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

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Message-ID: <5b1eb21c-92bf-59bc-25c5-f8c38c42b375@linaro.org>
Date:   Thu, 24 Nov 2022 14:57:35 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     neil.armstrong@...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 23/11/2022 11:19, neil.armstrong@...aro.org wrote:
>>>> 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.
>>
>> I don't see why. None of devices in your DTS reference this memory
>> region, so it is purely to keep it mapped for Modem, right? In such case
>> I still do not get why PAS/PIL, who starts and stops the remote
>> processor, could not prepare the memory and share it with modem.
> 
> OK you've got a point, but this is still an implementation detail.
> 
> I got some more background about why this memory zone appeared, before it
> was including in the modem reserved-memories, but for flexibility reasons
> this is now in the hands of the APPS runtime (linux) to setup this memory
> zone and share it to the MPSS subsystem.
> 
> So the only requirement for the PAS is to make sure this zone is shared
> before starting the startup process, not to actually do the share setup.
> 
> The zone will need to be shared whatever the PAS state is (probe, startup, remove, ...).

I don't understand here. The memory region should be shared before
remote processor start or before the PAS probe? If the second, why? What
if your driver probes a bit later - after PAS probe? Then all is lost...

Best regards,
Krzysztof