[PATCH] FRV: Use virtual interrupt disablement

Make the FRV arch use virtual interrupt disablement because accesses to the
processor status register (PSR) are relatively slow and because we will
soon have the need to deal with multiple interrupt controls at the same
time (separate h/w and inter-core interrupts).

The way this is done is to dedicate one of the four integer condition code
registers (ICC2) to maintaining a virtual interrupt disablement state
whilst inside the kernel.  This uses the ICC2.Z flag (Zero) to indicate
whether the interrupts are virtually disabled and the ICC2.C flag (Carry)
to indicate whether the interrupts are physically disabled.

ICC2.Z is set to indicate interrupts are virtually disabled.  ICC2.C is set
to indicate interrupts are physically enabled.  Under normal running
conditions Z==0 and C==1.

Disabling interrupts with local_irq_disable() doesn't then actually
physically disable interrupts - it merely sets ICC2.Z to 1.  Should an
interrupt then happen, the exception prologue will note ICC2.Z is set and
branch out of line using one instruction (an unlikely BEQ).  Here it will
physically disable interrupts and clear ICC2.C.

When it comes time to enable interrupts (local_irq_enable()), this simply
clears the ICC2.Z flag and invokes a trap #2 if both Z and C flags are
clear (the HI integer condition).  This can be done with the TIHI
conditional trap instruction.

The trap then physically reenables interrupts and sets ICC2.C again.  Upon
returning the interrupt will be taken as interrupts will then be enabled.
Note that whilst processing the trap, the whole exceptions system is
disabled, and so an interrupt can't happen till it returns.

If no pending interrupt had happened, ICC2.C would still be set, the HI
condition would not be fulfilled, and no trap will happen.

Saving interrupts (local_irq_save) is simply a matter of pulling the ICC2.Z
flag out of the CCR register, shifting it down and masking it off.  This
gives a result of 0 if interrupts were enabled and 1 if they weren't.

Restoring interrupts (local_irq_restore) is then a matter of taking the
saved value mentioned previously and XOR'ing it against 1.  If it was one,
the result will be zero, and if it was zero the result will be non-zero.
This result is then used to affect the ICC2.Z flag directly (it is a
condition code flag after all).  An XOR instruction does not affect the
Carry flag, and so that bit of state is unchanged.  The two flags can then
be sampled to see if they're both zero using the trap (TIHI) as for the
unconditional reenablement (local_irq_enable).

This patch also:

 (1) Modifies the debugging stub (break.S) to handle single-stepping crossing
     into the trap #2 handler and into virtually disabled interrupts.

 (2) Removes superseded fixup pointers from the second instructions in the trap
     tables (there's no a separate fixup table for this).

 (3) Declares the trap #3 vector for use in .org directives in the trap table.

 (4) Moves irq_enter() and irq_exit() in do_IRQ() to avoid problems with
     virtual interrupt handling, and removes the duplicate code that has now
     been folded into irq_exit() (softirq and preemption handling).

 (5) Tells the compiler in the arch Makefile that ICC2 is now reserved.

 (6) Documents the in-kernel ABI, including the virtual interrupts.

 (7) Renames the old irq management functions to different names.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 58 insertions, 7 deletions

diff --git a/arch/frv/kernel/entry.S b/arch/frv/kernel/entry.S
index c69d499d28c..1d21c8d34d8 100644
--- a/arch/frv/kernel/entry.S
+++ b/arch/frv/kernel/entry.S
@@ -141,7 +141,10 @@ __entry_uspace_external_interrupt_reentry:
 
 	movsg		gner0,gr4
 	movsg		gner1,gr5
-	stdi		gr4,@(gr28,#REG_GNER0)
+	stdi.p		gr4,@(gr28,#REG_GNER0)
+
+	# interrupts start off fully disabled in the interrupt handler
+	subcc		gr0,gr0,gr0,icc2		/* set Z and clear C */
 
 	# set up kernel global registers
 	sethi.p		%hi(__kernel_current_task),gr5
@@ -193,9 +196,8 @@ __entry_uspace_external_interrupt_reentry:
         .type		__entry_kernel_external_interrupt,@function
 __entry_kernel_external_interrupt:
 	LEDS		0x6210
-
-	sub		sp,gr15,gr31
-	LEDS32
+//	sub		sp,gr15,gr31
+//	LEDS32
 
