Re: [PATCH 5/5] arm/arm64: smccc: Add ARCH_SOC_ID support

[John Garry]

On 01/05/2020 17:05, Sudeep Holla wrote:
> On Fri, May 01, 2020 at 04:25:27PM +0100, John Garry wrote:
> > On 30/04/2020 12:48, Sudeep Holla wrote:
> > > +static int __init smccc_soc_init(void)
> > > +{
> > > +	struct device *dev;
> > > +	int ret, soc_id_rev;
> > > +	struct arm_smccc_res res;
> > > +	static char soc_id_str[8], soc_id_rev_str[12];
> > > +
> > > +	if (arm_smccc_get_version() < ARM_SMCCC_VERSION_1_2)
> > > +		return 0;
> > > +
> > > +	ret = smccc_soc_id_support_check();
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_SOC_ID, 0, &res);
> > > +
> > > +	ret = smccc_map_error_codes(res.a0);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	soc_id_version = res.a0;
> > > +
> > > +	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_SOC_ID, 1, &res);
> > > +
> > > +	ret = smccc_map_error_codes(res.a0);
> > > +	if (ret)
> > > +		return ret;
> > > +
> > > +	soc_id_rev = res.a0;
> > > +
> > > +	soc_dev_attr = kzalloc(sizeof(*soc_dev_attr), GFP_KERNEL);
> > > +	if (!soc_dev_attr)
> > > +		return -ENOMEM;
> > > +
> > > +	sprintf(soc_id_str, "0x%04x", IMP_DEF_SOC_ID(soc_id_version));
> > > +	sprintf(soc_id_rev_str, "0x%08x", soc_id_rev);
> > > +
> > > +	soc_dev_attr->soc_id = soc_id_str;
> > > +	soc_dev_attr->revision = soc_id_rev_str;
> > > +
> > > +	soc_dev = soc_device_register(soc_dev_attr);
> > > +	if (IS_ERR(soc_dev)) {
> > > +		ret = PTR_ERR(soc_dev);
> > > +		goto free_soc;
> > > +	}
> > > +
> > > +	dev = soc_device_to_device(soc_dev);
> > > +
> >
> > Just wondering, what about if the platform already had a SoC driver - now it
> > could have another one, such that we may have multiple sysfs soc devices,
> > right?
>
> Yes I had a quick look at that.
>
> 1. Such platform has option not to implement this SOC_ID if it doesn't
>     really require it.

True

> 2. If the firmware starts implementing it on some variants, then we can
>     distinguish them with compatibles and blacklist them from the other
>     SoC driver if having both is an issue
>
> 3. SoC bus layer supports adding multiple SoC ID driver and it may show
>     up as /sys/devices/soc<n> which may or may not be fine.

Yeah, it's this scenario which I'm concerned about, where some userspace 
expects, for example, soc0 to have a soc id from a known, expected list, 
and now may get something else. However it could be argued then that 
userspace is just too fragile then and there is no read problem here.

 But this
>     happens only if neither [1] nor [2] is done. I am happy to see if there's
>     any solution for this. Any suggestions ?

Not sure, but taking a slight deviation, maybe a way could be found to 
supplement this dev attribute info to other ARM soc drivers.

Cheers,
John