linux-kernel - Re: [PATCH v10 15/26] gunyah: rsc_mgr: Add platform ops on mem_lend/mem

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Date:   Wed, 22 Feb 2023 10:21:53 +0000
From:   Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:     Elliot Berman <quic_eberman@...cinc.com>,
        Alex Elder <elder@...aro.org>,
        Prakruthi Deepak Heragu <quic_pheragu@...cinc.com>
Cc:     Murali Nalajala <quic_mnalajal@...cinc.com>,
        Trilok Soni <quic_tsoni@...cinc.com>,
        Srivatsa Vaddagiri <quic_svaddagi@...cinc.com>,
        Carl van Schaik <quic_cvanscha@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Jassi Brar <jassisinghbrar@...il.com>,
        linux-arm-msm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v10 15/26] gunyah: rsc_mgr: Add platform ops on
 mem_lend/mem_reclaim

On 21/02/2023 21:22, Elliot Berman wrote:
> 
> 
> On 2/21/2023 6:51 AM, Srinivas Kandagatla wrote:
>>
>>
>> On 14/02/2023 21:24, Elliot Berman wrote:
> [snip]
>>> +
>>> +static struct gunyah_rm_platform_ops *rm_platform_ops;
>>> +static DECLARE_RWSEM(rm_platform_ops_lock);
>>
>> Why do we need this read/write lock or this global rm_platform_ops 
>> here, AFAIU, there will be only one instance of platform_ops per 
>> platform.
>>
>> This should be a core part of the gunyah and its driver early setup, 
>> that should give us pretty much lock less behaviour.
>>
>> We should be able to determine by Hypervisor UUID that its on Qualcomm 
>> platform or not, during early gunyah setup which should help us setup 
>> the platfrom ops accordingly.
>>
>> This should also help cleanup some of the gunyah code that was added 
>> futher down in this patchset.
> 
> I'm guessing the direction to take is:
> 
>    config GUNYAH
>      select QCOM_SCM if ARCH_QCOM

This is how other kernel drivers use SCM.

> 
> and have vm_mgr call directly into qcom_scm driver if the UID matches?

Yes that is the plan, we could have these callbacks as part key data 
structure like struct gh_rm and update it at very early in setup stage 
based on UUID match.

> 
> We have an Android requirement to enable CONFIG_GUNYAH=y and 
> CONFIG_QCOM_SCM=m, but it wouldn't be possible with this design. The 

Am not sure how this will work, if gunyah for QCOM Platform is depended 
on SCM then there is no way that gunyah could be a inbuilt and make scm 
a module.

On the other hand with the existing design gunyah will not be functional 
until scm driver is loaded and platform hooks are registered. This 
runtime dependency design does not express the dependency correctly and 
the only way to know if gunyah is functional is keep trying which can 
only work after scm driver is probed.

This also raises the design question on how much of platform hooks 
dependency is captured at gunyah core and api level, with state of 
current code /dev/gunyah will be created even without platform hooks and 
let the userspace use it which then only fail at hyp call level.

Other issue with current design is, scm module can be unloaded under the 
hood leaving gunyah with NULL pointers to those platform hook functions. 
This is the kind of issues we could see if the dependency is not 
expressed from bottom up.

The current design is not really capturing the depended components 
accurately.

Considering platform hooks as a core resource to gunyah on Qualcomm 
platform is something that needs attention. If we can fix that then it 
might be doable to have QCOM_SCM=m and CONFIG_GUNYAH=y.

--srini
> platform hooks implementation allows GUNYAH and QCOM_SCM to be enabled 
> without setting lower bound of the other.
> 
> - Elliot