Message-ID: <50546635-1870-499d-8cdc-eb4e9fa1a510@kernel.org>
Date: Thu, 5 Sep 2024 23:20:55 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Andrea della Porta <andrea.porta@...e.com>
Cc: Michael Turquette <mturquette@...libre.com>,
 Stephen Boyd <sboyd@...nel.org>, Rob Herring <robh@...nel.org>,
 Krzysztof Kozlowski <krzk+dt@...nel.org>, Conor Dooley
 <conor+dt@...nel.org>, Florian Fainelli <florian.fainelli@...adcom.com>,
 Broadcom internal kernel review list
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Subject: Re: [PATCH 08/11] misc: rp1: RaspberryPi RP1 misc driver

On 05/09/2024 20:54, Andrea della Porta wrote:
> Hi Krzysztof,
> 
> On 18:52 Thu 05 Sep     , Krzysztof Kozlowski wrote:
>> On 05/09/2024 18:33, Andrea della Porta wrote:
>>> Hi Krzysztof,
>>>
>>> On 20:27 Tue 03 Sep     , Krzysztof Kozlowski wrote:
>>>> On 03/09/2024 17:15, Andrea della Porta wrote:
>>>>>>>>> +
>>>>>>>>> +				rp1_clocks: clocks@...0018000 {
>>>>>>>>
>>>>>>>> Why do you mix MMIO with non-MMIO nodes? This really does not look
>>>>>>>> correct.
>>>>>>>>
>>>>>>>
>>>>>>> Right. This is already under discussion here:
>>>>>>> https://lore.kernel.org/all/ZtBzis5CzQMm8loh@apocalypse/
>>>>>>>
>>>>>>> IIUC you proposed to instantiate the non-MMIO nodes (the three clocks) by
>>>>>>> using CLK_OF_DECLARE.
>>>>>>
>>>>>> Depends. Where are these clocks? Naming suggests they might not be even
>>>>>> part of this device. But if these are part of the device, then why this
>>>>>> is not a clock controller (if they are controllable) or even removed
>>>>>> (because we do not represent internal clock tree in DTS).
>>>>>
>>>>> xosc is a crystal connected to the oscillator input of the RP1, so I would
>>>>> consider it an external fixed-clock. If we were in the entire dts, I would have
>>>>> put it in root under /clocks node, but here we're in the dtbo so I'm not sure
>>>>> where else should I put it.
>>>>
>>>> But physically, on which PCB, where is this clock located?
>>>
>>> xosc is a crystal, feeding the reference clock oscillator input pins of the RP1,
>>> please see page 12 of the following document:
>>> https://datasheets.raspberrypi.com/rp1/rp1-peripherals.pdf
>>
>> That's not the answer. Where is it physically located?
> 
> Please see below.
> 
>>
>>> On Rpi5, the PCB is the very same as the one on which the BCM2712 (SoC) and RP1
>>> are soldered. Would you consider it external (since the crystal is outside the RP1)
>>> or internal (since the oscillator feeded by the crystal is inside the RP1)?
>>
>> So it is on RPi 5 board? Just like every other SoC and every other
>> vendor? Then just like every other SoC and every other vendor it is in
>> board DTS file.
> 
> Yes it's on the Rpi5 board. These are two separate thing, though: one is where

Finally.

> to put it (DTS, DTSO) and another is in what target path relative to root. I
> was trying to understand the latter.

It is already or should be part of DTS, not DTSO. You are duplicating
device nodes.

> The clock node should be put in the DTBO since we are loading this driver at
> runtime and we probably don't want to depend on some specific node name to be
> present in the DTS. This is also true because this driver should possibly work
> also on ACPI system and on hypothetical PCI card on which the RP1 could be mounted

Not really. ACPI and thus DT in such case will take it as clock input.
Basically what you need here is to provide clock inputs to this device.
It's then firmware independent.

> in the future, and in that case a DTS could be not even there. 

No problem. Whatever firmware mechanism you have, it will provide you clock.

> After all, those clocks must be in the immediate proximity to the RP1, and on the
> same board, which may or may not be the main board as the Rpi5 case.
> I think that, since this application is a little bit peculiar, maybe some
> compromises could be legit.

Application is not peculiar but completely standard. You have standard
PCI device which has some inputs. One of these inputs, maybe on some
reserved M.2 pins or whatver connector you have there, is the clock.

...

>>>
>>> If externals, I would place them in the same position as xosc, so root node
>>> or some other node under root (eg.: /rp1-clocks)
>>
>> Just like /clocks, /rp1-clocks is not better. Neither /rp1-foo-clocks.
> 
> Right. So in this case, since xosc seems to be on the same level and on the same
> board of the RP1 and the SoC, and it's also external to the RP1, can I assume that
> placing xosc node in root is ok?

root node of the DTS yes. Root node of DTSO of course not, because it is
not part of DTSO and you are duplicating DTS. It would not even work.

That's why you need to apply the overlay to proper target as I asked
long time ago.

> 
>>
>> I think there is some sort of big misunderstanding here. Is this RP1
>> co-processor on the RP board, connected over PCI to Broadcom SoC?
> 
> Yes. 
> 
>  ---------------Rpi5 board---------------------
>  |                                            |
>  |    SoC ==pci bus==> RP1 <== xosc crystal   |
>  |                                            |
>  ----------------------------------------------
> 
>>
>>>
>>> If internals, I would leave them just where they are, i.e. inside the rp1 node
>>>
>>> Does it make sense?
>>
>> No, because you do not have xosc there, according to my knowledge.
> 
> Hmmm sorry, not sure what this negation was referring to... I was talking about
> hclk and pclk, not xosc here. Could you please add some details?

If considering hclk and pclk, then depends where are they. If they come
as inputs, then same as xosc. If they are not, then it is also obvious -
we do not represent internal device clocks as fixed clocks in DTS,
because it makes absolutely no sense at all. No benefits, no help,
nothing at all.

It's just device's internal clock.

This is again nothing peculiar. Many other devices have some internal
stuff. Do we add fixed clocks for these? Fixed regulators? No, of course
not.

Best regards,
Krzysztof

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