[MIPS] SMTC: Interrupt mask backstop hack

To support multiple TC microthreads acting as "CPUs" within a VPE,
VPE-wide interrupt mask bits must be specially manipulated during
interrupt handling. To support legacy drivers and interrupt controller
management code, SMTC has a "backstop" to track and if necessary restore
the interrupt mask. This has some performance impact on interrupt service
overhead. Disable it only if you know what you are doing.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/Kconfig

@@ -1404,6 +1404,19 @@ config MIPS_MT_SMTC_INSTANT_REPLAY
 	  it off), but ensures that IPIs are handled promptly even under
 	  heavy I/O interrupt load.
 
+config MIPS_MT_SMTC_IM_BACKSTOP
+	bool "Use per-TC register bits as backstop for inhibited IM bits"
+	depends on MIPS_MT_SMTC
+	default y
+	help
+	  To support multiple TC microthreads acting as "CPUs" within
+	  a VPE, VPE-wide interrupt mask bits must be specially manipulated
+	  during interrupt handling. To support legacy drivers and interrupt
+	  controller management code, SMTC has a "backstop" to track and
+	  if necessary restore the interrupt mask. This has some performance
+	  impact on interrupt service overhead. Disable it only if you know
+	  what you are doing.
+
 config MIPS_VPE_LOADER_TOM
 	bool "Load VPE program into memory hidden from linux"
 	depends on MIPS_VPE_LOADER

arch/mips/kernel/entry.S

@@ -84,6 +84,7 @@ FEXPORT(restore_all)			# restore full frame
 	LONG_S	sp, TI_REGS($28)
 	jal	deferred_smtc_ipi
 	LONG_S	s0, TI_REGS($28)
+#ifdef CONFIG_MIPS_MT_SMTC_IM_BACKSTOP
 /* Re-arm any temporarily masked interrupts not explicitly "acked" */
 	mfc0	v0, CP0_TCSTATUS
 	ori	v1, v0, TCSTATUS_IXMT
@@ -110,6 +111,7 @@ FEXPORT(restore_all)			# restore full frame
 	_ehb
 	xor	t0, t0, t3
 	mtc0	t0, CP0_TCCONTEXT
+#endif /* CONFIG_MIPS_MT_SMTC_IM_BACKSTOP */
 #endif /* CONFIG_MIPS_MT_SMTC */
 	.set	noat
 	RESTORE_TEMP

arch/mips/kernel/genex.S

@@ -243,9 +243,11 @@ NESTED(except_vec_vi_handler, 0, sp)
 	 */
 	mfc0	t1, CP0_STATUS
 	and	t0, a0, t1
+#ifdef CONFIG_MIPS_MT_SMTC_IM_BACKSTOP
 	mfc0	t2, CP0_TCCONTEXT
 	or	t0, t0, t2
 	mtc0	t0, CP0_TCCONTEXT
+#endif /* CONFIG_MIPS_MT_SMTC_IM_BACKSTOP */
 	xor	t1, t1, t0
 	mtc0	t1, CP0_STATUS
 	_ehb

include/asm-mips/irq.h

@@ -24,7 +24,7 @@ static inline int irq_canonicalize(int irq)
 #define irq_canonicalize(irq) (irq)	/* Sane hardware, sane code ... */
 #endif
 
-#ifdef CONFIG_MIPS_MT_SMTC
+#ifdef CONFIG_MIPS_MT_SMTC_IM_BACKSTOP
 /*
  * Clear interrupt mask handling "backstop" if irq_hwmask
  * entry so indicates. This implies that the ack() or end()