linux-kernel - Re: [PATCH 2/2] firmware: arm_scmi: Add qcom hvc/shmem transport

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Date:   Tue, 18 Jul 2023 15:05:16 -0700
From:   Nikunj Kela <quic_nkela@...cinc.com>
To:     Bjorn Andersson <andersson@...nel.org>
CC:     <sudeep.holla@....com>, <cristian.marussi@....com>,
        <robh+dt@...nel.org>, <krzysztof.kozlowski+dt@...aro.org>,
        <conor+dt@...nel.org>, <agross@...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 2/2] firmware: arm_scmi: Add qcom hvc/shmem transport


On 7/18/2023 12:30 PM, Bjorn Andersson wrote:
> On Tue, Jul 18, 2023 at 12:10:16PM -0700, Nikunj Kela wrote:
>> On 7/18/2023 12:07 PM, Bjorn Andersson wrote:
>>> On Tue, Jul 18, 2023 at 11:53:24AM -0700, Nikunj Kela wrote:
>>>> On 7/18/2023 11:29 AM, Bjorn Andersson wrote:
>>>>> On Tue, Jul 18, 2023 at 09:08:33AM -0700, Nikunj Kela wrote:
>>>>>> diff --git a/drivers/firmware/arm_scmi/qcom_hvc.c b/drivers/firmware/arm_scmi/qcom_hvc.c
>>> [..]
> [..]
>>>>>> +#ifdef CONFIG_ARM64
>>>>>> +	cap_id = readq((void *)(scmi_info->shmem) + size +
>>>>>> +		       sizeof(unsigned long));
>>>>>> +#else
>>>>>> +	cap_id = readl((void *)(scmi_info->shmem) + size +
>>>>>> +		       sizeof(unsigned long));
>>>>>> +#endif
>>>>> Please don't make the in-memory representation depend on architecture
>>>>> specific data types. Quite likely you didn't compile test one of these
>>>>> variants?
>>>>>
>>>>> Just define the in-memory representation as u32 + u64.
>>>> I tested this for ARM64, I didn't test it for 32bit since Hypervisor doesn't
>>>> support it currently. In future, it may add 32 bit support too.
>>> I'd not be surprised if the capability id is 64 bit on a 32-bit machine
>>> as well, it's not really a property of the architecture.
>> on 32bit machine, you will have to use SMC32 convention. lt will mean that
>> the parameters can only be 32 bit long. If you keep cap-id 64 bit in 32bit
>> machines, then it has to be passed in two parameters. Are you suggesting, I
>> make this driver dependent on ARM64 and only care about 64 bit for now?
>>
> I'm suggesting that the calling convention is one thing and the
> in-memory format for passing the information to the driver is a
> different thing.
>
> Keep the arguments passed in memory architecture-independent (i.e. make
> it u64).
>
> If you're saying that the calling convention would be different on a
> 32-bit system, then you are also saying that your driver _is_ 64-bit
> specific. Please confirm what the size of your capability id would be in
> such a system and make sure the Kconfig and/or the code, reflects
> reality.
>
> Thanks,
> Bjorn
It is confirmed that cap-id will be 32 bit long on 32bit machines. That 
said, I will make changes to use last 16bytes irrespective of the 
architecture.