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authorDavid Howells <dhowells@redhat.com>2006-02-14 16:53:20 -0500
committerLinus Torvalds <torvalds@g5.osdl.org>2006-02-14 19:09:35 -0500
commit28baebae73c3ea8b75c7cae225a7db817ab825a9 (patch)
tree940476b4d03b96480d451b7b5b6f3df3f0ff18dc /Documentation
parent68f624fc8b9fa50de9cc0ebd612ef7b7b9fa32d0 (diff)
[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>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/fujitsu/frv/kernel-ABI.txt234
1 files changed, 234 insertions, 0 deletions
diff --git a/Documentation/fujitsu/frv/kernel-ABI.txt b/Documentation/fujitsu/frv/kernel-ABI.txt
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1 =================================
2 INTERNAL KERNEL ABI FOR FR-V ARCH
3 =================================
4
5The internal FRV kernel ABI is not quite the same as the userspace ABI. A number of the registers
6are used for special purposed, and the ABI is not consistent between modules vs core, and MMU vs
7no-MMU.
8
9This partly stems from the fact that FRV CPUs do not have a separate supervisor stack pointer, and
10most of them do not have any scratch registers, thus requiring at least one general purpose
11register to be clobbered in such an event. Also, within the kernel core, it is possible to simply
12jump or call directly between functions using a relative offset. This cannot be extended to modules
13for the displacement is likely to be too far. Thus in modules the address of a function to call
14must be calculated in a register and then used, requiring two extra instructions.
15
16This document has the following sections:
17
18 (*) System call register ABI
19 (*) CPU operating modes
20 (*) Internal kernel-mode register ABI
21 (*) Internal debug-mode register ABI
22 (*) Virtual interrupt handling
23
24
25========================
26SYSTEM CALL REGISTER ABI
27========================
28
29When a system call is made, the following registers are effective:
30
31 REGISTERS CALL RETURN
32 =============== ======================= =======================
33 GR7 System call number Preserved
34 GR8 Syscall arg #1 Return value
35 GR9-GR13 Syscall arg #2-6 Preserved
36
37
38===================
39CPU OPERATING MODES
40===================
41
42The FR-V CPU has three basic operating modes. In order of increasing capability:
43
44 (1) User mode.
45
46 Basic userspace running mode.
47
48 (2) Kernel mode.
49
50 Normal kernel mode. There are many additional control registers available that may be
51 accessed in this mode, in addition to all the stuff available to user mode. This has two
52 submodes:
53
54 (a) Exceptions enabled (PSR.T == 1).
55
56 Exceptions will invoke the appropriate normal kernel mode handler. On entry to the
57 handler, the PSR.T bit will be cleared.
58
59 (b) Exceptions disabled (PSR.T == 0).
60
61 No exceptions or interrupts may happen. Any mandatory exceptions will cause the CPU to
62 halt unless the CPU is told to jump into debug mode instead.
63
64 (3) Debug mode.
65
66 No exceptions may happen in this mode. Memory protection and management exceptions will be
67 flagged for later consideration, but the exception handler won't be invoked. Debugging traps
68 such as hardware breakpoints and watchpoints will be ignored. This mode is entered only by
69 debugging events obtained from the other two modes.
70
71 All kernel mode registers may be accessed, plus a few extra debugging specific registers.
72
73
74=================================
75INTERNAL KERNEL-MODE REGISTER ABI
76=================================
77
78There are a number of permanent register assignments that are set up by entry.S in the exception
79prologue. Note that there is a complete set of exception prologues for each of user->kernel
80transition and kernel->kernel transition. There are also user->debug and kernel->debug mode
81transition prologues.
82
83
84 REGISTER FLAVOUR USE
85 =============== ======= ====================================================
86 GR1 Supervisor stack pointer
87 GR15 Current thread info pointer
88 GR16 GP-Rel base register for small data
89 GR28 Current exception frame pointer (__frame)
90 GR29 Current task pointer (current)
91 GR30 Destroyed by kernel mode entry
92 GR31 NOMMU Destroyed by debug mode entry
93 GR31 MMU Destroyed by TLB miss kernel mode entry
94 CCR.ICC2 Virtual interrupt disablement tracking
95 CCCR.CC3 Cleared by exception prologue (atomic op emulation)
96 SCR0 MMU See mmu-layout.txt.
97 SCR1 MMU See mmu-layout.txt.
98 SCR2 MMU Save for EAR0 (destroyed by icache insns in debug mode)
99 SCR3 MMU Save for GR31 during debug exceptions
100 DAMR/IAMR NOMMU Fixed memory protection layout.
101 DAMR/IAMR MMU See mmu-layout.txt.
102
103
104Certain registers are also used or modified across function calls:
105
106 REGISTER CALL RETURN
107 =============== =============================== ===============================
108 GR0 Fixed Zero -
109 GR2 Function call frame pointer
110 GR3 Special Preserved
111 GR3-GR7 - Clobbered
112 GR8 Function call arg #1 Return value (or clobbered)
113 GR9 Function call arg #2 Return value MSW (or clobbered)
114 GR10-GR13 Function call arg #3-#6 Clobbered
115 GR14 - Clobbered
116 GR15-GR16 Special Preserved
117 GR17-GR27 - Preserved
118 GR28-GR31 Special Only accessed explicitly
119 LR Return address after CALL Clobbered
120 CCR/CCCR - Mostly Clobbered
121
122
123================================
124INTERNAL DEBUG-MODE REGISTER ABI
125================================
126
127This is the same as the kernel-mode register ABI for functions calls. The difference is that in
128debug-mode there's a different stack and a different exception frame. Almost all the global
129registers from kernel-mode (including the stack pointer) may be changed.
