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Date:   Fri, 1 May 2020 17:40:08 +0100
From:   John Garry <john.garry@...wei.com>
To:     Sudeep Holla <sudeep.holla@....com>
CC:     "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Steven Price <steven.price@....com>,
        "harb@...erecomputing.com" <harb@...erecomputing.com>,
        Will Deacon <will@...nel.org>
Subject: Re: [PATCH 5/5] arm/arm64: smccc: Add ARCH_SOC_ID support

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

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