From: David van Moolenbroek <david@minix3.org>
Date: Mon, 12 Jul 2010 23:14:40 +0000 (+0000)
Subject: libsys: add standard condition spinning primitives
X-Git-Tag: v3.1.8~253
X-Git-Url: http://zhaoyanbai.com/repos/rndc-confgen.html?a=commitdiff_plain;h=1ecdac623a21665ac78f6f8d351a8b68d7e6b1c0;p=minix.git

libsys: add standard condition spinning primitives
---

diff --git a/drivers/at_wini/at_wini.c b/drivers/at_wini/at_wini.c
index 4cbd3f22b..3a2b96516 100644
--- a/drivers/at_wini/at_wini.c
+++ b/drivers/at_wini/at_wini.c
@@ -44,7 +44,7 @@ struct command {
 };
 
 /* Timeouts and max retries. */
-PRIVATE int timeout_ticks = DEF_TIMEOUT_TICKS;
+PRIVATE int timeout_usecs = DEF_TIMEOUT_USECS;
 PRIVATE int max_errors = MAX_ERRORS;
 PRIVATE long w_standard_timeouts = 0;
 PRIVATE long w_pci_debug = 0;
@@ -941,12 +941,12 @@ PRIVATE int w_io_test(void)
 	save_dev = w_device;
 
 	/* Reduce timeout values for this test transaction. */
-	save_timeout = timeout_ticks;
+	save_timeout = timeout_usecs;
 	save_errors = max_errors;
 	save_wakeup = wakeup_ticks;
 
 	if (!w_standard_timeouts) {
-		timeout_ticks = system_hz * 4;
+		timeout_usecs = 4000000;
 		wakeup_ticks = system_hz * 6;
 		max_errors = 3;
 	}
@@ -964,7 +964,7 @@ PRIVATE int w_io_test(void)
  		panic("Couldn't switch back devices");
 
  	/* Restore parameters. */
-	timeout_ticks = save_timeout;
+	timeout_usecs = save_timeout;
 	max_errors = save_errors;
 	wakeup_ticks = save_wakeup;
 	w_testing = 0;
@@ -1920,24 +1920,19 @@ int mask;			/* status mask */
 int value;			/* required status */
 {
 /* Wait until controller is in the required state.  Return zero on timeout.
- * An alarm that set a timeout flag is used. TIMEOUT is in micros, we need
- * ticks. Disabling the alarm is not needed, because a static flag is used
- * and a leftover timeout cannot do any harm.
  */
   unsigned long w_status;
-  clock_t t0, t1;
+  spin_t spin;
   int s;
 
-  getuptime(&t0);
-  do {
+  SPIN_FOR(&spin, timeout_usecs) {
 	if ((s=sys_inb(w_wn->base_cmd + REG_STATUS, &w_status)) != OK)
 		panic("Couldn't read register: %d", s);
 	w_wn->w_status= w_status;
 	if ((w_wn->w_status & mask) == value) {
         	return 1;
 	}
-  } while ((s=getuptime(&t1)) == OK && (t1-t0) < timeout_ticks );
-  if (OK != s) printf("AT_WINI: warning, get_uptime failed: %d\n",s);
+  }
 
   w_need_reset();			/* controller gone deaf */
   return(0);
@@ -1951,23 +1946,18 @@ int mask;			/* status mask */
 int value;			/* required status */
 {
 /* Wait until controller is in the required state.  Return zero on timeout.
- * An alarm that set a timeout flag is used. TIMEOUT is in micros, we need
- * ticks. Disabling the alarm is not needed, because a static flag is used
- * and a leftover timeout cannot do any harm.
  */
   unsigned long w_status;
-  clock_t t0, t1;
+  spin_t spin;
   int s;
 
-  getuptime(&t0);
-  do {
+  SPIN_FOR(&spin, timeout_usecs) {
 	if ((s=sys_inb(w_wn->base_dma + DMA_STATUS, &w_status)) != OK)
 		panic("Couldn't read register: %d", s);
 	if ((w_status & mask) == value) {
         	return 1;
 	}
-  } while ((s=getuptime(&t1)) == OK && (t1-t0) < timeout_ticks );
-  if (OK != s) printf("AT_WINI: warning, get_uptime failed: %d\n",s);
+  }
 
   return(0);
 }
@@ -2320,7 +2310,7 @@ message *m;
 	
 		if (timeout == 0) {
 			/* Restore defaults. */
-			timeout_ticks = DEF_TIMEOUT_TICKS;
+			timeout_usecs = DEF_TIMEOUT_USECS;
 			max_errors = MAX_ERRORS;
 			wakeup_ticks = WAKEUP_TICKS;
 			w_silent = 0;
@@ -2336,9 +2326,11 @@ message *m;
 				wakeup_ticks = timeout;
 				max_errors = 3;
 				w_silent = 1;
+
+				timeout = timeout * 1000000 / sys_hz();
 	
