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[ Upstream commit b407460ee9 ]
It is considered good practice to call cpu_relax() in busy loops, see
Documentation/process/volatile-considered-harmful.rst. This can not
only lower CPU power consumption or yield to a hyperthreaded twin
processor, but also allows an architecture to mitigate hardware issues
(e.g. ARM Erratum 754327 for Cortex-A9 prior to r2p0) in the
architecture-specific cpu_relax() implementation.
In addition, cpu_relax() is also a compiler barrier. It is not
immediately obvious that the @op argument "function" will result in an
actual function call (e.g. in case of inlining).
Where a function call is a C sequence point, this is lost on inlining.
Therefore, with agressive enough optimization it might be possible for
the compiler to hoist the:
(val) = op(args);
"load" out of the loop because it doesn't see the value changing. The
addition of cpu_relax() would inhibit this.
As the iopoll helpers lack calls to cpu_relax(), people are sometimes
reluctant to use them, and may fall back to open-coded polling loops
(including cpu_relax() calls) instead.
Fix this by adding calls to cpu_relax() to the iopoll helpers:
- For the non-atomic case, it is sufficient to call cpu_relax() in
case of a zero sleep-between-reads value, as a call to
usleep_range() is a safe barrier otherwise. However, it doesn't
hurt to add the call regardless, for simplicity, and for similarity
with the atomic case below.
- For the atomic case, cpu_relax() must be called regardless of the
sleep-between-reads value, as there is no guarantee all
architecture-specific implementations of udelay() handle this.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/45c87bec3397fdd704376807f0eec5cc71be440f.1685692810.git.geert+renesas@glider.be
Signed-off-by: Sasha Levin <sashal@kernel.org>
193 lines
7.6 KiB
C
193 lines
7.6 KiB
C
/* SPDX-License-Identifier: GPL-2.0-only */
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/*
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* Copyright (c) 2012-2014 The Linux Foundation. All rights reserved.
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*/
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#ifndef _LINUX_IOPOLL_H
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#define _LINUX_IOPOLL_H
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#include <linux/kernel.h>
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#include <linux/types.h>
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#include <linux/ktime.h>
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#include <linux/delay.h>
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#include <linux/errno.h>
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#include <linux/io.h>
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/**
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* read_poll_timeout - Periodically poll an address until a condition is
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* met or a timeout occurs
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* @op: accessor function (takes @args as its arguments)
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* @val: Variable to read the value into
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* @cond: Break condition (usually involving @val)
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* @sleep_us: Maximum time to sleep between reads in us (0
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* tight-loops). Should be less than ~20ms since usleep_range
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* is used (see Documentation/timers/timers-howto.rst).
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* @timeout_us: Timeout in us, 0 means never timeout
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* @sleep_before_read: if it is true, sleep @sleep_us before read.
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* @args: arguments for @op poll
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*
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* Returns 0 on success and -ETIMEDOUT upon a timeout. In either
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* case, the last read value at @args is stored in @val. Must not
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* be called from atomic context if sleep_us or timeout_us are used.
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*
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* When available, you'll probably want to use one of the specialized
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* macros defined below rather than this macro directly.
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*/
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#define read_poll_timeout(op, val, cond, sleep_us, timeout_us, \
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sleep_before_read, args...) \
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({ \
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u64 __timeout_us = (timeout_us); \
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unsigned long __sleep_us = (sleep_us); \
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ktime_t __timeout = ktime_add_us(ktime_get(), __timeout_us); \
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might_sleep_if((__sleep_us) != 0); \
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if (sleep_before_read && __sleep_us) \
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usleep_range((__sleep_us >> 2) + 1, __sleep_us); \
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for (;;) { \
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(val) = op(args); \
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if (cond) \
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break; \
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if (__timeout_us && \
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ktime_compare(ktime_get(), __timeout) > 0) { \
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(val) = op(args); \
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break; \
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} \
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if (__sleep_us) \
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usleep_range((__sleep_us >> 2) + 1, __sleep_us); \
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cpu_relax(); \
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} \
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(cond) ? 0 : -ETIMEDOUT; \
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})
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/**
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* read_poll_timeout_atomic - Periodically poll an address until a condition is
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* met or a timeout occurs
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* @op: accessor function (takes @args as its arguments)
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* @val: Variable to read the value into
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* @cond: Break condition (usually involving @val)
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* @delay_us: Time to udelay between reads in us (0 tight-loops). Should
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* be less than ~10us since udelay is used (see
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* Documentation/timers/timers-howto.rst).
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* @timeout_us: Timeout in us, 0 means never timeout
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* @delay_before_read: if it is true, delay @delay_us before read.
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* @args: arguments for @op poll
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*
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* Returns 0 on success and -ETIMEDOUT upon a timeout. In either
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* case, the last read value at @args is stored in @val.
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*
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* When available, you'll probably want to use one of the specialized
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* macros defined below rather than this macro directly.
