// SPDX-License-Identifier: GPL-2.0+
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/*
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* Test cases for using floating point operations inside a kernel module.
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*
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* This tests kernel_fpu_begin() and kernel_fpu_end() functions, especially
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* when userland has modified the floating point control registers. The kernel
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* state might depend on the state set by the userland thread that was active
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* before a syscall.
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*
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* To facilitate the test, this module registers file
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* /sys/kernel/debug/selftest_helpers/test_fpu, which when read causes a
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* sequence of floating point operations. If the operations fail, either the
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* read returns error status or the kernel crashes.
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* If the operations succeed, the read returns "1\n".
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*/
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#include <linux/module.h>
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#include <linux/kernel.h>
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#include <linux/debugfs.h>
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#include <asm/fpu/api.h>
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static int test_fpu(void)
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{
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/*
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* This sequence of operations tests that rounding mode is
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* to nearest and that denormal numbers are supported.
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* Volatile variables are used to avoid compiler optimizing
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* the calculations away.
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*/
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volatile double a, b, c, d, e, f, g;
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a = 4.0;
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b = 1e-15;
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c = 1e-310;
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/* Sets precision flag */
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d = a + b;
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/* Result depends on rounding mode */
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e = a + b / 2;
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/* Denormal and very large values */
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f = b / c;
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/* Depends on denormal support */
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g = a + c * f;
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if (d > a && e > a && g > a)
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return 0;
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else
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return -EINVAL;
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}
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static int test_fpu_get(void *data, u64 *val)
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{
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int status = -EINVAL;
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kernel_fpu_begin();
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status = test_fpu();
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kernel_fpu_end();
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*val = 1;
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return status;
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}
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DEFINE_SIMPLE_ATTRIBUTE(test_fpu_fops, test_fpu_get, NULL, "%lld\n");
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static struct dentry *selftest_dir;
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static int __init test_fpu_init(void)
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{
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selftest_dir = debugfs_create_dir("selftest_helpers", NULL);
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if (!selftest_dir)
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return -ENOMEM;
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debugfs_create_file("test_fpu", 0444, selftest_dir, NULL,
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&test_fpu_fops);
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return 0;
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}
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static void __exit test_fpu_exit(void)
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{
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debugfs_remove(selftest_dir);
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}
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module_init(test_fpu_init);
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module_exit(test_fpu_exit);
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MODULE_LICENSE("GPL");
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