Linux-libre 4.2-gnu

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+			DSCR (Data Stream Control Register)
+		================================================
+
+DSCR register in powerpc allows user to have some control of prefetch of data
+stream in the processor. Please refer to the ISA documents or related manual
+for more detailed information regarding how to use this DSCR to attain this
+control of the pefetches . This document here provides an overview of kernel
+support for DSCR, related kernel objects, it's functionalities and exported
+user interface.
+
+(A) Data Structures:
+
+	(1) thread_struct:
+		dscr		/* Thread DSCR value */
+		dscr_inherit	/* Thread has changed default DSCR */
+
+	(2) PACA:
+		dscr_default	/* per-CPU DSCR default value */
+
+	(3) sysfs.c:
+		dscr_default	/* System DSCR default value */
+
+(B) Scheduler Changes:
+
+	Scheduler will write the per-CPU DSCR default which is stored in the
+	CPU's PACA value into the register if the thread has dscr_inherit value
+	cleared which means that it has not changed the default DSCR till now.
+	If the dscr_inherit value is set which means that it has changed the
+	default DSCR value, scheduler will write the changed value which will
+	now be contained in thread struct's dscr into the register instead of
+	the per-CPU default PACA based DSCR value.
+
+	NOTE: Please note here that the system wide global DSCR value never
+	gets used directly in the scheduler process context switch at all.
+
+(C) SYSFS Interface:
+
+	Global DSCR default:		/sys/devices/system/cpu/dscr_default
+	CPU specific DSCR default:	/sys/devices/system/cpu/cpuN/dscr
+
+	Changing the global DSCR default in the sysfs will change all the CPU
+	specific DSCR defaults immediately in their PACA structures. Again if
+	the current process has the dscr_inherit clear, it also writes the new
+	value into every CPU's DSCR register right away and updates the current
+	thread's DSCR value as well.
+
+	Changing the CPU specif DSCR default value in the sysfs does exactly
+	the same thing as above but unlike the global one above, it just changes
+	stuff for that particular CPU instead for all the CPUs on the system.
+
+(D) User Space Instructions:
+
+	The DSCR register can be accessed in the user space using any of these
+	two SPR numbers available for that purpose.
+
+	(1) Problem state SPR:		0x03	(Un-privileged, POWER8 only)
+	(2) Privileged state SPR:	0x11	(Privileged)
+
+	Accessing DSCR through privileged SPR number (0x11) from user space
+	works, as it is emulated following an illegal instruction exception
+	inside the kernel. Both mfspr and mtspr instructions are emulated.
+
+	Accessing DSCR through user level SPR (0x03) from user space will first
+	create a facility unavailable exception. Inside this exception handler
+	all mfspr isntruction based read attempts will get emulated and returned
+	where as the first mtspr instruction based write attempts will enable
+	the DSCR facility for the next time around (both for read and write) by
+	setting DSCR facility in the FSCR register.
+
+(E) Specifics about 'dscr_inherit':
+
+	The thread struct element 'dscr_inherit' represents whether the thread
+	in question has attempted and changed the DSCR itself using any of the
+	following methods. This element signifies whether the thread wants to
+	use the CPU default DSCR value or its own changed DSCR value in the
+	kernel.
+
+		(1) mtspr instruction	(SPR number 0x03)
+		(2) mtspr instruction	(SPR number 0x11)
+		(3) ptrace interface	(Explicitly set user DSCR value)
+
+	Any child of the process created after this event in the process inherits
+	this same behaviour as well.