 	# set up the stack pointer
 	or.p		sp,gr0,gr30
@@ -231,7 +233,10 @@ __entry_kernel_external_interrupt_reentry:
 	stdi		gr24,@(gr28,#REG_GR(24))
 	stdi		gr26,@(gr28,#REG_GR(26))
 	sti		gr29,@(gr28,#REG_GR(29))
-	stdi		gr30,@(gr28,#REG_GR(30))
+	stdi.p		gr30,@(gr28,#REG_GR(30))
+
+	# note virtual interrupts will be fully enabled upon return
+	subicc		gr0,#1,gr0,icc2			/* clear Z, set C */
 
 	movsg		tbr ,gr20
 	movsg		psr ,gr22
@@ -267,7 +272,10 @@ __entry_kernel_external_interrupt_reentry:
 
 	movsg		gner0,gr4
 	movsg		gner1,gr5
-	stdi		gr4,@(gr28,#REG_GNER0)
+	stdi.p		gr4,@(gr28,#REG_GNER0)
+
+	# interrupts start off fully disabled in the interrupt handler
+	subcc		gr0,gr0,gr0,icc2			/* set Z and clear C */
 
 	# set the return address
 	sethi.p		%hi(__entry_return_from_kernel_interrupt),gr4
@@ -291,6 +299,45 @@ __entry_kernel_external_interrupt_reentry:
 
 	.size		__entry_kernel_external_interrupt,.-__entry_kernel_external_interrupt
 
+###############################################################################
+#
+# deal with interrupts that were actually virtually disabled
+# - we need to really disable them, flag the fact and return immediately
+# - if you change this, you must alter break.S also
+#
+###############################################################################
+	.balign		L1_CACHE_BYTES
+	.globl		__entry_kernel_external_interrupt_virtually_disabled
+	.type		__entry_kernel_external_interrupt_virtually_disabled,@function
+__entry_kernel_external_interrupt_virtually_disabled:
+	movsg		psr,gr30
+	andi		gr30,#~PSR_PIL,gr30
+	ori		gr30,#PSR_PIL_14,gr30		; debugging interrupts only
+	movgs		gr30,psr
+	subcc		gr0,gr0,gr0,icc2		; leave Z set, clear C
+	rett		#0
+
+	.size		__entry_kernel_external_interrupt_virtually_disabled,.-__entry_kernel_external_interrupt_virtually_disabled
+
+###############################################################################
+#
+# deal with re-enablement of interrupts that were pending when virtually re-enabled
+# - set ICC2.C, re-enable the real interrupts and return
+# - we can clear ICC2.Z because we shouldn't be here if it's not 0 [due to TIHI]
+# - if you change this, you must alter break.S also
+#
+###############################################################################
+	.balign		L1_CACHE_BYTES
+	.globl		__entry_kernel_external_interrupt_virtual_reenable
+	.type		__entry_kernel_external_interrupt_virtual_reenable,@function
+__entry_kernel_external_interrupt_virtual_reenable:
+	movsg		psr,gr30
+	andi		gr30,#~PSR_PIL,gr30		; re-enable interrupts
+	movgs		gr30,psr
+	subicc		gr0,#1,gr0,icc2			; clear Z, set C
+	rett		#0
+
+	.size		__entry_kernel_external_interrupt_virtual_reenable,.-__entry_kernel_external_interrupt_virtual_reenable
 
 ###############################################################################
 #
@@ -335,6 +382,7 @@ __entry_uspace_softprog_interrupt_reentry:
 
 	sethi.p		%hi(__entry_return_from_user_exception),gr23
 	setlo		%lo(__entry_return_from_user_exception),gr23
+
 	bra		__entry_common
 
 	.size		__entry_uspace_softprog_interrupt,.-__entry_uspace_softprog_interrupt
@@ -495,7 +543,10 @@ __entry_common:
 
 	movsg		gner0,gr4
 	movsg		gner1,gr5
-	stdi		gr4,@(gr28,#REG_GNER0)
+	stdi.p		gr4,@(gr28,#REG_GNER0)
+
+	# set up virtual interrupt disablement
+	subicc		gr0,#1,gr0,icc2			/* clear Z flag, set C flag */
 
 	# set up kernel global registers
 	sethi.p		%hi(__kernel_current_task),gr5