130
131 REGISTER FLAVOUR USE
132 =============== ======= ====================================================
133 GR1 Debug stack pointer
134 GR16 GP-Rel base register for small data
135 GR31 Current debug exception frame pointer (__debug_frame)
136 SCR3 MMU Saved value of GR31
137
138
139Note that debug mode is able to interfere with the kernel's emulated atomic ops, so it must be
140exceedingly careful not to do any that would interact with the main kernel in this regard. Hence
141the debug mode code (gdbstub) is almost completely self-contained. The only external code used is
142the sprintf family of functions.
143
144Futhermore, break.S is so complicated because single-step mode does not switch off on entry to an
145exception. That means unless manually disabled, single-stepping will blithely go on stepping into
146things like interrupts. See gdbstub.txt for more information.
147
148
149==========================
150VIRTUAL INTERRUPT HANDLING
151==========================
152
153Because accesses to the PSR is so slow, and to disable interrupts we have to access it twice (once
154to read and once to write), we don't actually disable interrupts at all if we don't have to. What
155we do instead is use the ICC2 condition code flags to note virtual disablement, such that if we
156then do take an interrupt, we note the flag, really disable interrupts, set another flag and resume
157execution at the point the interrupt happened. Setting condition flags as a side effect of an
158arithmetic or logical instruction is really fast. This use of the ICC2 only occurs within the
159kernel - it does not affect userspace.
160
161The flags we use are:
162
163 (*) CCR.ICC2.Z [Zero flag]
164
165 Set to virtually disable interrupts, clear when interrupts are virtually enabled. Can be
166 modified by logical instructions without affecting the Carry flag.
167
168 (*) CCR.ICC2.C [Carry flag]
169
170 Clear to indicate hardware interrupts are really disabled, set otherwise.
171
172
173What happens is this:
174
175 (1) Normal kernel-mode operation.
176
177 ICC2.Z is 0, ICC2.C is 1.
178
179 (2) An interrupt occurs. The exception prologue examines ICC2.Z and determines that nothing needs
180 doing. This is done simply with an unlikely BEQ instruction.
181
182 (3) The interrupts are disabled (local_irq_disable)
183
184 ICC2.Z is set to 1.
185
186 (4) If interrupts were then re-enabled (local_irq_enable):
187
188 ICC2.Z would be set to 0.
189
190 A TIHI #2 instruction (trap #2 if condition HI - Z==0 && C==0) would be used to trap if
191 interrupts were now virtually enabled, but physically disabled - which they're not, so the
192 trap isn't taken. The kernel would then be back to state (1).
193
194 (5) An interrupt occurs. The exception prologue examines ICC2.Z and determines that the interrupt
195 shouldn't actually have happened. It jumps aside, and there disabled interrupts by setting
196 PSR.PIL to 14 and then it clears ICC2.C.
197
198 (6) If interrupts were then saved and disabled again (local_irq_save):
199
200 ICC2.Z would be shifted into the save variable and masked off (giving a 1).
201
202 ICC2.Z would then be set to 1 (thus unchanged), and ICC2.C would be unaffected (ie: 0).
203
204 (7) If interrupts were then restored from state (6) (local_irq_restore):
205
206 ICC2.Z would be set to indicate the result of XOR'ing the saved value (ie: 1) with 1, which
207 gives a result of 0 - thus leaving ICC2.Z set.
208
209 ICC2.C would remain unaffected (ie: 0).
210
211 A TIHI #2 instruction would be used to again assay the current state, but this would do
212 nothing as Z==1.
213
214 (8) If interrupts were then enabled (local_irq_enable):
215
216 ICC2.Z would be cleared. ICC2.C would be left unaffected. Both flags would now be 0.
217
218 A TIHI #2 instruction again issued to assay the current state would then trap as both Z==0
219 [interrupts virtually enabled] and C==0 [interrupts really disabled] would then be true.
220
221 (9) The trap #2 handler would simply enable hardware interrupts (set PSR.PIL to 0), set ICC2.C to
222 1 and return.
223
224(10) Immediately upon returning, the pending interrupt would be taken.
225
226(11) The interrupt handler would take the path of actually processing the interrupt (ICC2.Z is
227 clear, BEQ fails as per step (2)).
228
229(12) The interrupt handler would then set ICC2.C to 1 since hardware interrupts are definitely
230 enabled - or else the kernel wouldn't be here.
231
232(13) On return from the interrupt handler, things would be back to state (1).
233
234This trap (#2) is only available in kernel mode. In user mode it will result in SIGILL.