-				if (timeout_ticks > timeout)
-					timeout_ticks = timeout;
+				if (timeout_usecs > timeout)
+					timeout_usecs = timeout;
 			}
 	
 		  	r= sys_safecopyto(m->IO_ENDPT,
diff --git a/drivers/at_wini/at_wini.h b/drivers/at_wini/at_wini.h
index aefa1826b..95a26bbeb 100644
--- a/drivers/at_wini/at_wini.h
+++ b/drivers/at_wini/at_wini.h
@@ -200,7 +200,7 @@
 #define NR_SUBDEVS	(MAX_DRIVES * SUB_PER_DRIVE)
 #define DELAY_USECS     1000	/* controller timeout in microseconds */
 #define DELAY_TICKS 	   1	/* controller timeout in ticks */
-#define DEF_TIMEOUT_TICKS 	300	/* controller timeout in ticks */
+#define DEF_TIMEOUT_USECS 5000000L  /* controller timeout in microseconds */
 #define RECOVERY_USECS 500000	/* controller recovery time in microseconds */
 #define RECOVERY_TICKS    30	/* controller recovery time in ticks */
 #define INITIALIZED	0x01	/* drive is initialized */
diff --git a/drivers/floppy/floppy.c b/drivers/floppy/floppy.c
index 2a706e5a1..d79c0fe9c 100644
--- a/drivers/floppy/floppy.c
+++ b/drivers/floppy/floppy.c
@@ -130,8 +130,7 @@
 #define NR_DRIVES          2	/* maximum number of drives */
 #define DIVISOR          128	/* used for sector size encoding */
 #define SECTOR_SIZE_CODE   2	/* code to say "512" to the controller */
-#define TIMEOUT_MICROS   500000L	/* microseconds waiting for FDC */
-#define TIMEOUT_TICKS     30	/* ticks waiting for FDC */
+#define TIMEOUT_MICROS   5000000L	/* microseconds waiting for FDC */
 #define NT                 7	/* number of diskette/drive combinations */
 #define UNCALIBRATED       0	/* drive needs to be calibrated at next use */
 #define CALIBRATED         1	/* no calibration needed */
@@ -950,15 +949,14 @@ PRIVATE int fdc_results(void)
 
   int s, result_nr;
   unsigned long status;
-  clock_t t0,t1;
+  spin_t spin;
 
   /* Extract bytes from FDC until it says it has no more.  The loop is
    * really an outer loop on result_nr and an inner loop on status. 
    * A timeout flag alarm is set.
    */
   result_nr = 0;
-  getuptime(&t0);
-  do {
+  SPIN_FOR(&spin, TIMEOUT_MICROS) {
 	/* Reading one byte is almost a mirror of fdc_out() - the DIRECTION
 	 * bit must be set instead of clear, but the CTL_BUSY bit destroys
 	 * the perfection of the mirror.
@@ -981,8 +979,7 @@ PRIVATE int fdc_results(void)
 
 		return(OK);			/* only good exit */
 	}
-  } while ( (s=getuptime(&t1))==OK && (t1-t0) < TIMEOUT_TICKS );
-  if (OK!=s) printf("FLOPPY: warning, getuptime failed: %d\n", s); 
+  }
   need_reset = TRUE;		/* controller chip must be reset */
 
   if ((s=sys_irqenable(&irq_hook_id)) != OK)
@@ -1026,27 +1023,26 @@ PRIVATE void fdc_out(
  * can only write to it when it is listening, and it decides when to listen.
  * If the controller refuses to listen, the FDC chip is given a hard reset.
  */
-  clock_t t0, t1;
+  spin_t spin;
   int s;
   unsigned long status;
 
