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

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Date:   Tue, 18 Jul 2023 12:30:04 -0700
From:   Bjorn Andersson <andersson@...nel.org>
To:     Nikunj Kela <quic_nkela@...cinc.com>
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 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