Re: [PATCH v14 4/5] hw/riscv: virt: Add PMU DT node to the device tree

[<Conor.Dooley@microchip.com>]

+CC Rob, which I probably should've done earlier, so
context all preserved

On 29/11/2022 09:42, Conor Dooley wrote:
> On 29/11/2022 09:27, Atish Kumar Patra wrote:
>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>
>> On Mon, Nov 28, 2022 at 11:32 PM <Conor.Dooley@microchip.com> wrote:
>>>
>>> On 29/11/2022 07:08, Andrew Jones wrote:
>>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>>>
>>>> On Mon, Nov 28, 2022 at 09:10:03PM +0000, Conor.Dooley@microchip.com wrote:
>>>>> On 28/11/2022 20:41, Atish Kumar Patra wrote:
>>>>>> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
>>>>>>
>>>>>> On Mon, Nov 28, 2022 at 12:38 PM <Conor.Dooley@microchip.com> wrote:
>>>>>>>
>>>>>>> On 28/11/2022 20:16, Atish Kumar Patra wrote:
>>>>>>>> On Thu, Nov 24, 2022 at 5:17 AM Conor Dooley <conor.dooley@microchip.com> wrote:
>>>>>>>>>
>>>>>>>>> On Wed, Aug 24, 2022 at 03:17:00PM -0700, Atish Patra wrote:
>>>>>>>>>> Qemu virt machine can support few cache events and cycle/instret counters.
>>>>>>>>>> It also supports counter overflow for these events.
>>>>>>>>>>
>>>>>>>>>> Add a DT node so that OpenSBI/Linux kernel is aware of the virt machine
>>>>>>>>>> capabilities. There are some dummy nodes added for testing as well.
>>>>>>>>>
>>>>>>>>> Hey Atish!
>>>>>>>>>
>>>>>>>>> I was fiddling with dumping the virt machine dtb again today to check
>>>>>>>>> some dt-binding changes I was making for the isa string would play
>>>>>>>>> nicely with the virt machine & I noticed that this patch has introduced
>>>>>>>>> a new validation failure:
>>>>>>>>>
>>>>>>>>> ./build/qemu-system-riscv64 -nographic -machine virt,dumpdtb=qemu.dtb
>>>>>>>>>
>>>>>>>>> dt-validate -p ../linux/Documentation/devicetree/bindings/processed-schema.json qemu.dtb
>>>>>>>>> /home/conor/stuff/qemu/qemu.dtb: soc: pmu: {'riscv,event-to-mhpmcounters': [[1, 1, 524281, 2, 2, 524284, 65561, 65561, 524280, 65563, 65563, 524280, 65569, 65569, 524280, 0, 0, 0, 0, 0]], 'compatible': ['riscv,pmu']} should not be valid under {'type': 'object'}
>>>>>>>>>           From schema: /home/conor/.local/lib/python3.10/site-packages/dtschema/schemas/simple-bus.yaml
>>>>>>>>>
>>>>>>>>> I assume this is the aforementioned "dummy" node & you have no intention
>>>>>>>>> of creating a binding for this?
>>>>>>>>>
>>>>>>>>
>>>>>>>> It is a dummy node from Linux kernel perspective. OpenSbi use this
>>>>>>>> node to figure out the hpmcounter mappings.
>>>>>>>
>>>>>>> Aye, but should it not have a binding anyway, since they're not
>>>>>>> meant to be linux specific?
>>>>>>>
>>>>>> It is documented in OpenSBI.
>>>>>> https://github.com/riscv-software-src/opensbi/blob/master/docs/pmu_support.md
>>>>>>
>>>>>> Are you suggesting that any non-Linux specific DT nodes should be part
>>>>>> of Linux DT binding as well ?
>>>>>
>>>>> I thought the point was that they were *not* meant to be linux specific,
>>>>> just happening to be housed there.
>>>>>
>>>>
>>>> I'm not sure if there's an official policy on where DT nodes should be
>>>> specified, but it looks like Samuel's opinion is that they should live
>>>> in the Linux kernel, whether they're used there or not [1].
>>>>
>>>> [1] http://lists.infradead.org/pipermail/opensbi/2022-October/003522.html
>>>
>>> Yah, that was also my understanding. See also U-Boot moving to unify
>>> their custom bindings into the linux repo:
>>> https://lore.kernel.org/linux-devicetree/20220930001410.2802843-1-sjg@chromium.org/
>>>
>>
>> This adds the U-Boot specific DT properties to the dts schema itself,
>> not Linux kernel DT bindings.
> 
> Yeah, sorry. I muddled things up a little there. My point was that they
> are trying to get to a stage where dt-validate and those tools work for
> them too. I'm not sure were I said "linux repo" rather than "dt-schema
> repo" when I double checked the file paths in the link before pasting it
> to make sure it was the dt-schema one.. I blame it being early.
> 
>> I am not opposed to adding PMU DT bindings to Linux but there should
>> be a clear policy on this.
>> What about OpenSBI domain DT bindings ?
>> If every other DT based open source project starts adding their DT
>> binding to the Linux kernel, that may go downhill pretty soon.

Rob, perhaps you can be a source of clarity here :) My early morning
muddling didn't help things.


> Maybe I am misunderstanding, but I had thought the goal was to get to
> user-independent bindings. Rob and Krzysztof certainly labour the point
> that the bindings should not reflect how one operating system's drivers
> would like to see them & u-boot or FreeBSD using a property is grounds
> for it not being removed from the bindings in the linux tree.
> 
> I'll go and actually ask Rob.

I did go & ask Rob, to which he said "I'll apply it even if no driver."

Do you want to whip up a binding, or shall I?