Re: [PATCH 2/5] dt-bindings: mfd: syscon: Add ti,am62-ddr-pmctrl

[Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>]

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@baylibre.com>
>>>>>> ---
>>>>>>   Documentation/devicetree/bindings/mfd/syscon.yaml | 2 ++
>>>>>>   1 file changed, 2 insertions(+)
>>>>>
>>>>> Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.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.


Best regards,
Krzysztof