The Fuchsia Authors
89 linhas
2.5 KiB
C
89 linhas
2.5 KiB
C
// Copyright 2016 The Fuchsia Authors
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// Copyright (c) 2013 Google Inc.
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//
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// Use of this source code is governed by a MIT-style
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// license that can be found in the LICENSE file or at
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// https://opensource.org/licenses/MIT
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#include <compiler.h>
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#include <kernel/debug.h>
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__ALIGNED(8) __NAKED
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#if (__CORTEX_M >= 0x03) || (CORTEX_SC >= 300)
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void spin_cycles(uint32_t cycles)
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{
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asm (
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/* 4 cycles per loop, subtract out 8 cycles for the overhead of the next
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* 4 instructions, plus the call into and return from the function.
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* Then, add 3 then >> 2 to round up to the number of loop iterations.
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*/
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"subs r1, %[cycles], #5\n"
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"asrs r1, r1, #2\n"
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"ble .Ldone\n"
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/* Padding to stay aligned on an 8 byte boundary, also has the added
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* advantage of normalizing the overhead (1+1+2 cycles if the branch is
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* take, or 1+1+1+1 cycles if the branch is skipped and the nop is
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* executed)
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*/
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"nop\n"
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/* Main delay loop.
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* sub is 1 cycle
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* nop is 1 cycle
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* branch is 2 cycles
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*/
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".Lloop:\n"
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"subs r1, r1, #1\n"
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"nop\n"
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"bne .Lloop\n"
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".Ldone:\n"
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"bx lr\n"
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: /* no output */
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: [cycles] "r" (cycles) /* input is cycles */
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: "r1" /* r1 gets clobbered */
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);
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}
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#else
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/* Cortex-M0 & Cortex-M0+ */
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void spin_cycles(uint32_t cycles)
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{
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asm (
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/* 4 cycles per loop, subtract out 8 cycles for the overhead of the next
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* 4 instructions, plus the call into and return from the function.
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* Then, add 3 then >> 2 to round up to the number of loop iterations.
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*/
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"sub r1, %[cycles], #5\n"
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"asr r1, r1, #2\n"
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"cmp r1, #0\n"
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"ble .Ldone\n"
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/* Padding to stay aligned on an 8 byte boundary, also has the added
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* advantage of normalizing the overhead (1+1+2 cycles if the branch is
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* take, or 1+1+1+1 cycles if the branch is skipped and the nop is
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* executed)
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*/
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"nop\n"
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/* Main delay loop.
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* sub is 1 cycle
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* nop is 1 cycle
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* branch is 2 cycles
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*/
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".Lloop:\n"
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"sub r1, r1, #1\n"
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"cmp r1,#0\n"
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"bne .Lloop\n"
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".Ldone:\n"
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"bx lr\n"
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: /* no output */
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: [cycles] "r" (cycles) /* input is cycles */
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: "r1" /* r1 gets clobbered */
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);
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}
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#endif
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