linux-kernel - Re: [PATCH 1/2] dt-bindings: reserved-memory: document Qualcomm MPSS DSM memory

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Date:   Fri, 18 Nov 2022 14:30:23 +0100
From:   neil.armstrong@...aro.org
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...ainline.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Andy Gross <agross@...nel.org>,
        Frank Rowand <frowand.list@...il.com>
Cc:     linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH 1/2] dt-bindings: reserved-memory: document Qualcomm MPSS
 DSM memory

On 18/11/2022 11:45, Krzysztof Kozlowski wrote:
> On 17/11/2022 10:47, Neil Armstrong wrote:
>>>
>>>> +
>>>> +properties:
>>>> +  compatible:
>>>> +    const: qcom,mpss-dsm-mem
>>>
>>> Why do we need dedicated binding and compatible for it instead of using
>>> memory-region phandle in the device?
>>
>> So like rmtfs, this memory zone is shared between APPS and the MPSS subsystem.
>>
>> Like rmtfs it makes no sense to link it to the MPSS PAS, since it's only a launcher,
>> it doesn't represent the MPSS subsystem.
> 
> This also does not represent a device. Memory region is not a device, so
> this is as well not correct representation of hardware.

I never used the term device so far, but a shared memory region with a platform
specific process to share the region between subsystems.

> 
>>
>> In the PAS startup process, the resources are released from APPS once the MPSS subsystem
>> is running, which is not the case with the MPSS DSM where it must be shared during the whole
>> lifetime of the system.
> 
> I don't think that PAS releases the region. I checked the
> qcom_q6v5_pas.c and there is only ioremap. The device stays loaded thus
> the memory stays mapped.
Yes PAS does release the firmware region when the firmware is started,
qcom_scm_pas_metadata_release() does that.

> 
> We have already three of such "memory region devices" and we keep
> growing it. It's not scalable.

If we want to properly describe this, we must then represent the MPSS subsystem
and associate this memory region.

> 
> Best regards,
> Krzysztof

Thanks,
Neil

>