/sys/class/bsr

Field	Value
Subsystem	IBM POWER Barrier Synchronization Register support (Kconfig title, `drivers/char/Kconfig` line 113 of the fetched copy); implementation file not in cached tree
Config gate	`CONFIG_IBM_BSR` – verified from the fetched `char-Kconfig`: tristate “IBM POWER Barrier Synchronization Register support”, depends on PPC_PSERIES
ABI documentation	`Documentation/ABI/testing/sysfs-class-bsr`
Since	2.6.27 (entries dated Jul 2008)
Live (5.10.134 host)	class absent: no `bsr` row in the cached live enumeration of 46 classes (x86_64 host; the driver is POWER-only)
Ledger ID	SYSC-bsr

What it represents

The sysfs view of IBM POWER Barrier Synchronization Registers: a hardware mechanism for fast synchronization of threads across a large system that, per the Kconfig help text, “avoids bouncing a cacheline between several cores on a system”. Each bsr* directory in the class describes one BSR device and exposes the geometry a userspace program needs before mapping it: the register size, the mappable memory-region length, and the stride at which the allocated BSR bytes repeat within the mapping. The class exists only on IBM POWER systems (pSeries); the actual synchronization object is reached by mapping the device, not by writing sysfs.

Relationship to the device model

The class directories are attachments over platform/OF-discovered devices on pSeries machines; the ABI documents only the three geometry attributes and no device-management path. On non-POWER systems – including the x86_64 container host – neither the driver nor the class is built. The char-device side (mapping the registers) lives outside sysfs; the class is the discovery layer.

Attributes

All documented attributes are read-only, per the “(RO)” markers in the ABI file.

Attribute	R/W	Unit/format	Conditional?	Source
`bsr*/bsr_size`	RO	size of the barrier-synchronization register, in bytes	always	ABI testing `sysfs-class-bsr`
`bsr*/bsr_length`	RO	length of memory region that can be mapped, in bytes	always	ABI testing `sysfs-class-bsr`
`bsr*/bsr_stride`	RO	stride, i.e. interval at which the allocated BSR bytes repeat within the mapping	always	ABI testing `sysfs-class-bsr`

Safety and writes

No sysfs writes are documented; all three attributes are read-only. Reads are safe. The synchronization registers themselves are memory-mapped, so the hazard story (concurrent clobbering of shared registers by other partitions or threads) belongs to the mapping interface, not this class. On this x86_64 host the class is absent, consistent with the PPC_PSERIES dependency.

References

Documentation/ABI/testing/sysfs-class-bsr (cached copy; three What: entries, contacts Arnd Bergmann / Greg Kroah-Hartman)
ABI doc online
Fetched char-Kconfig (drivers/char/Kconfig), config IBM_BSR block
sysfs overview, ledger
Ledger row SYSC-bsr