arm64/sme: Expose SMIDR through sysfs

We currently expose MIDR and REVID to userspace through sysfs to enable it
to make decisions based on the specific implementation. Since SME supports
implementations where streaming mode is provided by a separate hardware
unit called a SMCU it provides a similar ID register SMIDR. Expose it to
userspace via sysfs when the system supports SME along with the other ID
registers.

Since we disable the SME priority mapping feature if it is supported by
hardware we currently mask out the SMPS bit which reports that it is
supported.

Signed-off-by: Mark Brown <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Will Deacon <[e-mail]>

Documentation/ABI/testing/sysfs-devices-system-cpu

···
 		/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
+		/sys/devices/system/cpu/cpuX/regs/identification/smidr_el1
 Date:		June 2016
 Contact:	Linux ARM Kernel Mailing list <[e-mail]>
 Description:	AArch64 CPU registers
 
 		'identification' directory exposes the CPU ID registers for
-		identifying model and revision of the CPU.
+		identifying model and revision of the CPU and SMCU.
 
 What:		/sys/devices/system/cpu/aarch32_el0
 Date:		May 2021

arch/arm64/include/asm/cpu.h

···
 	u64		reg_midr;
 	u64		reg_revidr;
 	u64		reg_gmid;
+	u64		reg_smidr;
 
 	u64		reg_id_aa64dfr0;
 	u64		reg_id_aa64dfr1;

arch/arm64/kernel/cpuinfo.c

···
 
 CPUREGS_ATTR_RO(midr_el1, midr);
 CPUREGS_ATTR_RO(revidr_el1, revidr);
+CPUREGS_ATTR_RO(smidr_el1, smidr);
 
 static struct attribute *cpuregs_id_attrs[] = {
 	&cpuregs_attr_midr_el1.attr,
···
 
 static const struct attribute_group cpuregs_attr_group = {
 	.attrs = cpuregs_id_attrs,
+	.name = "identification"
+};
+
+static struct attribute *sme_cpuregs_id_attrs[] = {
+	&cpuregs_attr_smidr_el1.attr,
+	NULL
+};
+
+static const struct attribute_group sme_cpuregs_attr_group = {
+	.attrs = sme_cpuregs_id_attrs,
 	.name = "identification"
 };
 
···
 	rc = sysfs_create_group(&info->kobj, &cpuregs_attr_group);
 	if (rc)
 		kobject_del(&info->kobj);
+	if (system_supports_sme())
+		rc = sysfs_merge_group(&info->kobj, &sme_cpuregs_attr_group);
 out:
 	return rc;
 }
···
 		info->reg_zcr = read_zcr_features();
 
 	if (IS_ENABLED(CONFIG_ARM64_SME) &&
-	    id_aa64pfr1_sme(info->reg_id_aa64pfr1))
+	    id_aa64pfr1_sme(info->reg_id_aa64pfr1)) {
 		info->reg_smcr = read_smcr_features();
+
+		/*
+		 * We mask out SMPS since even if the hardware
+		 * supports priorities the kernel does not at present
+		 * and we block access to them.
+		 */
+		info->reg_smidr = read_cpuid(SMIDR_EL1) & ~SMIDR_EL1_SMPS;
+	}
 
 	cpuinfo_detect_icache_policy(info);
 }