1 2 Real Time Clock (RTC) Drivers for Linux 3 ======================================= 4 5When Linux developers talk about a "Real Time Clock", they usually mean 6something that tracks wall clock time and is battery backed so that it 7works even with system power off. Such clocks will normally not track 8the local time zone or daylight savings time -- unless they dual boot 9with MS-Windows -- but will instead be set to Coordinated Universal Time 10(UTC, formerly "Greenwich Mean Time"). 11 12The newest non-PC hardware tends to just count seconds, like the time(2) 13system call reports, but RTCs also very commonly represent time using 14the Gregorian calendar and 24 hour time, as reported by gmtime(3). 15 16Linux has two largely-compatible userspace RTC API families you may 17need to know about: 18 19 * /dev/rtc ... is the RTC provided by PC compatible systems, 20 so it's not very portable to non-x86 systems. 21 22 * /dev/rtc0, /dev/rtc1 ... are part of a framework that's 23 supported by a wide variety of RTC chips on all systems. 24 25Programmers need to understand that the PC/AT functionality is not 26always available, and some systems can do much more. That is, the 27RTCs use the same API to make requests in both RTC frameworks (using 28different filenames of course), but the hardware may not offer the 29same functionality. For example, not every RTC is hooked up to an 30IRQ, so they can't all issue alarms; and where standard PC RTCs can 31only issue an alarm up to 24 hours in the future, other hardware may 32be able to schedule one any time in the upcoming century. 33 34 35 Old PC/AT-Compatible driver: /dev/rtc 36 -------------------------------------- 37 38All PCs (even Alpha machines) have a Real Time Clock built into them. 39Usually they are built into the chipset of the computer, but some may 40actually have a Motorola MC146818 (or clone) on the board. This is the 41clock that keeps the date and time while your computer is turned off. 42 43ACPI has standardized that MC146818 functionality, and extended it in 44a few ways (enabling longer alarm periods, and wake-from-hibernate). 45That functionality is NOT exposed in the old driver. 46 47However it can also be used to generate signals from a slow 2Hz to a 48relatively fast 8192Hz, in increments of powers of two. These signals 49are reported by interrupt number 8. (Oh! So *that* is what IRQ 8 is 50for...) It can also function as a 24hr alarm, raising IRQ 8 when the 51alarm goes off. The alarm can also be programmed to only check any 52subset of the three programmable values, meaning that it could be set to 53ring on the 30th second of the 30th minute of every hour, for example. 54The clock can also be set to generate an interrupt upon every clock 55update, thus generating a 1Hz signal. 56 57The interrupts are reported via /dev/rtc (major 10, minor 135, read only 58character device) in the form of an unsigned long. The low byte contains 59the type of interrupt (update-done, alarm-rang, or periodic) that was 60raised, and the remaining bytes contain the number of interrupts since 61the last read. Status information is reported through the pseudo-file 62/proc/driver/rtc if the /proc filesystem was enabled. The driver has 63built in locking so that only one process is allowed to have the /dev/rtc 64interface open at a time. 65 66A user process can monitor these interrupts by doing a read(2) or a 67select(2) on /dev/rtc -- either will block/stop the user process until 68the next interrupt is received. This is useful for things like 69reasonably high frequency data acquisition where one doesn't want to 70burn up 100% CPU by polling gettimeofday etc. etc. 71 72At high frequencies, or under high loads, the user process should check 73the number of interrupts received since the last read to determine if 74there has been any interrupt "pileup" so to speak. Just for reference, a 75typical 486-33 running a tight read loop on /dev/rtc will start to suffer 76occasional interrupt pileup (i.e. > 1 IRQ event since last read) for 77frequencies above 1024Hz. So you really