   if (need_reset) return;	/* if controller is not listening, return */
 
   /* It may take several tries to get the FDC to accept a command.  */
-  getuptime(&t0);
-  do {
-  	if ( (s=getuptime(&t1))==OK && (t1-t0) > TIMEOUT_TICKS ) {
-  		if (OK!=s) printf("FLOPPY: warning, getuptime failed: %d\n", s); 
-		need_reset = TRUE;	/* hit it over the head */
-		return;
-	}
+  SPIN_FOR(&spin, TIMEOUT_MICROS) {
   	if ((s=sys_inb(FDC_STATUS, &status)) != OK)
   		panic("Sys_inb in fdc_out() failed: %d", s);
+
+  	if ((status & (MASTER | DIRECTION)) == (MASTER | 0)) {
+		if ((s=sys_outb(FDC_DATA, val)) != OK)
+			panic("Sys_outb in fdc_out() failed: %d", s);
+
+		return;
+	}
   }
-  while ((status & (MASTER | DIRECTION)) != (MASTER | 0)); 
-  
-  if ((s=sys_outb(FDC_DATA, val)) != OK)
-	panic("Sys_outb in fdc_out() failed: %d", s);
+
+  need_reset = TRUE;	/* hit it over the head */
 }
 
 /*===========================================================================*
diff --git a/drivers/fxp/fxp.c b/drivers/fxp/fxp.c
index 3271c6620..544ba53bc 100644
--- a/drivers/fxp/fxp.c
+++ b/drivers/fxp/fxp.c
@@ -946,7 +946,6 @@ static void fxp_confaddr(fxp_t *fp)
 {
 	static char eakey[]= FXP_ENVVAR "#_EA";
 	static char eafmt[]= "x:x:x:x:x:x";
-	clock_t t0,t1;
 	int i, r;
 	u32_t bus_addr;
 	long v;
@@ -987,12 +986,8 @@ static void fxp_confaddr(fxp_t *fp)
 
 	fxp_cu_ptr_cmd(fp, SC_CU_START, bus_addr, TRUE /* check idle */);
 
-	getuptime(&t0);
-	do {
-		/* Wait for CU command to complete */
-		if (tmpbufp->ias.ias_status & CBL_F_C)
-			break;
-	} while (getuptime(&t1)==OK && (t1-t0) < micros_to_ticks(1000));
+	/* Wait for CU command to complete */
+	SPIN_UNTIL(tmpbufp->ias.ias_status & CBL_F_C, 1000);
 
 	if (!(tmpbufp->ias.ias_status & CBL_F_C))
 		panic("fxp_confaddr: CU command failed to complete");
@@ -1356,7 +1351,6 @@ fxp_t *fp;
 {
 	int r;
 	u32_t bus_addr;
-	clock_t t0,t1;
 
 	/* Configure device */
 	tmpbufp->cc.cc_status= 0;
@@ -1372,12 +1366,8 @@ fxp_t *fp;
 
 	fxp_cu_ptr_cmd(fp, SC_CU_START, bus_addr, TRUE /* check idle */);
 
-	getuptime(&t0);
-	do {
-		/* Wait for CU command to complete */
-		if (tmpbufp->cc.cc_status & CBL_F_C)
-			break;
-	} while (getuptime(&t1)==OK && (t1-t0) < micros_to_ticks(100000));
+	/* Wait for CU command to complete */
+	SPIN_UNTIL(tmpbufp->cc.cc_status & CBL_F_C, 100000);
 
 	if (!(tmpbufp->cc.cc_status & CBL_F_C))
 		panic("fxp_do_conf: CU command failed to complete");
@@ -1395,7 +1385,7 @@ int cmd;
 phys_bytes bus_addr;
 int check_idle;
 {
-	clock_t t0,t1;
+	spin_t spin;
 	port_t port;
 	u8_t scb_cmd;
 
@@ -1412,15 +1402,15 @@ int check_idle;
 	fxp_outb(port, SCB_CMD, cmd);
 
