linux-kernel - Re: [PATCH v2 3/3] firmware: arm_scmi: Add qcom hvc/shmem transport

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Date:   Tue, 25 Jul 2023 10:12:19 -0700
From:   Nikunj Kela <quic_nkela@...cinc.com>
To:     Cristian Marussi <cristian.marussi@....com>
CC:     <sudeep.holla@....com>, <robh+dt@...nel.org>,
        <krzysztof.kozlowski+dt@...aro.org>, <conor+dt@...nel.org>,
        <agross@...nel.org>, <andersson@...nel.org>,
        <konrad.dybcio@...aro.org>, <linux-arm-kernel@...ts.infradead.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH v2 3/3] firmware: arm_scmi: Add qcom hvc/shmem transport


On 7/25/2023 10:03 AM, Cristian Marussi wrote:
> On Mon, Jul 24, 2023 at 09:44:19AM -0700, Nikunj Kela wrote:
>> Add a new transport channel to the SCMI firmware interface driver for
>> SCMI message exchange on Qualcomm virtual platforms.
>>
>> The hypervisor associates an object-id also known as capability-id
>> with each hvc doorbell object. The capability-id is used to identify the
>> doorbell from the VM's capability namespace, similar to a file-descriptor.
>>
>> The hypervisor, in addition to the function-id, expects the capability-id
>> to be passed in x1 register when HVC call is invoked.
>>
>> The qcom hvc doorbell/shared memory transport uses a statically defined
>> shared memory region that binds with "arm,scmi-shmem" device tree node.
>>
>> The function-id & capability-id are allocated by the hypervisor on bootup
>> and are stored in the shmem region by the firmware before starting Linux.
>>
>> Currently, there is no usecase for the atomic support therefore this driver
>> doesn't include the changes for the same.
>>
> Hi Nikunj,
>
> so basically this new SCMI transport that you are introducing is just
> exactly like the existing SMC transport with the only difference that
> you introduced even another new way to configure func_id, a new cap_id
> param AND the fact that you use HVC instead of SMC... all of this tied
> to a new compatible to identify this new transport mechanism....
> ..but all in all is just a lot of plain duplicated code to maintain...
>
> ...why can't you fit this other smc/hvc transport variant into the
> existing SMC transport by properly picking and configuring func_id/cap_id
> and "doorbell" method (SMC vs HVC) in the chan_setup() step ?
>
> ..I mean ... you can decide where to pick your params based on
> compatibles and also you can setup your invokation method (SMC vs HVC)
> based on those...while keeping all the other stuff exactly the same...
> ...including support for atomic exchanges...if not, when you'll need that
> too in your QC_HVC transport you'll have to duplicate also that (and my
> bugs too probably :P)
>
> (... well maybe in this scenario also the transport itself should be
> renamed from SMC to something more general...)
>
> Not sure if I am missing something, or if Sudeep will be horrified by
> this unifying proposal of mine, but in this series as it stands now I
> just see a lot of brutally duplicated stuff that just differs by naming
> and a very minimal change in logic that could be addressed changing and
> generalizing the original SMC transport code instead.
>
> Thanks,
> Cristian

Hi Christian,

I totally agree with you and will be happy to include my changes in 
smc.c if Sudeep agrees with that approach.

Thanks