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Measuring Interrupt Latency on the RPX-CLLF

Measuring Interrupt Latency on the RPX-CLLF

This is an excerpt from Chapter 11 of Embedded Linux by Craig Hollabaugh, published by Addison-Wesley. ISBN 0672322269. No part of this excerpt may be reproduced, in any form or by any means without the prior written permission of the publisher. (c) 2002 Addison-Wesley

The Project Trailblazer engineers want to use embedded Linux for race timing to reduce data entry errors, increase efficiency and enhance the racers' experience.

This chapter series explores using Linux as an event timer with 1-millisecond resolution. In Part 1 we discussed Linux timing sources and interrupt latency. Part 2 covered measuring interrupt latency on the MZ104, and Part 3 was about measuring interrupt latency on the MediaEngine. In this part we discuss measuring interrupt latency on the RPX-CLLF.

The RPX-CLLF's MPC860 has seven external interrupt pins. The Project Trailblazers want to drive IRQ2 with PA0, so they configure the interrupt controller for IRQ2 rising-edge operation. Asserting PA0 should generate Interrupt 2. Figure 5 shows the interrupt signal connection between the RPX-CLLF's PA0 and IRQ2 and the oscilloscope.

Listing 3 shows the source code for the interrupt_latency_rpxcllf.c device driver.

The interrupt_latency_rpxcllf.c device driver performs steps similar to those of the interrupt_latency_x86.c device driver. However, significant differences exist between the x86 and PowerPC interrupt controllers. The request_8xxirq function is a specialized PowerPC 8xx version of request_irq. request_8xxirq sets all the required MPC860 interrupt registers. When using an 8xx processor, you must explicitly call the request_8xxirq function. The following console output shows an instantaneous latency test result for the RPX-CLLF target board:

bash-2.04# insmod interrupt_latency_rpxcllf.o SIEL = 0x08000000
interrupt_latency_rpxcllf 1.0 initialized bash-2.04# cat/proc/interrupts

3:08xx SIUEdgefec
4:08xx SIUEdgeinterrupt_latency
5:31118xx SIUEdgecpm
15:08xx SIUEdgetbint
bash-2.04# cat /proc/interrupt_latency
Start 112.477152
Finish 112.477187
Latency 35
Count 1

The /proc/interrupts file shows that the interrupt_latency routine is registered on Interrupt 4, not on Interrupt 2. This kernel version incorrectly reports interrupt values. The /proc/interrupt_latency file shows an instantaneous latency of 35ms. Figure 6 shows an oscilloscope capture of the interrupt signal.

The interrupt signal measurement shows a latency of 30.75ms, whereas the driver returns a calculated latency of 35ms. Code execution accounts for the 4.25ms discrepancy. As with the x86, this discrepancy is not a factor in the race timer.

The Project Trailblazer engineers calculated the RPX-CLLF's average interrupt latency to be approximately 35ms. The PowerPC version of the do_gettimeofday function has near microsecond accuracy.

Interrupt Latency Test Summary
The MZ104, the MediaEngine, and the RPX-CLLF exhibit fast average interrupt latencies, and their do_gettimeofday functions return near-microsecond accuracy. By using the interrupt latency device drivers and an oscilloscope, the engineers found the average interrupt latency for the MZ104, the MediaEngine, and the RPX-CLLF to be 50ms, 10ms, and 35ms, respectively. With average interrupt latencies at or below 50ms, each target board is capable of providing millisecond timing accuracy for the race timer. The engineers breathe a sign of relief because they feared that they would have to move to a real-time solution, such as RTLinux or Real Time Application Interface (RTAI), which would mean a significant increase in their development time. However, the stock Linux kernel running on the Project Trailblazer target boards provides their required functionality. The engineers are now ready to tackle the design of the race timer.

In my next and last part we'll go over implementing the race timer.

Listing 3: The interrupt_latency_rpxcllf.c Device Driver

* interrupt_latency_rpxcllf v1.0 11/25/01
* www.embeddedlinuxinterfacing.com
* The original location of this code is
* http://www.embeddedlinuxinterfacing.com/chapters/11/
* Copyright (C) 2001 by Craig Hollabaugh
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU Library General Public License as
* published by the Free Software Foundation; either version 2 of the
* License, or (at your option) any later version.
* This program is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of
* Library General Public License for more details.
* You should have received a copy of the GNU Library General Public
* License along with this program; if not, write to the
* Free Software Foundation, Inc.,
* 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA

* interrupt_latency_rpxcllf.c is based on procfs_example.c by Erik Mouw.
* For more information, please see, The Linux Kernel Procfs Guide, Erik Mouw
* http://kernelnewbies.org/documents/kdoc/procfs-guide/lkprocfsguide.html

powerpc-linux-gcc -O2 -D__KERNEL__ -DMODULE -I/usr/src/powerpc-linux/include \
-c interrupt_latency_rpxcllf.c \
-o /tftpboot/powerpc-rootfs/tmp/interrupt_latency_rpxcllf.o

/* interrupt_latency_rpxcllf
* This module measures instantaneous interrupt latency of the RPX-CLLF
* using PA0 and IRQ2. Configure the SIU for IRQ2 edge interrupt
* generation. Configure PA0 as an output
then setting PA0 causes a
* rising edge that generates the interrupt.
* Section 11.5.1 of Motorola MPC860 PowerQUICC User's Manual
* After module insertion, reading /proc/interrupt_latency will assert PA0
* generating the interrupt. The interrupt handler will deassert this signal.
* View on scope. An interrupt counter is included to help debug noisy a
* interrupt line.