 	/* What is a reasonable time-out? There is nothing in the
-	 * documentation. 1 ms should be enough.
+	 * documentation. 1 ms should be enough. We use 100 ms.
 	 */
-	getuptime(&t0);
+	spin_init(&spin, 100000);
 	do {
 		/* Wait for CU command to be accepted */
 		scb_cmd= fxp_inb(port, SCB_CMD);
 		if ((scb_cmd & SC_CUC_MASK) == SC_CU_NOP)
 			break;
-	} while (getuptime(&t1)==OK && (t1-t0) < micros_to_ticks(100000));
+	} while (spin_check(&spin));
 
 	if ((scb_cmd & SC_CUC_MASK) != SC_CU_NOP)
 		panic("fxp_cu_ptr_cmd: CU does not accept command");
@@ -1435,7 +1425,7 @@ int cmd;
 phys_bytes bus_addr;
 int check_idle;
 {
-	clock_t t0,t1;
+	spin_t spin;
 	port_t port;
 	u8_t scb_cmd;
 
@@ -1451,13 +1441,13 @@ int check_idle;
 	fxp_outl(port, SCB_POINTER, bus_addr);
 	fxp_outb(port, SCB_CMD, cmd);
 
-	getuptime(&t0);
+	spin_init(&spin, 1000);
 	do {
 		/* Wait for RU command to be accepted */
 		scb_cmd= fxp_inb(port, SCB_CMD);
 		if ((scb_cmd & SC_RUC_MASK) == SC_RU_NOP)
 			break;
-	} while (getuptime(&t1)==OK && (t1-t0) < micros_to_ticks(1000));
+	} while (spin_check(&spin));
 
 	if ((scb_cmd & SC_RUC_MASK) != SC_RU_NOP)
 		panic("fxp_ru_ptr_cmd: RU does not accept command");
@@ -1501,7 +1491,6 @@ fxp_t *fp;
  *===========================================================================*/
 static void fxp_getstat_s(message *mp)
 {
-	clock_t t0,t1;
 	int r;
 	fxp_t *fp;
 	u32_t *p;
@@ -1520,12 +1509,8 @@ static void fxp_getstat_s(message *mp)
 	 */
 	fxp_cu_ptr_cmd(fp, SC_CU_DUMP_SC, 0, FALSE /* do not check idle */);
 
-	getuptime(&t0);
-	do {
-		/* Wait for CU command to complete */
-		if (*p != 0)
-			break;
-	} while (getuptime(&t1)==OK && (t1-t0) < micros_to_ticks(1000));
+	/* Wait for CU command to complete */
+	SPIN_UNTIL(*p != 0, 1000);
 
 	if (*p == 0)
 		panic("fxp_getstat: CU command failed to complete");
@@ -2220,7 +2205,7 @@ PRIVATE u16_t mii_read(fp, reg)
 fxp_t *fp;
 int reg;
 {
-	clock_t t0,t1;
+	spin_t spin;
 	port_t port;
 	u32_t v;
 
@@ -2234,12 +2219,12 @@ int reg;
 	fxp_outl(port, CSR_MDI_CTL, CM_READ | (1 << CM_PHYADDR_SHIFT) |
 		(reg << CM_REG_SHIFT));
 
-	getuptime(&t0);
+	spin_init(&spin, 100000);
 	do {
 		v= fxp_inl(port, CSR_MDI_CTL);
 		if (v & CM_READY)
 			break;
-	} while (getuptime(&t1)==OK && (t1-t0) < micros_to_ticks(100000));
+	} while (spin_check(&spin));
 
 	if (!(v & CM_READY))
 		panic("mii_read: MDI not ready after command");
diff --git a/drivers/rtl8139/rtl8139.c b/drivers/rtl8139/rtl8139.c
index c7cef8c84..c7c182e74 100644
--- a/drivers/rtl8139/rtl8139.c
+++ b/drivers/rtl8139/rtl8139.c
@@ -689,18 +689,13 @@ re_t *rep;
 	u32_t t;
 	phys_bytes bus_buf;
 	int i;
-	clock_t t0,t1;
 
 	port= rep->re_base_port;
 
 #if 0
 	/* Reset the PHY */
 	rl_outb(port, RL_BMCR, MII_CTRL_RST);
-	getuptime(&t0);
-	do {
-		if (!(rl_inb(port, RL_BMCR) & MII_CTRL_RST))
-			break;
-	} while (getuptime(&t1)==OK && (t1-t0) < system_hz);
+	SPIN_UNTIL(!(rl_inb(port, RL_BMCR) & MII_CTRL_RST), 1000000);
 	if (rl_inb(port, RL_BMCR) & MII_CTRL_RST)
 		panic("reset PHY failed to complete");
 #endif
@@ -711,11 +706,7 @@ re_t *rep;
 		port, rl_inb(port, RL_CR));
 #endif
 	rl_outb(port, RL_CR, RL_CR_RST);
-	getuptime(&t0);
-	do {
-		if (!(rl_inb(port, RL_CR) & RL_CR_RST))
-			break;
-	} while (getuptime(&t1)==OK && (t1-t0) < system_hz);
+	SPIN_UNTIL(!(rl_inb(port, RL_CR) & RL_CR_RST), 1000000);
 #if VERBOSE
 	printf("rl_reset_hw: (after reset) port = 0x%x, RL_CR = 0x%x\n",
 		port, rl_inb(port, RL_CR));
@@ -1572,7 +1563,6 @@ static void rl_clear_rx(re_t *rep)
 {
 	port_t port;
 	u8_t cr;
-	clock_t t0,t1;
 
 	rep->re_clear_rx= FALSE;
 	port= rep->re_base_port;
@@ -1581,11 +1571,7 @@ static void rl_clear_rx(re_t *rep)
 	cr= rl_inb(port, RL_CR);
 	cr &= ~RL_CR_RE;
 	rl_outb(port, RL_CR, cr);
-	getuptime(&t0);
-	do {
-		if (!(rl_inb(port, RL_CR) & RL_CR_RE))
-			break;
-	} while (getuptime(&t1)==OK && (t1-t0) < system_hz);
+	SPIN_UNTIL(!(rl_inb(port, RL_CR) & RL_CR_RE), 1000000);
 	if (rl_inb(port, RL_CR) & RL_CR_RE)
 		panic("cannot disable receiver");
 
@@ -1819,7 +1805,6 @@ static int rl_handler(re_t *rep)
 #if 0
 	u8_t cr;
 #endif
-	clock_t t0,t1;
 	int_event_check = FALSE;	/* disable check by default */
 
 	port= rep->re_base_port;
@@ -1912,11 +1897,7 @@ static int rl_handler(re_t *rep)
 			cr= rl_inb(port, RL_CR);
 			cr &= ~RL_CR_TE;
 			rl_outb(port, RL_CR, cr);
-			getuptime(&t0);
-			do {
-				if (!(rl_inb(port, RL_CR) & RL_CR_TE))
-					break;
-			} while (getuptime(&t1)==OK && (t1-t0) < system_hz);
+			SPIN_UNTIL(!(rl_inb(port, RL_CR) & RL_CR_TE), 1000000);
 			if (rl_inb(port, RL_CR) & RL_CR_TE) {
 				panic("cannot disable transmitter");
 			}
@@ -2289,7 +2270,6 @@ int a;
 u16_t w;
 {
 	int b, i, cmd;
-	clock_t t0, t1;
 
 	outb_reg3(dep, 1, 0x80 | 0x8);		/* Set CS */
 
@@ -2316,11 +2296,7 @@ u16_t w;
 	outb_reg3(dep, 1, 0x80);	/* Drop CS */
 	/* micro_delay(1); */			/* Is this required? */
 	outb_reg3(dep, 1, 0x80 | 0x8);		/* Set CS */
-	getuptime(&t0);
-	do {
-		if (inb_reg3(dep, 1) & 1)
-			break;
-	} while (getuptime(&t1) == OK && (t1 == t0));
+	SPIN_UNTIL(inb_reg3(dep, 1) & 1, 10000);
 	if (!(inb_reg3(dep, 1) & 1))
 		panic("device remains busy");
 }
diff --git a/drivers/rtl8169/rtl8169.c b/drivers/rtl8169/rtl8169.c
index 7141d9a48..f802992db 100644
--- a/drivers/rtl8169/rtl8169.c
+++ b/drivers/rtl8169/rtl8169.c
@@ -1015,11 +1015,7 @@ re_t *rep;
 	printf("rl_reset_hw: (before reset) port = 0x%x, RL_CR = 0x%x\n",
 		port, rl_inb(port, RL_CR));
 	rl_outb(port, RL_CR, RL_CR_RST);
-	getuptime(&t0);
-	do {
-		if (!(rl_inb(port, RL_CR) & RL_CR_RST))
-			break;
-	} while (getuptime(&t1) == OK && (t1 - t0) < system_hz);
+	SPIN_UNTIL(!(rl_inb(port, RL_CR) & RL_CR_RST), 1000000);
 	printf("rl_reset_hw: (after reset) port = 0x%x, RL_CR = 0x%x\n",
 		port, rl_inb(port, RL_CR));
 	if (rl_inb(port, RL_CR) & RL_CR_RST)
diff --git a/drivers/ti1225/ti1225.c b/drivers/ti1225/ti1225.c
index 1d590c88b..81ef98ea3 100644
--- a/drivers/ti1225/ti1225.c
+++ b/drivers/ti1225/ti1225.c
@@ -316,7 +316,7 @@ PRIVATE void do_int(struct port *pp)
 {
 	int r, devind, vcc_5v, vcc_3v, vcc_Xv, vcc_Yv,
 		socket_5v, socket_3v, socket_Xv, socket_Yv;
-	clock_t t0, t1;
+	spin_t spin;
 	u32_t csr_event, csr_present, csr_control;
 	u8_t v8;
 	u16_t v16;
@@ -453,12 +453,12 @@ PRIVATE void do_int(struct port *pp)
 		printf("TI_CARD_CTRL: 0x%02x\n", v8);
 	}
 
