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a7f4df4e21
Add user-mode implementations of gettimeofday() and clock_gettime() to
the VDSO. This is currently usable with 2 clocksources: the CP0 count
register, which is accessible to user-mode via RDHWR on R2 and later
cores, or the MIPS Global Interrupt Controller (GIC) timer, which
provides a "user-mode visible" section containing a mirror of its
counter registers. This section must be mapped into user memory, which
is done below the VDSO data page.
When a supported clocksource is not in use, the VDSO functions will
return -ENOSYS, which causes libc to fall back on the standard syscall
path.
When support for neither of these clocksources is compiled into the
kernel at all, the VDSO still provides clock_gettime(), as the coarse
realtime/monotonic clocks can still be implemented. However,
gettimeofday() is not provided in this case as nothing can be done
without a suitable clocksource. This causes the symbol lookup to fail
in libc and it will then always use the standard syscall path.
This patch includes a workaround for a bug in QEMU which results in
RDHWR on the CP0 count register always returning a constant (incorrect)
value. A fix for this has been submitted, and the workaround can be
removed after the fix has been in stable releases for a reasonable
amount of time.
A simple performance test which calls gettimeofday() 1000 times in a
loop and calculates the average execution time gives the following
results on a Malta + I6400 (running at 20MHz):
- Syscall: ~31000 ns
- VDSO (GIC): ~15000 ns
- VDSO (CP0): ~9500 ns
[markos.chandras@imgtec.com:
- Minor code re-arrangements in order for mappings to be made
in the order they appear to the process' address space.
- Move do_{monotonic, realtime} outside of the MIPS_CLOCK_VSYSCALL ifdef
- Use gic_get_usm_range so we can do the GIC mapping in the
arch/mips/kernel/vdso instead of the GIC irqchip driver]
Signed-off-by: Alex Smith <alex.smith@imgtec.com>
Signed-off-by: Markos Chandras <markos.chandras@imgtec.com>
Cc: linux-kernel@vger.kernel.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/11338/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
89 lines
2.0 KiB
C
89 lines
2.0 KiB
C
/*
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* This file is subject to the terms and conditions of the GNU General Public
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* License. See the file "COPYING" in the main directory of this archive
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* for more details.
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*
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* Copyright (C) 2007 by Ralf Baechle
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*/
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#include <linux/clocksource.h>
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#include <linux/init.h>
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#include <linux/sched_clock.h>
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#include <asm/time.h>
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static cycle_t c0_hpt_read(struct clocksource *cs)
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{
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return read_c0_count();
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}
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static struct clocksource clocksource_mips = {
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.name = "MIPS",
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.read = c0_hpt_read,
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.mask = CLOCKSOURCE_MASK(32),
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.flags = CLOCK_SOURCE_IS_CONTINUOUS,
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};
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static u64 notrace r4k_read_sched_clock(void)
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{
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return read_c0_count();
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}
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static inline unsigned int rdhwr_count(void)
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{
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unsigned int count;
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__asm__ __volatile__(
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" .set push\n"
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" .set mips32r2\n"
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" rdhwr %0, $2\n"
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" .set pop\n"
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: "=r" (count));
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return count;
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}
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static bool rdhwr_count_usable(void)
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{
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unsigned int prev, curr, i;
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/*
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* Older QEMUs have a broken implementation of RDHWR for the CP0 count
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* which always returns a constant value. Try to identify this and don't
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* use it in the VDSO if it is broken. This workaround can be removed
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* once the fix has been in QEMU stable for a reasonable amount of time.
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*/
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for (i = 0, prev = rdhwr_count(); i < 100; i++) {
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curr = rdhwr_count();
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if (curr != prev)
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return true;
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prev = curr;
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}
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pr_warn("Not using R4K clocksource in VDSO due to broken RDHWR\n");
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return false;
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}
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int __init init_r4k_clocksource(void)
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{
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if (!cpu_has_counter || !mips_hpt_frequency)
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return -ENXIO;
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/* Calculate a somewhat reasonable rating value */
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clocksource_mips.rating = 200 + mips_hpt_frequency / 10000000;
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/*
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* R2 onwards makes the count accessible to user mode so it can be used
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* by the VDSO (HWREna is configured by configure_hwrena()).
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*/
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if (cpu_has_mips_r2_r6 && rdhwr_count_usable())
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clocksource_mips.archdata.vdso_clock_mode = VDSO_CLOCK_R4K;
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clocksource_register_hz(&clocksource_mips, mips_hpt_frequency);
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sched_clock_register(r4k_read_sched_clock, 32, mips_hpt_frequency);
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return 0;
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}
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