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usleep_range is a finer precision implementations of msleep and is designed to be a drop-in replacement for udelay where a precise sleep / busy-wait is unnecessary. Since an easy interface to hrtimers could lead to an undesired proliferation of interrupts, we provide only a "range" API, forcing the caller to think about an acceptable tolerance on both ends and hopefully avoiding introducing another interrupt. INTRO As discussed here ( http://lkml.org/lkml/2007/8/3/250 ), msleep(1) is not precise enough for many drivers (yes, sleep precision is an unfair notion, but consistently sleeping for ~an order of magnitude greater than requested is worth fixing). This patch adds a usleep API so that udelay does not have to be used. Obviously not every udelay can be replaced (those in atomic contexts or being used for simple bitbanging come to mind), but there are many, many examples of mydriver_write(...) /* Wait for hardware to latch */ udelay(100) in various drivers where a busy-wait loop is neither beneficial nor necessary, but msleep simply does not provide enough precision and people are using a busy-wait loop instead. CONCERNS FROM THE RFC Why is udelay a problem / necessary? Most callers of udelay are in device/ driver initialization code, which is serial... As I see it, there is only benefit to sleeping over a delay; the notion of "refactoring" areas that use udelay was presented, but I see usleep as the refactoring. Consider i2c, if the bus is busy, you need to wait a bit (say 100us) before trying again, your current options are: * udelay(100) * msleep(1) <-- As noted above, actually as high as ~20ms on some platforms, so not really an option * Manually set up an hrtimer to try again in 100us (which is what usleep does anyway...) People choose the udelay route because it is EASY; we need to provide a better easy route. Device / driver / boot code is *currently* serial, but every few months someone makes noise about parallelizing boot, and IMHO, a little forward-thinking now is one less thing to worry about if/when that ever happens udelay's could be preempted Sure, but if udelay plans on looping 1000 times, and it gets preempted on loop 200, whenever it's scheduled again, it is going to do the next 800 loops. Is the interruptible case needed? Probably not, but I see usleep as a very logical parallel to msleep, so it made sense to include the "full" API. Processors are getting faster (albeit not as quickly as they are becoming more parallel), so if someone wanted to be interruptible for a few usecs, why not let them? If this is a contentious point, I'm happy to remove it. OTHER THOUGHTS I believe there is also value in exposing the usleep_range option; it gives the scheduler a lot more flexibility and allows the programmer to express his intent much more clearly; it's something I would hope future driver writers will take advantage of. To get the results in the NUMBERS section below, I literally s/udelay/usleep the kernel tree; I had to go in and undo the changes to the USB drivers, but everything else booted successfully; I find that extremely telling in and of itself -- many people are using a delay API where a sleep will suit them just fine. SOME ATTEMPTS AT NUMBERS It turns out that calculating quantifiable benefit on this is challenging, so instead I will simply present the current state of things, and I hope this to be sufficient: How many udelay calls are there in 2.6.35-rc5? udealy(ARG) >= | COUNT 1000 | 319 500 | 414 100 | 1146 20 | 1832 I am working on Android, so that is my focus for this. The following table is a modified usleep that simply printk's the amount of time requested to sleep; these tests were run on a kernel with udelay >= 20 --> usleep "boot" is power-on to lock screen "power collapse" is when the power button is pushed and the device suspends "resume" is when the power button is pushed and the lock screen is displayed (no touchscreen events or anything, just turning on the display) "use device" is from the unlock swipe to clicking around a bit; there is no sd card in this phone, so fail loading music, video, camera ACTION | TOTAL NUMBER OF USLEEP CALLS | NET TIME (us) boot | 22 | 1250 power-collapse | 9 | 1200 resume | 5 | 500 use device | 59 | 7700 The most interesting category to me is the "use device" field; 7700us of busy-wait time that could be put towards better responsiveness, or at the least less power usage. Signed-off-by: Patrick Pannuto <ppannuto@codeaurora.org> Cc: apw@canonical.com Cc: corbet@lwn.net Cc: arjan@linux.intel.com Cc: Randy Dunlap <rdunlap@xenotime.net> Cc: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
55 lines
1.4 KiB
C
55 lines
1.4 KiB
C
#ifndef _LINUX_DELAY_H
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#define _LINUX_DELAY_H
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/*
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* Copyright (C) 1993 Linus Torvalds
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*
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* Delay routines, using a pre-computed "loops_per_jiffy" value.
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*/
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#include <linux/kernel.h>
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extern unsigned long loops_per_jiffy;
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#include <asm/delay.h>
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/*
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* Using udelay() for intervals greater than a few milliseconds can
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* risk overflow for high loops_per_jiffy (high bogomips) machines. The
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* mdelay() provides a wrapper to prevent this. For delays greater
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* than MAX_UDELAY_MS milliseconds, the wrapper is used. Architecture
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* specific values can be defined in asm-???/delay.h as an override.
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* The 2nd mdelay() definition ensures GCC will optimize away the
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* while loop for the common cases where n <= MAX_UDELAY_MS -- Paul G.
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*/
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#ifndef MAX_UDELAY_MS
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#define MAX_UDELAY_MS 5
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#endif
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#ifndef mdelay
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#define mdelay(n) (\
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(__builtin_constant_p(n) && (n)<=MAX_UDELAY_MS) ? udelay((n)*1000) : \
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({unsigned long __ms=(n); while (__ms--) udelay(1000);}))
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#endif
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#ifndef ndelay
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static inline void ndelay(unsigned long x)
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{
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udelay(DIV_ROUND_UP(x, 1000));
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}
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#define ndelay(x) ndelay(x)
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#endif
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extern unsigned long lpj_fine;
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void calibrate_delay(void);
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void msleep(unsigned int msecs);
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unsigned long msleep_interruptible(unsigned int msecs);
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void usleep_range(unsigned long min, unsigned long max);
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static inline void ssleep(unsigned int seconds)
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{
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msleep(seconds * 1000);
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}
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#endif /* defined(_LINUX_DELAY_H) */
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