linux-kernel - Re: [RESEND PATCH 1/2] dt-bindings: dma: Add reg-names to nvidia,tegra210-adma

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Message-ID: <774df64c-56a1-461a-82fa-a0340732b779@kernel.org>
Date: Wed, 22 May 2024 13:29:30 +0200
From: Krzysztof Kozlowski <krzk@...nel.org>
To: Sameer Pujar <spujar@...dia.com>, vkoul@...nel.org, robh@...nel.org,
 krzk+dt@...nel.org, conor+dt@...nel.org, thierry.reding@...il.com,
 jonathanh@...dia.com, dmaengine@...r.kernel.org, devicetree@...r.kernel.org
Cc: linux-tegra@...r.kernel.org, linux-kernel@...r.kernel.org,
 ldewangan@...dia.com, mkumard@...dia.com
Subject: Re: [RESEND PATCH 1/2] dt-bindings: dma: Add reg-names to
 nvidia,tegra210-adma

On 22/05/2024 09:43, Sameer Pujar wrote:
> 
> 
> On 22-05-2024 12:17, Krzysztof Kozlowski wrote:
>> On 22/05/2024 07:35, Sameer Pujar wrote:
>>> On 21-05-2024 17:23, Krzysztof Kozlowski wrote:
>>>> On 21/05/2024 13:08, Sameer Pujar wrote:
>>>>> From: Mohan Kumar <mkumard@...dia.com>
>>>>>
>>>>> For Non-Hypervisor mode, Tegra ADMA driver requires the register
>>>>> resource range to include both global and channel page in the reg
>>>>> entry. For Hypervisor more, Tegra ADMA driver requires only the
>>>>> channel page and global page range is not allowed for access.
>>>>>
>>>>> Add reg-names DT binding for Hypervisor mode to help driver to
>>>>> differentiate the config between Hypervisor and Non-Hypervisor
>>>>> mode of execution.
>>>>>
>>>>> Signed-off-by: Mohan Kumar <mkumard@...dia.com>
>>>>> Signed-off-by: Sameer Pujar <spujar@...dia.com>
>>>>> ---
>>>>>    .../devicetree/bindings/dma/nvidia,tegra210-adma.yaml  | 10 ++++++++++
>>>>>    1 file changed, 10 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/devicetree/bindings/dma/nvidia,tegra210-adma.yaml b/Documentation/devicetree/bindings/dma/nvidia,tegra210-adma.yaml
>>>>> index 877147e95ecc..ede47f4a3eec 100644
>>>>> --- a/Documentation/devicetree/bindings/dma/nvidia,tegra210-adma.yaml
>>>>> +++ b/Documentation/devicetree/bindings/dma/nvidia,tegra210-adma.yaml
>>>>> @@ -29,8 +29,18 @@ properties:
>>>>>              - const: nvidia,tegra186-adma
>>>>>
>>>>>      reg:
>>>>> +    description: |
>>>>> +      For hypervisor mode, the address range should include a
>>>>> +      ADMA channel page address range, for non-hypervisor mode
>>>>> +      it starts with ADMA base address covering Global and Channel
>>>>> +      page address range.
>>>>>        maxItems: 1
>>>>>
>>>>> +  reg-names:
>>>>> +    description: only required for Hypervisor mode.
>>>> This does not work like that. I provide vm entry for non-hypervisor mode
>>>> and what? You claim it is virtualized?
>>>>
>>>> Drop property.
>>> With 'vm' entry added for hypervisor mode, the 'reg' address range needs
>>> to be updated to use channel specific region only. This is used to
>>> inform driver to skip global regions which is taken care by hypervisor.
>>> This is expected to be used in the scenario where Linux acts as a
>>> virtual machine (VM). May be the hypervisor mode gives a different
>>> impression here? Sorry, I did not understand what dropping the property
>>> exactly means here.
>> It was imperative. Drop it. Remove it. I provided explanation why.
> 
> The driver doesn't know if it is operated in a native config or in the 
> hypervisor config based on the 'reg' address range alone. So 'vm' entry 
> with restricted 'reg' range is used to differentiate here for the 
> hypervisor config. Just adding 'vm' entry won't be enough, the 'reg' 
> region must be updated as well to have expected behavior. Not sure how 
> this dependency can be enforced in the schema.

That's not a unusual problem, so please come with a solution for your
entire subarch. We've been discussing similar topic in terms of SCMI
controlled resources (see talk on Linaro Connect a week ago:
https://www.kitefor.events/events/linaro-connect-24/submissions/161 I
don't know where is recording or slides, see also discussions on mailing
lists about it), which is not that far away from the problem here. Other
platforms and maybe nvidia had as well changes in IO space for
virtualized configuration.

Come with unified approach FOR ALL your devices, not only this one
(that's kind of basic thing we keep repeating... don't solve only one
your problem), do not abuse the regular property, because as I said:
reg-names will be provided as well in non-vm case and then your entire
logic is wrong. The purpose of reg-names is not to tell whether you have
or have not virtualized environment.


Best regards,
Krzysztof