[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, 73 insertions, 4 deletions

diff --git a/arch/frv/kernel/break.S b/arch/frv/kernel/break.S
index 33233dc..687c48d 100644
--- a/arch/frv/kernel/break.S
+++ b/arch/frv/kernel/break.S
@@ -200,12 +200,20 @@ __break_step:
 	movsg		bpcsr,gr2
 	sethi.p		%hi(__entry_kernel_external_interrupt),gr3
 	setlo		%lo(__entry_kernel_external_interrupt),gr3
-	subcc		gr2,gr3,gr0,icc0
+	subcc.p		gr2,gr3,gr0,icc0
+	sethi		%hi(__entry_uspace_external_interrupt),gr3
+	setlo.p		%lo(__entry_uspace_external_interrupt),gr3
 	beq		icc0,#2,__break_step_kernel_external_interrupt
-	sethi.p		%hi(__entry_uspace_external_interrupt),gr3
-	setlo		%lo(__entry_uspace_external_interrupt),gr3
-	subcc		gr2,gr3,gr0,icc0
+	subcc.p		gr2,gr3,gr0,icc0
+	sethi		%hi(__entry_kernel_external_interrupt_virtually_disabled),gr3
+	setlo.p		%lo(__entry_kernel_external_interrupt_virtually_disabled),gr3
 	beq		icc0,#2,__break_step_uspace_external_interrupt
+	subcc.p		gr2,gr3,gr0,icc0
+	sethi		%hi(__entry_kernel_external_interrupt_virtual_reenable),gr3
+	setlo.p		%lo(__entry_kernel_external_interrupt_virtual_reenable),gr3
+	beq		icc0,#2,__break_step_kernel_external_interrupt_virtually_disabled
+	subcc		gr2,gr3,gr0,icc0
+	beq		icc0,#2,__break_step_kernel_external_interrupt_virtual_reenable
 
 	LEDS		0x2007,gr2
 
@@ -254,6 +262,9 @@ __break_step_kernel_softprog_interrupt:
 # step through an external interrupt from kernel mode
 	.globl		__break_step_kernel_external_interrupt
 __break_step_kernel_external_interrupt:
+	# deal with virtual interrupt disablement
+	beq		icc2,#0,__break_step_kernel_external_interrupt_virtually_disabled
+
 	sethi.p		%hi(__entry_kernel_external_interrupt_reentry),gr3
 	setlo		%lo(__entry_kernel_external_interrupt_reentry),gr3
 
@@ -294,6 +305,64 @@ __break_return_as_kernel_prologue:
 #endif
 	rett		#1
 
+# we single-stepped into an interrupt handler whilst interrupts were merely virtually disabled
+# need to really disable interrupts, set flag, fix up and return
+__break_step_kernel_external_interrupt_virtually_disabled:
+	movsg		psr,gr2
+	andi		gr2,#~PSR_PIL,gr2
+	ori		gr2,#PSR_PIL_14,gr2	/* debugging interrupts only */
+	movgs		gr2,psr
+
+	ldi		@(gr31,#REG_CCR),gr3
+	movgs		gr3,ccr
+	subcc.p		gr0,gr0,gr0,icc2	/* leave Z set, clear C */
+
+	# exceptions must've been enabled and we must've been in supervisor mode
+	setlos		BPSR_BET|BPSR_BS,gr3
+	movgs		gr3,bpsr
+
+	# return to where the interrupt happened
+	movsg		pcsr,gr2
+	movgs		gr2,bpcsr
+
+	lddi.p		@(gr31,#REG_GR(2)),gr2
+
+	xor		gr31,gr31,gr31
+	movgs		gr0,brr
+#ifdef CONFIG_MMU
+	movsg		scr3,gr31
+#endif
+	rett		#1
+
+# we stepped through into the virtual interrupt reenablement trap
+#
+# we also want to single step anyway, but after fixing up so that we get an event on the
+# instruction after the broken-into exception returns
+	.globl		__break_step_kernel_external_interrupt_virtual_reenable
+__break_step_kernel_external_interrupt_virtual_reenable:
+	movsg		psr,gr2
+	andi		gr2,#~PSR_PIL,gr2
+	movgs		gr2,psr
+
+	ldi		@(gr31,#REG_CCR),gr3
+	movgs		gr3,ccr
+	subicc		gr0,#1,gr0,icc2		/* clear Z, set C */
+
+	# save the adjusted ICC2
+	movsg		ccr,gr3
+	sti		gr3,@(gr31,#REG_CCR)
+
+	# exceptions must've been enabled and we must've been in supervisor mode
+	setlos		BPSR_BET|BPSR_BS,gr3
+	movgs		gr3,bpsr
+
+	# return to where the trap happened
+	movsg		pcsr,gr2
+	movgs		gr2,bpcsr
+
+	# and then process the single step
+	bra		__break_continue
+
 # step through an internal exception from uspace mode
 	.globl		__break_step_uspace_softprog_interrupt
 __break_step_uspace_softprog_interrupt: