> the largest wkup_conf region node we have today has a grand
>>>>> total number sub-nodes of 6)
>>>>
>>>> But what is being added here is device per each register, not per feature.
>>>
>>> The register layout is like this:
>>
>> The register layout of what? How is the device called? Is datasheet
>> available anywhere?
> 
> Yes, it is available here: https://www.ti.com/de/lit/pdf/spruj16
> 
> 14 Registers
> 14.2 Device Configuration Registers
> 14.2.1 CTRL_MMR Registers
> 14.2.1.1 General Purpose Control Registers
> 14.2.1.1.3 WKUP_CTRL_MMR0 Registers
> 
> Each domain has their own set of general purpose control registers,
> CTRL_MMR for the main domain, MCU_CTRL_MMR0 for the MCU domain,
> WKUP_CTRL_MMR0 for the wakeup domain.


So according to the doc you have only one device - CTRL_MMR. All other
splits are superficial.

> 
> So I understand this to just be a collection of general purpose control
> registers. If you go by feature, then many of the registers can be
> grouped into units with a specific purpose or controlling a specific
> device which are also grouped by the offsets they represent. I assume

It could work if you have distinctive groups, but here:
1. You do not have this grouped, you just judge by yourself "oh, that's
group A, that's B".
2. Group per one register is not that.

For me this is one big block and even CLKSEL is spread all over so
cannot be really made distinctive.

> this is why the other nodes in this wkup_conf node were created. Also in

The other nodes represent some sort of fake or totally arbitrary
grouping. That's abuse of the syscon.

> my opinion this makes the relation between the original device and this
> general purpose control registers better understandable.
> 
> For this patch the ddr-pmctrl regsiter is just a single register, but it
> has the purpose of controlling the DDR device power management.

Sure, but that is NOT syscon. One register of entire block is not system
controller. The entire block is system controller.



Best regards,
Krzysztof

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