[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 c69d499d28cf..1d21c8d34d8a 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