Re: [PATCH v7] arm64: cpuinfo: Expose MIDR_EL1 and REVIDR_EL1 to sysfs

Suzuki K Poulose
2016-07-01T13:12:29Z

On 01/07/16 14:01, Catalin Marinas wrote:
> On Thu, Jun 30, 2016 at 06:36:44PM +0100, Suzuki K. Poulose wrote:
>> From: Steve Capper <steve.capper@linaro.org>
>>
>> It can be useful for JIT software to be aware of MIDR_EL1 and
>> REVIDR_EL1 to ascertain the presence of any core errata that could
>> affect code generation.
>>
>> This patch exposes these registers through sysfs:
>>
>> /sys/devices/system/cpu/cpu$ID/regs/identification/midr_el1
>> /sys/devices/system/cpu/cpu$ID/regs/identification/revidr_el1

>> +What: /sys/devices/system/cpu/cpuX/regs/
>> + /sys/devices/system/cpu/cpuX/regs/identification/
>> + /sys/devices/system/cpu/cpuX/regs/identification/midr_el1
>> + /sys/devices/system/cpu/cpuX/regs/identification/revidr_el1
>> +Date: June 2016
>> +Contact: Linux ARM Kernel Mailing list <linux-arm-kernel@lists.infradead.org>
>> + Linux Kernel mailing list <linux-kernel@vger.kernel.org>
>> +Description: ARM64 CPU identification registers
>
> s/ARM64/AArch64/.

Ok.

>
>> + 'identification' directory exposes the CPU ID registers for
>> + identifying model and revision of the CPU.
>> + - midr_el1 : This file gives contents of Main ID Register (MIDR_EL1).
>> + - revidr_el1 : This file gives contents of the Revision ID register
>> + (REVIDR_EL1).
> [...]

I will remove those superfluous explanation.

>> --- a/arch/arm64/kernel/cpuinfo.c
>> +++ b/arch/arm64/kernel/cpuinfo.c
>> @@ -183,6 +183,140 @@ const struct seq_operations cpuinfo_op = {
>> .show = c_show
>> };
>>
>> +
>> +static struct kobj_type cpuregs_kobj_type = {
>> + .sysfs_ops = &kobj_sysfs_ops,
>> +};
>> +
>> +/*
>> + * The ARM ARM uses the phrase "32-bit register" to describe a register
>> + * whose upper 32 bits are RES0 (per C5.1.1, ARM DDI 0487A.i), however
>> + * no statement is made as to whether the upper 32 bits will or will not
>> + * be made use of in future, and between ARM DDI 0487A.c and ARM DDI
>> + * 0487A.d CLIDR_EL1 was expanded from 32-bit to 64-bit.
>> + *
>> + * Thus, while both MIDR_EL1 and REVIDR_EL1 are described as 32-bit
>> + * registers, we expose them both as 64 bit values to cater for possible
>> + * future expansion without an ABI break.
>> + */
>> +#define kobj_to_cpuinfo(kobj) container_of(kobj, struct cpuinfo_arm64, kobj)
>> +#define CPUREGS_ATTR_RO(_name, _field) \
>> + static ssize_t _name##_show(struct kobject *kobj, \
>> + struct kobj_attribute *attr, char *buf) \
>> + { \
>> + struct cpuinfo_arm64 *info = kobj_to_cpuinfo(kobj); \
>> + \
>> + if (info->reg_midr) \
>> + return sprintf(buf, "0x%016x\n", info->reg_##_field); \
>> + else \
>> + return 0; \
>> + } \
>> + static struct kobj_attribute cpuregs_attr_##_name = __ATTR_RO(_name)
>> +
>> +CPUREGS_ATTR_RO(midr_el1, midr);
>> +CPUREGS_ATTR_RO(revidr_el1, revidr);
>> +
>> +static struct attribute *cpuregs_id_attrs[] = {
>> + &cpuregs_attr_midr_el1.attr,
>> + &cpuregs_attr_revidr_el1.attr,
>> + NULL
>> +};
>> +
>> +static struct attribute_group cpuregs_attr_group = {
>> + .attrs = cpuregs_id_attrs,
>> + .name = "identification"
>> +};
>> +
>> +static int cpuid_add_regs(int cpu)
>> +{
>> + int rc;
>> + struct device *dev;
>> + struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
>> +
>> + dev = get_cpu_device(cpu);
>> + if (dev) {
>> + rc = kobject_add(&info->kobj, &dev->kobj, "regs");
>> + if (!rc)
>> + rc = sysfs_create_group(&info->kobj, &cpuregs_attr_group);
>> + } else {
>> + return -ENODEV;
>> + }
>> +
>> + return rc;
>> +}
>> +
>> +static int cpuid_remove_regs(int cpu)
>> +{
>> + int rc = 0;
>> + struct device *dev;
>> + struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
>> +
>> + dev = get_cpu_device(cpu);
>> + if (dev) {
>> + sysfs_remove_group(&info->kobj, &cpuregs_attr_group);
>> + kobject_del(&info->kobj);
>> + } else {
>> + rc = -ENODEV;
>> + }
>> +
>> + return rc;
>> +}
>> +
>> +static int cpuid_callback(struct notifier_block *nb,
>> + unsigned long action, void *hcpu)
>> +{
>> + int rc = 0;
>> + unsigned long cpu = (unsigned long)hcpu;
>> +
>> + switch (action & ~CPU_TASKS_FROZEN) {
>> + case CPU_ONLINE:
>> + rc = cpuid_add_regs(cpu);
>> + break;
>> + case CPU_DEAD:
>> + rc = cpuid_remove_regs(cpu);
>> + break;
>> + }
>> +
>> + return notifier_from_errno(rc);
>> +}
>> +
>> +static int __init cpuinfo_regs_init(void)
>> +{
>> + int cpu, finalcpu, ret;
>> +
>> + cpu_notifier_register_begin();
>> +
>> + for_each_possible_cpu(cpu) {
>> + struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
>> +
>> + kobject_init(&info->kobj, &cpuregs_kobj_type);
>> + if (cpu_online(cpu)) {
>> + ret = cpuid_add_regs(cpu);
>> + if (ret)
>> + break;
>> + }
>> + }
>> +
>> +
>> + /*
>> + * We were unable to put down sysfs groups for all the CPUs, revert
>> + * all the groups we have placed down s.t. none are visible.
>> + * Otherwise we could give a misleading picture of what's present.
>> + */
>> + if (ret) {
>> + finalcpu = cpu;
>> + for_each_online_cpu(cpu) {
>> + if (cpu == finalcpu)
>> + break;
>> + cpuid_remove_regs(cpu);
>> + }
>> + } else {
>> + __hotcpu_notifier(cpuid_callback, 0);
>> + }
>> +
>> + cpu_notifier_register_done();
>> + return ret;
>
> What is the failure scenario here and do we expect it to fail in the
> middle of a for_each_possible_cpu()? You don't do any such clean-up if
> this fails during CPU_ONLINE, so I think we should either ignore this

This was done to decide whether we should report the registers at all at
init time. i.e, if we failed to setup the sysfs attributes at init time
for at least one CPU, we don't enable these attributes (and we skip the
notifiers.) for any.

But you're right. I think we could remove this check altogether.
It was initially put in place when we didn't have the protection against
hotplug.

> altogether or have a different clean-up function that handles the
> CPU_ONLINE cases. I prefer the former.

Thanks
Suzuki