#define MODULE_VERSION "1.0"
#define MODULE_NAME "interrupt_latency_rpxcllf"

volatile immap_t *immap;
static void *io_base;

#define PA0 0x8000
#define SIEL_ED2 0x08000000

int interruptcount = 0;

struct timeval tv1, tv2; /* do_gettimeofday fills these */

static struct proc_dir_entry *interrupt_latency_file;

* function interrupt_interrupt_latency
* This function is the interrupt handler for interrupt 2. It sets the tv2
* structure using do_gettimeofday. It then clears PA0.
void interrupt_interrupt_latency(int irq, void *dev_id, struct pt_regs *regs)
immap->im_ioport.iop_padat |= PA0; /* deassert the interrupt signal */

* function proc_read_interrupt_latency
* The kernel executes this function when a read operation occurs on
* /proc/interrupt_latency. This function sets the tv1 structure. It asserts
* PA0 which should immediately cause interrupt 2 to occur. The handler
* records tv2 and deasserts PA0. This function returns the time
* differential between tv2 and tv1.
static int proc_read_interrupt_latency(char *page, char **start, off_t off,
int count, int *eof, void *data)
int len;

immap->im_ioport.iop_padat &= ~PA0; /* assert the interrupt signal */

len = sprintf(page, "Start %9i.%06i\nFinish %9i.%06i\nLatency %16i\n\
Count %18i\n",(int) tv1.tv_sec, (int) tv1.tv_usec,
(int) tv2.tv_sec, (int) tv2.tv_usec,
(int) (tv2.tv_usec - tv1.tv_usec),

return len;

* function init_interrupt_latency
* This function creates the /proc directory entry interrupt_latency. It
* requests interrupt 2 from Linux then configures the interrupt controller.
static int __init init_interrupt_latency(void)
unsigned long r;
int rv = 0;

interrupt_latency_file = create_proc_entry("interrupt_latency", 0444, NULL);
if(interrupt_latency_file == NULL) {
return -ENOMEM;

interrupt_latency_file->data = NULL;
interrupt_latency_file->read_proc = &proc_read_interrupt_latency;
interrupt_latency_file->write_proc = NULL;
interrupt_latency_file->owner = THIS_MODULE;

/* request interrupt from linux */
rv = request_8xxirq(INTERRUPT, interrupt_interrupt_latency, 0,
if ( rv ) {
printk("Can't get interrupt %d\n", INTERRUPT);
goto no_interrupt_latency;

/* get the IMMAP register address */
immap = (immap_t *)(mfspr(IMMR) & 0xFFFF0000);
immap->im_ioport.iop_papar &= ~PA0; /* set PA0 to general I/O */
immap->im_ioport.iop_padir |= PA0; /* set PA0 as output */

/* set IRQ2 to edge triggering */
immap->im_siu_conf.sc_siel |= SIEL_ED2;
printk("SIEL = 0x%08X\n",immap->im_siu_conf.sc_siel);

immap->im_ioport.iop_padat |= PA0; /* deassert the interrupt signal */

/* everything initialized */
printk(KERN_INFO "%s %s initialized\n",MODULE_NAME, MODULE_VERSION);
return 0;

remove_proc_entry("interrupt_latency", NULL);

* function cleanup_interrupt_latency
* This function frees interrupt 2 then removes the
* /proc directory entry interrupt_latency.
static void __exit cleanup_interrupt_latency(void)
free_irq(INTERRUPT,NULL); /* free the interrupt */

remove_proc_entry("interrupt_latency", NULL);

printk(KERN_INFO "%s %s removed\n", MODULE_NAME, MODULE_VERSION);


MODULE_AUTHOR("Craig Hollabaugh");
MODULE_DESCRIPTION("interrupt_latency proc module");


More Stories By Maureen O'Gara

Maureen O'Gara the most read technology reporter for the past 20 years, is the Cloud Computing and Virtualization News Desk editor of SYS-CON Media. She is the publisher of famous "Billygrams" and the editor-in-chief of "Client/Server News" for more than a decade. One of the most respected technology reporters in the business, Maureen can be reached by email at maureen(at)sys-con.com or paperboy(at)g2news.com, and by phone at 516 759-7025. Twitter: @MaureenOGara

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