should check the high bytes 78of the value you read, especially at frequencies above that of the 79normal timer interrupt, which is 100Hz. 80 81Programming and/or enabling interrupt frequencies greater than 64Hz is 82only allowed by root. This is perhaps a bit conservative, but we don't want 83an evil user generating lots of IRQs on a slow 386sx-16, where it might have 84a negative impact on performance. This 64Hz limit can be changed by writing 85a different value to /proc/sys/dev/rtc/max-user-freq. Note that the 86interrupt handler is only a few lines of code to minimize any possibility 87of this effect. 88 89Also, if the kernel time is synchronized with an external source, the 90kernel will write the time back to the CMOS clock every 11 minutes. In 91the process of doing this, the kernel briefly turns off RTC periodic 92interrupts, so be aware of this if you are doing serious work. If you 93don't synchronize the kernel time with an external source (via ntp or 94whatever) then the kernel will keep its hands off the RTC, allowing you 95exclusive access to the device for your applications. 96 97The alarm and/or interrupt frequency are programmed into the RTC via 98various ioctl(2) calls as listed in ./include/linux/rtc.h 99Rather than write 50 pages describing the ioctl() and so on, it is 100perhaps more useful to include a small test program that demonstrates 101how to use them, and demonstrates the features of the driver. This is 102probably a lot more useful to people interested in writing applications 103that will be using this driver. See the code at the end of this document. 104 105(The original /dev/rtc driver was written by Paul Gortmaker.) 106 107 108 New portable "RTC Class" drivers: /dev/rtcN 109 -------------------------------------------- 110 111Because Linux supports many non-ACPI and non-PC platforms, some of which 112have more than one RTC style clock, it needed a more portable solution 113than expecting a single battery-backed MC146818 clone on every system. 114Accordingly, a new "RTC Class" framework has been defined. It offers 115three different userspace interfaces: 116 117 * /dev/rtcN ... much the same as the older /dev/rtc interface 118 119 * /sys/class/rtc/rtcN ... sysfs attributes support readonly 120 access to some RTC attributes. 121 122 * /proc/driver/rtc ... the first RTC (rtc0) may expose itself 123 using a procfs interface. More information is (currently) shown 124 here than through sysfs. 125 126The RTC Class framework supports a wide variety of RTCs, ranging from those 127integrated into embeddable system-on-chip (SOC) processors to discrete chips 128using I2C, SPI, or some other bus to communicate with the host CPU. There's 129even support for PC-style RTCs ... including the features exposed on newer PCs 130through ACPI. 131 132The new framework also removes the "one RTC per system" restriction. For 133example, maybe the low-power battery-backed RTC is a discrete I2C chip, but 134a high functionality RTC is integrated into the SOC. That system might read 135the system clock from the discrete RTC, but use the integrated one for all 136other tasks, because of its greater functionality. 137 138The ioctl() calls supported by /dev/rtc are also supported by the RTC class 139framework. However, because the chips and systems are not standardized, 140some PC/AT functionality might not be provided. And in the same way, some 141newer features -- including those enabled by ACPI -- are exposed by the 142RTC class framework, but can't be supported by the older driver. 143 144 * RTC_RD_TIME, RTC_SET_TIME ... every RTC supports at least reading 145 time, returning the result as a Gregorian calendar date and 24 hour 146 wall clock time. To be most useful, this time may also be updated. 147 148 * RTC_AIE_ON, RTC_AIE_OFF, RTC_ALM_SET, RTC_ALM_READ ... when the RTC 149 is connected to an IRQ line, it can often issue an alarm IRQ up to 150 24 hours in the future. (Use RTC_WKALM_* by preference.) 