-	getuptime(&t0);
+	spin_init(&spin, 100000);
 	do {
 		csr_present= pp->csr_ptr->csr_present;
 		if (csr_present & CP_PWRCYCLE)
 			break;
-	} while (getuptime(&t1)==OK && (t1-t0) < micros_to_ticks(100000));
+	} while (spin_check(&spin));
 
 	if (!(csr_present & CP_PWRCYCLE))
 	{
diff --git a/include/Makefile b/include/Makefile
index 13f1855e2..f17616515 100644
--- a/include/Makefile
+++ b/include/Makefile
@@ -22,7 +22,7 @@ INCS+=	minix/a.out.h minix/bitmap.h minix/callnr.h minix/cdrom.h \
 	minix/netdriver.h minix/partition.h minix/paths.h \
 	minix/portio.h minix/profile.h minix/queryparam.h \
 	minix/rs.h minix/safecopies.h minix/sched.h minix/sef.h minix/sound.h \
-	minix/sys_config.h minix/sysinfo.h minix/syslib.h \
+	minix/spin.h minix/sys_config.h minix/sysinfo.h minix/syslib.h \
 	minix/sysutil.h minix/timers.h minix/tty.h minix/type.h minix/types.h \
 	minix/u64.h minix/vfsif.h minix/vm.h \
 	minix/compiler.h minix/compiler-ack.h minix/sha2.h
diff --git a/include/minix/drivers.h b/include/minix/drivers.h
index fd2ae6cb6..39a5b8320 100644
--- a/include/minix/drivers.h
+++ b/include/minix/drivers.h
@@ -22,6 +22,7 @@
 #include <minix/syslib.h>
 #include <minix/sysutil.h>
 #include <minix/timers.h>
+#include <minix/spin.h>
 #include <minix/bitmap.h>
 
 #include <machine/interrupt.h>	/* IRQ vectors and miscellaneous ports */
diff --git a/include/minix/spin.h b/include/minix/spin.h
new file mode 100644
index 000000000..c445e0e55
--- /dev/null
+++ b/include/minix/spin.h
@@ -0,0 +1,38 @@
+/* Prototypes for condition spinning helper functions (part of libsys). */
+#ifndef _MINIX_SPIN_H
+#define _MINIX_SPIN_H
+
+/* Opaque spin state structure. */
+typedef struct {
+	int s_state;
+	u32_t s_usecs;
+	u64_t s_base_tsc;
+	clock_t s_base_uptime;
+	int s_timeout;
+} spin_t;
+
+/* Functions. */
+_PROTOTYPE( void spin_init, (spin_t *s, u32_t usecs)			);
+_PROTOTYPE( int spin_check, (spin_t *s)					);
+
+/* Macros. */
+
+/* Execute a loop for at least 'u' microseconds, using spin object 's'.
+ * The body of the loop is guaranteed to be executed at least once.
+ */
+#define SPIN_FOR(s,u)							\
+	for (spin_init((s), (u)); spin_check((s)); )
+
+/* Return whether spin object 's' timed out after a loop. */
+#define SPIN_TIMEOUT(s) ((s)->s_timeout)
+
+/* Spin until the given condition becomes true, or 'u' microseconds expired.
+ * The condition is guaranteed to be checked at least once.
+ */
+#define SPIN_UNTIL(c,u) do {						\
+	spin_t s;							\
+	SPIN_FOR(&s,(u))						\
+		if (c) break;						\
+} while (0)
+
+#endif /* _MINIX_SPIN_H */
diff --git a/lib/libsys/Makefile b/lib/libsys/Makefile
index cc8e7773c..a4c1dfab2 100644
--- a/lib/libsys/Makefile
+++ b/lib/libsys/Makefile
@@ -121,7 +121,8 @@ SRCS=  \
 	profile_extern.c \
 	profile.c \
 	vprintf.c \
-	timers.c
+	timers.c \
+	spin.c
 
