From patchwork Tue Jun 26 16:03:57 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Arnd Bergmann X-Patchwork-Id: 140003 Delivered-To: patch@linaro.org Received: by 2002:a2e:970d:0:0:0:0:0 with SMTP id r13-v6csp5424849lji; Tue, 26 Jun 2018 09:05:40 -0700 (PDT) X-Google-Smtp-Source: ADUXVKK31aG2y/s9iH0qyb6Uut6q5sInxP5ACvfQ7S/84iN7hUSSuSVYGoPsTlaK/lxU/9Qbgcmh X-Received: by 2002:a63:3686:: with SMTP id d128-v6mr1913730pga.18.1530029140063; Tue, 26 Jun 2018 09:05:40 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1530029140; cv=none; d=google.com; s=arc-20160816; b=UCKRWIE5inB+VyHNERp6O0nvcbO1JyD6bgwCGxPt+dFEqGu/jcOCugAdM/9aqDleEj HQsIniJUkSJbNiuGQUWLUOizVbByIJXU0Ahq2SwhDJkTfxnrXDhsl9wuhOZayslyHvkm vAxvNPYBsjCPhOrybdf+c6CismX8EQw5yWgEy5fuvfGvKZBVPTzpBi0+UeNHO4JzQAPf XEv9LpojP9ceNDIzq7iNfQMgOP7YmKHBGd+QKh/gfedoBJMNNyqzRxhAAwWSFjQdxxe4 zxPXVkN4MidHddLTHbzNH2JW+hIx0SW5XNg5/+91ZMhtBfsSMuq+HHVDBNxULqrUVbuj RwcA== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:sender:message-id:date:subject:cc:to:from :arc-authentication-results; bh=g+SajwEUq/RHUOsrFaYBp6uBCST3QXNfJkXkIdKr9BE=; b=RFcVUNHyDGUm9ZARy+k4NkOvo+QmzY+GEebYAjB88U3WDpzDihBVLq86ja/AH/4to6 5J5nycy+cZ+SeW3WQ72VrtsPyAl6Ye+MvfWF9txmGC2lB7/Mw0AaGZdD34UJF4TsaJQ5 XHFux0W7iJk0GIKEyMQp4v4Qfly5C7GZIP7upkUqZFodfFG4I0t3d6qj0DvhVVn9asqg Ec90iCWaVgeO5MVBH9FKLSny/6UlOVFgMqqMSZOeFPOTJPc9wlBO1K07gOL4QiyJQuQW OgLxkH4qmrk9f0XkzFCSJV+aNoSK7QBgtodirM6fznUM3pM3OPc1Wfntdz2vEv4ZJhTq GUIg== ARC-Authentication-Results: i=1; mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Return-Path: Received: from vger.kernel.org (vger.kernel.org. [209.132.180.67]) by mx.google.com with ESMTP id p186-v6si1889873pfg.57.2018.06.26.09.05.39; Tue, 26 Jun 2018 09:05:40 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752952AbeFZQFi (ORCPT + 31 others); Tue, 26 Jun 2018 12:05:38 -0400 Received: from mout.kundenserver.de ([217.72.192.75]:53103 "EHLO mout.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752927AbeFZQFf (ORCPT ); Tue, 26 Jun 2018 12:05:35 -0400 Received: from wuerfel.lan ([95.208.111.237]) by mrelayeu.kundenserver.de (mreue105 [212.227.15.145]) with ESMTPA (Nemesis) id 0MG9Yd-1fLS800rMF-00F88a; Tue, 26 Jun 2018 18:04:47 +0200 From: Arnd Bergmann To: Jonathan Corbet Cc: y2038@lists.linaro.org, Arnd Bergmann , Dave Chinner , John Stultz , Thomas Gleixner , Stephen Boyd , Linus Walleij , Mauro Carvalho Chehab , Matthew Wilcox , Michal Hocko , Randy Dunlap , Ingo Molnar , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH] Documentation: document ktime_get_*() APIs Date: Tue, 26 Jun 2018 18:03:57 +0200 Message-Id: <20180626160442.572358-1-arnd@arndb.de> X-Mailer: git-send-email 2.9.0 X-Provags-ID: V03:K1:ut17xDfi06YenRCbZzQJgVuyHLvbh1cUPm1TtCHOmw0P4xJUmLb aSRSXAN1fftIRZizdcd/Yv49lZDFEnKLuvbdPfPx6nKGGMedUGnhROZKfAlIdstBca+I9s/ e7PpNw2ciX2l/nwNX6d2/BgfHeo1GQALHVQj3jKw9MoogLzMopET2M7fNMd0LTxz0c45XSV E96MBfHJSmF6BkBpH5UvQ== X-UI-Out-Filterresults: notjunk:1; V01:K0:XRCzDgBT2SY=:5egptImQQp+QKEUYq+MFsJ QDdwup/TyNf/EA9hnsNuTPA0xhn0DxO5w/UerR1uv8k7ujbJHwU85zQypl5r+GBYulkiEsTdN X7lL3q8PknwaqQ4/LUg1jkTjZ2jrkT5O7t00wjVKmCq+GQJcTpU9TxBw6+gp9Gw3apQ9Njhkk a+QNW6m6uc2nuhEkmjjFWhMKxuLH3tIEKbT8i8UHsBYJE3ka0B9drXQjtYShdQ3PypghtTqzx jhsxUmdupuRS/bwlVK6TidsmwNXYZ+XIy8kcKo05Q1dbZfmqtrkgAcPi7yWZTKTR5L3OJCxYo pQZBUI6N+Xj5DYapMJw1ZlUkmFOGzz9VXfRwwRfZCVBXOQ31lrfilKfzWLCi/OUv/dV+A+JQN /HUpjACPJKHEKN3fJkT6OMc6jKpWS5VEGUGUrFj1ACc2FRUFsGqrdWJMNglylq4De73+1IC3b NAch0NoZsDolJZGzg6Az3M43Xwc1tubaVQNnhRE8lcYP+1hbI1nkdJTJ6hxDxHMEeR4Sq5/mS XQsZpUIkOVO75a/HBunBxrEtCwi1hiGnpVMg2loDr5qyKzyb7hyZN3G5612EnZLfNlr/8nvvQ rgiS847McYQ21c41PmAeKILZpSI40TC0p+NIS0Ji+mQiIGRvoLDMcAl8k7VwmcyalKV23kvOe SjJti0gMlwftmIBVtVFgRCTsGT8oKdUkxxAjC6pMt4nlDnPOtmb1RFEUdmVMb6B1kivQ= Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org As Dave Chinner points out, we don't have a proper documentation for the ktime_get() family of interfaces, making it rather unclear which of the over 30 (!) interfaces one should actually use in a driver or elsewhere in the kernel. I wrote up an explanation from how I personally see the interfaces, documenting what each of the functions do and hopefully making it a bit clearer which should be used where. This is the first time I tried writing .rst format documentation, so in addition to any mistakes in the content, I probably also introduce nonstandard formatting ;-) I first tried to add an extra section to Documentation/timers/timekeeping.txt, but this is currently not included in the generated API, and it seems useful to have the API docs as part of what gets generated in https://www.kernel.org/doc/html/latest/core-api/index.html#core-utilities instead, so I started a new file there. I also considered adding the documentation inline in the include/linux/timekeeping.h header, but couldn't figure out how to do that in a way that would result both in helpful inline comments as well as readable html output, so I settled for the latter, with a small note pointing to it from the header. Cc: Dave Chinner Cc: John Stultz Cc: Thomas Gleixner Cc: Stephen Boyd Cc: Linus Walleij Signed-off-by: Arnd Bergmann --- Documentation/core-api/index.rst | 1 + Documentation/core-api/timekeeping.rst | 185 +++++++++++++++++++++++++++++++++ include/linux/timekeeping.h | 15 +++ 3 files changed, 201 insertions(+) create mode 100644 Documentation/core-api/timekeeping.rst -- 2.9.0 Tested-by: Randy Dunlap Reviewed-by: Randy Dunlap diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst index f5a66b72f984..989c97cc232a 100644 --- a/Documentation/core-api/index.rst +++ b/Documentation/core-api/index.rst @@ -28,6 +28,7 @@ Core utilities printk-formats circular-buffers gfp_mask-from-fs-io + timekeeping Interfaces for kernel debugging =============================== diff --git a/Documentation/core-api/timekeeping.rst b/Documentation/core-api/timekeeping.rst new file mode 100644 index 000000000000..97dafa69dddf --- /dev/null +++ b/Documentation/core-api/timekeeping.rst @@ -0,0 +1,185 @@ +ktime access +============ + +Device drivers can read the current time using ktime_get() and the many +related functions declared in linux/timekeeping.h. As a rule of thumb, +using an accessor with a shorter name is preferred over one with a longer +name if both are equally fit for a particular use case. + +Basic ktime_t based interfaces +------------------------------ + +The recommended simplest form returns an opaque ktime_t, with variants +that return time for different clock references: + + +.. c:function:: ktime_t ktime_get( void ) + + CLOCK_MONOTONIC + + Useful for reliable timestamps and measuring short time intervals + accurately. Starts at system boot time but stops during suspend. + +.. c:function:: ktime_t ktime_get_boottime( void ) + + CLOCK_BOOTTIME + + Like ktime_get(), but does not stop when suspended. This can be + used e.g. for key expiration times that need to be synchronized + with other machines across a suspend operation. + +.. c:function:: ktime_t ktime_get_real( void ) + + CLOCK_REALTIME + + Returns the time in relative to the UNIX epoch starting in 1970 + using the Coordinated Universal Time (UTC), same as gettimeofday() + user space. This is used for all timestamps that need to + persist across a reboot, like inode times, but should be avoided + for internal uses, since it can jump backwards due to a leap + second update, NTP adjustment settimeofday() operation from user + space. + +.. c:function:: ktime_t ktime_get_clocktai( void ) + + CLOCK_TAI + + Like ktime_get_real(), but uses the International Atomic Time (TAI) + reference instead of UTC to avoid jumping on leap second updates. + This is rarely useful in the kernel. + +.. c:function:: ktime_t ktime_get_raw( void ) + + CLOCK_MONOTONIC_RAW + + Like ktime_get(), but runs at the same rate as the hardware + clocksource without (NTP) adjustments for clock drift. This is + also rarely needed in the kernel. + +nanosecond, timespec64, and second output +------------------------------------- + +For all of the above, there are variants that return the time in a +different format depending on what is required by the user: + +.. c:function:: u64 ktime_get_ns( void ) + u64 ktime_get_boottime_ns( void ) + u64 ktime_get_real_ns( void ) + u64 ktime_get_tai_ns( void ) + u64 ktime_get_raw_ns( void ) + + Same as the plain ktime_get functions, but returning a u64 number + of nanoseconds in the respective time reference, which may be + more convenient for some callers. + +.. c:function:: void ktime_get_ts64( struct timespec64 * ) + void ktime_get_boottime_ts64( struct timespec64 * ) + void ktime_get_real_ts64( struct timespec64 * ) + void ktime_get_clocktai_ts64( struct timespec64 * ) + void ktime_get_raw_ts64( struct timespec64 * ) + + Same above, but returns the time in a 'struct timespec64', split + into seconds and nanoseconds. This can avoid an extra division + when printing the time, or when passing it into an external + interface that expects a 'timespec' or 'timeval' structure. + +.. c:function:: time64_t ktime_get_seconds( void ) + time64_t ktime_get_boottime_seconds( void ) + time64_t ktime_get_real_seconds( void ) + time64_t ktime_get_clocktai_seconds( void ) + time64_t ktime_get_raw_seconds( void ) + + Return a coarse-grained version of the time as a scalar + time64_t. This avoids accessing the clock hardware and rounds + down the seconds to the full seconds of the last timer tick + using the respective reference. + +Coarse and fast_ns access +------------------------- + +Some additional variants exist for more specialized cases: + +.. c:function:: ktime_t ktime_get_coarse_boottime( void ) + ktime_t ktime_get_coarse_real( void ) + ktime_t ktime_get_coarse_clocktai( void ) + ktime_t ktime_get_coarse_raw( void ) + +.. c:function:: void ktime_get_coarse_ts64( struct timespec64 * ) + void ktime_get_coarse_boottime_ts64( struct timespec64 * ) + void ktime_get_coarse_real_ts64( struct timespec64 * ) + void ktime_get_coarse_clocktai_ts64( struct timespec64 * ) + void ktime_get_coarse_raw_ts64( struct timespec64 * ) + + These are quicker than the non-coarse versions, but less accurate, + corresponding to CLOCK_MONONOTNIC_COARSE and CLOCK_REALTIME_COARSE + in user space, along with the equivalent boottime/tai/raw + timebase not available in user space. + + The time returned here corresponds to the last timer tick, which + may be as much as 10ms in the past (for CONFIG_HZ=100), same as + reading the 'jiffies' variable. These are only useful when called + in a fast path and one still expects better than second accuracy, + but can't easily use 'jiffies', e.g. for inode timestamps. + Skipping the hardware clock access saves around 100 CPU cycles + on most modern machines with a reliable cycle counter, but + up to several microseconds on older hardware with an external + clocksource. + +.. c:function:: u64 ktime_get_mono_fast_ns( void ) + u64 ktime_get_raw_fast_ns( void ) + u64 ktime_get_boot_fast_ns( void ) + u64 ktime_get_real_fast_ns( void ) + + These variants are safe to call from any context, including from + a non-maskable interrupt (NMI) during a timekeeper update, and + while we are entering suspend with the clocksource powered down. + This is useful in some tracing or debugging code as well as + machine check reporting, but most drivers should never call them, + since the time is allowed to jump under certain conditions. + +Deprecated time interfaces +-------------------------- + +Older kernels used some other interfaces that are now being phased out +but may appear in third-party drivers being ported here. In particular, +all interfaces returning a 'struct timeval' or 'struct timespec' have +been replaced because the tv_sec member overflows in year 2038 on 32-bit +architectures. These are the recommended replacements: + +.. c:function:: void ktime_get_ts( struct timespec * ) + + Use ktime_get() or ktime_get_ts64() instead. + +.. c:function:: struct timeval do_gettimeofday( void ) + struct timespec getnstimeofday( void ) + struct timespec64 getnstimeofday64( void ) + void ktime_get_real_ts( struct timespec * ) + + ktime_get_real_ts64() is a direct replacement, but consider using + monotonic time (ktime_get_ts64()) and/or a ktime_t based interface + (ktime_get()/ktime_get_real()). + +.. c:function:: struct timespec current_kernel_time( void ) + struct timespec64 current_kernel_time64( void ) + struct timespec get_monotonic_coarse( void ) + struct timespec64 get_monotonic_coarse64( void ) + + These are replaced by ktime_get_coarse_real_ts64() and + ktime_get_coarse_ts64(). However, A lot of code that wants + coarse-grained times can use the simple 'jiffies' instead, while + some drivers may actually want the higher resolution accessors + these days. + +.. c:function:: struct timespec getrawmonotonic( void ) + struct timespec64 getrawmonotonic64( void ) + struct timespec timekeeping_clocktai( void ) + struct timespec64 timekeeping_clocktai64( void ) + struct timespec get_monotonic_boottime( void ) + struct timespec64 get_monotonic_boottime64( void ) + + These are replaced by ktime_get_raw()/ktime_get_raw_ts64(), + ktime_get_clocktai()/ktime_get_clocktai_ts64() as well + as ktime_get_boottime()/ktime_get_boottime_ts64(). + However, if the particular choice of clock source is not + important for the user, consider converting to + ktime_get()/ktime_get_ts64() instead for consistency. diff --git a/include/linux/timekeeping.h b/include/linux/timekeeping.h index 86bc2026efce..947b1b8d2d01 100644 --- a/include/linux/timekeeping.h +++ b/include/linux/timekeeping.h @@ -21,6 +21,21 @@ extern int do_sys_settimeofday64(const struct timespec64 *tv, const struct timezone *tz); /* + * ktime_get() family: read the current time in a multitude of ways, + * + * The default time reference is CLOCK_MONOTONIC, starting at + * boot time but not counting the time spent in suspend. + * For other references, use the functions with "real", "clocktai", + * "boottime" and "raw" suffixes. + * + * To get the time in a different format, use the ones wit + * "ns", "ts64" and "seconds" suffix. + * + * See Documentation/core-api/timekeeping.rst for more details. + */ + + +/* * timespec64 based interfaces */ extern void ktime_get_raw_ts64(struct timespec64 *ts);