151 152 * RTC_WKALM_SET, RTC_WKALM_RD ... RTCs that can issue alarms beyond 153 the next 24 hours use a slightly more powerful API, which supports 154 setting the longer alarm time and enabling its IRQ using a single 155 request (using the same model as EFI firmware). 156 157 * RTC_UIE_ON, RTC_UIE_OFF ... if the RTC offers IRQs, it probably 158 also offers update IRQs whenever the "seconds" counter changes. 159 If needed, the RTC framework can emulate this mechanism. 160 161 * RTC_PIE_ON, RTC_PIE_OFF, RTC_IRQP_SET, RTC_IRQP_READ ... another 162 feature often accessible with an IRQ line is a periodic IRQ, issued 163 at settable frequencies (usually 2^N Hz). 164 165In many cases, the RTC alarm can be a system wake event, used to force 166Linux out of a low power sleep state (or hibernation) back to a fully 167operational state. For example, a system could enter a deep power saving 168state until it's time to execute some scheduled tasks. 169 170Note that many of these ioctls need not actually be implemented by your 171driver. The common rtc-dev interface handles many of these nicely if your 172driver returns ENOIOCTLCMD. Some common examples: 173 174 * RTC_RD_TIME, RTC_SET_TIME: the read_time/set_time functions will be 175 called with appropriate values. 176 177 * RTC_ALM_SET, RTC_ALM_READ, RTC_WKALM_SET, RTC_WKALM_RD: the 178 set_alarm/read_alarm functions will be called. 179 180 * RTC_IRQP_SET, RTC_IRQP_READ: the irq_set_freq function will be called 181 to set the frequency while the framework will handle the read for you 182 since the frequency is stored in the irq_freq member of the rtc_device 183 structure. Also make sure you set the max_user_freq member in your 184 initialization routines so the framework can sanity check the user 185 input for you. 186 187If all else fails, check out the rtc-test.c driver! 188 189 190-------------------- 8< ---------------- 8< ----------------------------- 191 192/* 193 * Real Time Clock Driver Test/Example Program 194 * 195 * Compile with: 196 * gcc -s -Wall -Wstrict-prototypes rtctest.c -o rtctest 197 * 198 * Copyright (C) 1996, Paul Gortmaker. 199 * 200 * Released under the GNU General Public License, version 2, 201 * included herein by reference. 202 * 203 */ 204 205#include <stdio.h> 206#include <linux/rtc.h> 207#include <sys/ioctl.h> 208#include <sys/time.h> 209#include <sys/types.h> 210#include <fcntl.h> 211#include <unistd.h> 212#include <stdlib.h> 213#include <errno.h> 214 215 216/* 217 * This expects the new RTC class driver framework, working with 218 * clocks that will often not be clones of what the PC-AT had. 219 * Use the command line to specify another RTC if you need one. 220 */ 221static const char default_rtc[] = "/dev/rtc0"; 222 223 224int main(int argc, char **argv) 225{ 226 int i, fd, retval, irqcount = 0; 227 unsigned long tmp, data; 228 struct rtc_time rtc_tm; 229 const char *rtc = default_rtc; 230 231 switch (argc) { 232 case 2: 233 rtc = argv[1]; 234 /* FALLTHROUGH */ 235 case 1: 236 break; 237 default: 238 fprintf(stderr, "usage: rtctest [rtcdev]\n"); 239 return 1; 240 } 241 242 fd = open(rtc, O_RDONLY); 243 244 if (fd == -1) { 245 perror(rtc); 246 exit(errno); 247 } 248 249 fprintf(stderr, "\n\t\t\tRTC Driver Test Example.\n\n"); 250 251 /* Turn on update interrupts (one per second) */ 252 retval = ioctl(fd, RTC_UIE_ON, 0); 253 if (retval == -1) { 254 if (errno == ENOTTY) { 255 fprintf(stderr, 256 "\n...Update IRQs not supported.\n"); 257 goto test_READ; 258 } 259 perror("RTC_UIE_ON ioctl"); 260 exit(errno); 261 } 262 263 fprintf(stderr, "Counting 5 update (1/sec) interrupts from reading %s:", 264 rtc); 265 fflush(stderr); 266 for (i=1; i<6; i++) { 267 /* This read will block */ 268 retval = read(fd, &data, sizeof(unsigned long)); 269 if (retval == -1) { 270 perror("read"); 271 exit(errno); 272 } 273 fprintf(stderr, " %d",i); 274 fflush(stderr); 275 irqcount++; 276 } 277 278 fprintf(stderr, "\nAgain, from using select(2) on /dev/rtc:"); 279 fflush(stderr); 280 for (i=1; i<6; i++) { 281 struct timeval tv = {5, 0}; /* 5 second timeout on select */ 282 fd_set readfds; 283 284 FD_ZERO(&readfds); 285 FD_SET(fd, &readfds); 286 /* The select will