 CPPFLAGS.sched_start.c+=	-I${MINIXSRCDIR}
 
diff --git a/lib/libsys/spin.c b/lib/libsys/spin.c
new file mode 100644
index 000000000..43eaca7cb
--- /dev/null
+++ b/lib/libsys/spin.c
@@ -0,0 +1,99 @@
+/* Helper functions that allow driver writers to easily busy-wait (spin) for a
+ * condition to become satisfied within a certain maximum time span.
+ */
+/* This implementation first spins without making any system calls for a
+ * while, and then starts using system calls (specifically, the system call to
+ * obtain the current time) while spinning. The reason for this is that in
+ * many cases, the condition to be checked will become satisfied rather
+ * quickly, and we want to avoid getting descheduled in that case. However,
+ * after a while, running out of scheduling quantum will cause our priority to
+ * be lowered, and we can avoid this by voluntarily giving up the CPU, by
+ * making a system call.
+ */
+#include "sysutil.h"
+#include <minix/spin.h>
+
+/* Number of microseconds to keep spinning initially, without performing a
+ * system call. We pick a value somewhat smaller than a typical clock tick.
+ * Note that for the above reasons, we want to avoid using sys_hz() here.
+ */
+#define TSC_SPIN		1000		/* in microseconds */
+
+/* Internal spin states. */
+enum {
+	STATE_INIT,		/* simply check the condition (once) */
+	STATE_BASE_TS,		/* get the initial TSC value (once) */
+	STATE_TS,		/* use the TSC to spin (up to TSC_SPIN us) */
+	STATE_UPTIME		/* use the clock to spin */
+};
+
+PUBLIC void spin_init(spin_t *s, u32_t usecs)
+{
+	/* Initialize the given spin state structure, set to spin at most the
+	 * given number of microseconds.
+	 */
+	s->s_state = STATE_INIT;
+	s->s_usecs = usecs;
+	s->s_timeout = FALSE;
+}
+
+PUBLIC int spin_check(spin_t *s)
+{
+	/* Check whether a timeout has taken place. Return TRUE if the caller
+	 * should continue spinning, and FALSE if a timeout has occurred. The
+	 * implementation assumes that it is okay to spin a little bit too long
+	 * (up to a full clock tick extra).
+	 */
+	u64_t cur_tsc, tsc_delta;
+	clock_t now, micro_delta;
+
+	switch (s->s_state) {
+	case STATE_INIT:
+		s->s_state = STATE_BASE_TS;
+		break;
+
+	case STATE_BASE_TS:
+		s->s_state = STATE_TS;
+		read_tsc_64(&s->s_base_tsc);
+		break;
+
+	case STATE_TS:
+		read_tsc_64(&cur_tsc);
+
+		tsc_delta = sub64(cur_tsc, s->s_base_tsc);
+
+		micro_delta = tsc_64_to_micros(tsc_delta);
+
+		if (micro_delta >= s->s_usecs) {
+			s->s_timeout = TRUE;
+			return FALSE;
+		}
+
+		if (micro_delta >= TSC_SPIN) {
+			s->s_usecs -= micro_delta;
+			getuptime(&s->s_base_uptime);
+			s->s_state = STATE_UPTIME;
+		}
+
+		break;
+
+	case STATE_UPTIME:
+		getuptime(&now);
+
+		/* We assume that sys_hz() caches its return value. */
+		micro_delta = ((now - s->s_base_uptime) * 1000 / sys_hz()) *
+			1000;
+
+		if (micro_delta >= s->s_usecs) {
+			s->s_timeout = TRUE;
+			return FALSE;
+		}
+
+		break;
+
+	default:
+		panic("spin_check: invalid state %d", s->s_state);
+	}
+
+	return TRUE;
+}