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*/
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#define read_poll_timeout_atomic(op, val, cond, delay_us, timeout_us, \
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delay_before_read, args...) \
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({ \
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u64 __timeout_us = (timeout_us); \
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unsigned long __delay_us = (delay_us); \
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ktime_t __timeout = ktime_add_us(ktime_get(), __timeout_us); \
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if (delay_before_read && __delay_us) \
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udelay(__delay_us); \
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for (;;) { \
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(val) = op(args); \
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if (cond) \
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break; \
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if (__timeout_us && \
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ktime_compare(ktime_get(), __timeout) > 0) { \
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(val) = op(args); \
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break; \
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} \
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if (__delay_us) \
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udelay(__delay_us); \
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cpu_relax(); \
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} \
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(cond) ? 0 : -ETIMEDOUT; \
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})
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/**
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* readx_poll_timeout - Periodically poll an address until a condition is met or a timeout occurs
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* @op: accessor function (takes @addr as its only argument)
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* @addr: Address to poll
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* @val: Variable to read the value into
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* @cond: Break condition (usually involving @val)
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* @sleep_us: Maximum time to sleep between reads in us (0
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* tight-loops). Should be less than ~20ms since usleep_range
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* is used (see Documentation/timers/timers-howto.rst).
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* @timeout_us: Timeout in us, 0 means never timeout
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*
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* Returns 0 on success and -ETIMEDOUT upon a timeout. In either
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* case, the last read value at @addr is stored in @val. Must not
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* be called from atomic context if sleep_us or timeout_us are used.
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*
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* When available, you'll probably want to use one of the specialized
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* macros defined below rather than this macro directly.
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*/
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#define readx_poll_timeout(op, addr, val, cond, sleep_us, timeout_us) \
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read_poll_timeout(op, val, cond, sleep_us, timeout_us, false, addr)
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/**
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* readx_poll_timeout_atomic - Periodically poll an address until a condition is met or a timeout occurs
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* @op: accessor function (takes @addr as its only argument)
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* @addr: Address to poll
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* @val: Variable to read the value into
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* @cond: Break condition (usually involving @val)
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* @delay_us: Time to udelay between reads in us (0 tight-loops). Should
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* be less than ~10us since udelay is used (see
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* Documentation/timers/timers-howto.rst).
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* @timeout_us: Timeout in us, 0 means never timeout
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*
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* Returns 0 on success and -ETIMEDOUT upon a timeout. In either
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* case, the last read value at @addr is stored in @val.
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*
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* When available, you'll probably want to use one of the specialized
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* macros defined below rather than this macro directly.
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*/
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#define readx_poll_timeout_atomic(op, addr, val, cond, delay_us, timeout_us) \
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read_poll_timeout_atomic(op, val, cond, delay_us, timeout_us, false, addr)
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#define readb_poll_timeout(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout(readb, addr, val, cond, delay_us, timeout_us)
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#define readb_poll_timeout_atomic(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout_atomic(readb, addr, val, cond, delay_us, timeout_us)
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#define readw_poll_timeout(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout(readw, addr, val, cond, delay_us, timeout_us)
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#define readw_poll_timeout_atomic(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout_atomic(readw, addr, val, cond, delay_us, timeout_us)
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#define readl_poll_timeout(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout(readl, addr, val, cond, delay_us, timeout_us)
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#define readl_poll_timeout_atomic(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout_atomic(readl, addr, val, cond, delay_us, timeout_us)
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#define readq_poll_timeout(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout(readq, addr, val, cond, delay_us, timeout_us)
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#define readq_poll_timeout_atomic(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout_atomic(readq, addr, val, cond, delay_us, timeout_us)
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#define readb_relaxed_poll_timeout(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout(readb_relaxed, addr, val, cond, delay_us, timeout_us)
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#define readb_relaxed_poll_timeout_atomic(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout_atomic(readb_relaxed, addr, val, cond, delay_us, timeout_us)
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#define readw_relaxed_poll_timeout(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout(readw_relaxed, addr, val, cond, delay_us, timeout_us)
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#define readw_relaxed_poll_timeout_atomic(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout_atomic(readw_relaxed, addr, val, cond, delay_us, timeout_us)
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#define readl_relaxed_poll_timeout(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout(readl_relaxed, addr, val, cond, delay_us, timeout_us)
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#define readl_relaxed_poll_timeout_atomic(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout_atomic(readl_relaxed, addr, val, cond, delay_us, timeout_us)
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#define readq_relaxed_poll_timeout(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout(readq_relaxed, addr, val, cond, delay_us, timeout_us)
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#define readq_relaxed_poll_timeout_atomic(addr, val, cond, delay_us, timeout_us) \
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readx_poll_timeout_atomic(readq_relaxed, addr, val, cond, delay_us, timeout_us)
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#endif /* _LINUX_IOPOLL_H */
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