wait until an RTC interrupt happens. */ 287 retval = select(fd+1, &readfds, NULL, NULL, &tv); 288 if (retval == -1) { 289 perror("select"); 290 exit(errno); 291 } 292 /* This read won't block unlike the select-less case above. */ 293 retval = read(fd, &data, sizeof(unsigned long)); 294 if (retval == -1) { 295 perror("read"); 296 exit(errno); 297 } 298 fprintf(stderr, " %d",i); 299 fflush(stderr); 300 irqcount++; 301 } 302 303 /* Turn off update interrupts */ 304 retval = ioctl(fd, RTC_UIE_OFF, 0); 305 if (retval == -1) { 306 perror("RTC_UIE_OFF ioctl"); 307 exit(errno); 308 } 309 310test_READ: 311 /* Read the RTC time/date */ 312 retval = ioctl(fd, RTC_RD_TIME, &rtc_tm); 313 if (retval == -1) { 314 perror("RTC_RD_TIME ioctl"); 315 exit(errno); 316 } 317 318 fprintf(stderr, "\n\nCurrent RTC date/time is %d-%d-%d, %02d:%02d:%02d.\n", 319 rtc_tm.tm_mday, rtc_tm.tm_mon + 1, rtc_tm.tm_year + 1900, 320 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec); 321 322 /* Set the alarm to 5 sec in the future, and check for rollover */ 323 rtc_tm.tm_sec += 5; 324 if (rtc_tm.tm_sec >= 60) { 325 rtc_tm.tm_sec %= 60; 326 rtc_tm.tm_min++; 327 } 328 if (rtc_tm.tm_min == 60) { 329 rtc_tm.tm_min = 0; 330 rtc_tm.tm_hour++; 331 } 332 if (rtc_tm.tm_hour == 24) 333 rtc_tm.tm_hour = 0; 334 335 retval = ioctl(fd, RTC_ALM_SET, &rtc_tm); 336 if (retval == -1) { 337 if (errno == ENOTTY) { 338 fprintf(stderr, 339 "\n...Alarm IRQs not supported.\n"); 340 goto test_PIE; 341 } 342 perror("RTC_ALM_SET ioctl"); 343 exit(errno); 344 } 345 346 /* Read the current alarm settings */ 347 retval = ioctl(fd, RTC_ALM_READ, &rtc_tm); 348 if (retval == -1) { 349 perror("RTC_ALM_READ ioctl"); 350 exit(errno); 351 } 352 353 fprintf(stderr, "Alarm time now set to %02d:%02d:%02d.\n", 354 rtc_tm.tm_hour, rtc_tm.tm_min, rtc_tm.tm_sec); 355 356 /* Enable alarm interrupts */ 357 retval = ioctl(fd, RTC_AIE_ON, 0); 358 if (retval == -1) { 359 perror("RTC_AIE_ON ioctl"); 360 exit(errno); 361 } 362 363 fprintf(stderr, "Waiting 5 seconds for alarm..."); 364 fflush(stderr); 365 /* This blocks until the alarm ring causes an interrupt */ 366 retval = read(fd, &data, sizeof(unsigned long)); 367 if (retval == -1) { 368 perror("read"); 369 exit(errno); 370 } 371 irqcount++; 372 fprintf(stderr, " okay. Alarm rang.\n"); 373 374 /* Disable alarm interrupts */ 375 retval = ioctl(fd, RTC_AIE_OFF, 0); 376 if (retval == -1) { 377 perror("RTC_AIE_OFF ioctl"); 378 exit(errno); 379 } 380 381test_PIE: 382 /* Read periodic IRQ rate */ 383 retval = ioctl(fd, RTC_IRQP_READ, &tmp); 384 if (retval == -1) { 385 /* not all RTCs support periodic IRQs */ 386 if (errno == ENOTTY) { 387 fprintf(stderr, "\nNo periodic IRQ support\n"); 388 return 0; 389 } 390 perror("RTC_IRQP_READ ioctl"); 391 exit(errno); 392 } 393 fprintf(stderr, "\nPeriodic IRQ rate is %ldHz.\n", tmp); 394 395 fprintf(stderr, "Counting 20 interrupts at:"); 396 fflush(stderr); 397 398 /* The frequencies 128Hz, 256Hz, ... 8192Hz are only allowed for root. */ 399 for (tmp=2; tmp<=64; tmp*=2) { 400 401 retval = ioctl(fd, RTC_IRQP_SET, tmp); 402 if (retval == -1) { 403 /* not all RTCs can change their periodic IRQ rate */ 404 if (errno == ENOTTY) { 405 fprintf(stderr, 406 "\n...Periodic IRQ rate is fixed\n"); 407 goto done; 408 } 409 perror("RTC_IRQP_SET ioctl"); 410 exit(errno); 411 } 412 413 fprintf(stderr, "\n%ldHz:\t", tmp); 414 fflush(stderr); 415 416 /* Enable periodic interrupts */ 417 retval = ioctl(fd, RTC_PIE_ON, 0); 418 if (retval == -1) { 419 perror("RTC_PIE_ON ioctl"); 420 exit(errno); 421 } 422 423 for (i=1; i<21; i++) { 424 /* This blocks */ 425 retval = read(fd, &data, sizeof(unsigned long)); 426 if (retval == -1) { 427 perror("read"); 428 exit(errno); 429 } 430 fprintf(stderr, " %d",i); 431 fflush(stderr); 432 irqcount++; 433 } 434 435 /* Disable periodic interrupts */ 436 retval = ioctl(fd, RTC_PIE_OFF, 0); 437 if (retval == -1) { 438 perror("RTC_PIE_OFF ioctl"); 439 exit(errno); 440 } 441 } 442 443done: 444 fprintf(stderr, "\n\n\t\t\t *** Test complete ***\n"); 445 446 close(fd); 447 448 return 0; 449} 450