CSE 466 – Fall 2000 - Introduction - 1

Remainder of Syllabus

Lecture RTOS

Maestro In Linux Distributed Control Architecture – distributed state management

Quiz Embedded Networking Safety and Society Real Time UML Quiz/Final

Lab This week: streaming demo required…no report Next week: Two-way pilot-player interface, full “player” UI w/ pause, etc.

no resetting of processor for next song! Time to clean up.

I2C network I2C network upgrade?

CSE 466 – Fall 2000 - Introduction - 2

Linux Interprocess Communication

Semaphoreprocess 1: semid = getsem(semid); semop(semid, -1); // waitprocess 2: semid = getsem(semid); semop(semid,1); // signal

Pipes (queue)void main() {

int pfds[2];pipe(pfds);if (fork()) producer();else consumer();

}void producer() { // serial?

int q = pfds[0];while (1) write(q, data, n);

}void consumer() { // music?

int q = pfds[1];while (1) read(q, data, n);

}

CSE 466 – Fall 2000 - Introduction - 3

FIFO’s, which are named pipes

Process 1void main() { mknod(“myfifo”, S_IFIFO, ); // create a FIFO file node

f = open(“/usr/larrya/myfifo”, O_WRONLY);write(f, data, n);

}

Process 2void main() {

f = open(“/usr/larrya/myfifo”, O_RDONLY);read(f, data, n);

}

CSE 466 – Fall 2000 - Introduction - 4

A Player?

void main() {int pfds[2];pipe(pfds);if (fork()) serial();else consumer();

}void serialTask() { // serial?

int q = pfds[0];s = open(“/dev/com1”, O_RDWR);while (1) {read(s, data, 1); write(s,ack, 1); write(q, data, 1);

}void musicTask() { // music?

int q = pfds[1];while (1) {

semop(semid, -1);processNextPacket(read(q, data, n));

}

now all we need as an ISR, right?

CSE 466 – Fall 2000 - Introduction - 5

Not Quite

Not yet User programs are not allowed to access the physical devices (physical memory

space) – including hardware interrupts. Only the OS can do that. User programs can’t control when things happen

OS slice is about 10ms, maybe 1ms on some machines If there are many tasks, our musicTask might not meet its deadline..

Need a device driver Its part of the operating system It can include an interrupt service routine What is a device driver?

The interface is the same as a file on the disc supports standard file I/O interface

• open()• read()• write()• close()

Driver writer defines these system calls to provide a useful general device to the public

CSE 466 – Fall 2000 - Introduction - 6

Partition between the User App and The Driver

Device Driver should be general…what would a player look like? open() – makes sure that hardware resources are available…not being

used by another device for example. Registers the interrupt service routine

read() – nothing? can be defined to return current packet? write() – adds packets to the queue? so, maybe our SERIAL is the user

app, and our MUSIC/ISR is the OS/Device Driver. write should fail if it isn’t a proper music packet: three tones and duration.

close() – release the hardware resources

Now…how do we get a good trade-off between interrupt latency and timeliness+ Need an ISR for the tone generation and tone duration counting Where can we put the housekeeping?

ISR? device driver? User App?

CSE 466 – Fall 2000 - Introduction - 7

Answer: In the ISR…sort of

Linux Nomenclature for Interrupt handling Top Half – the actual ISR. Time critical stuff Bottom Half – the Time dependent, less critical stuff. Signaled by the ISR

User Appie. stream cse466

format music fromserial port to player

Top Half:Tone Generation

Bottom Half:Housekeeping

int

Device Driver Interface(open, read, etc.)

task queue

userspace

kernelspace

systemcall

CSE 466 – Fall 2000 - Introduction - 8

Schedule of task queues…bottom halves in Linux

blah()blah()blah()

TOPHALF

interrupt

BOT.HALF

blah()blah()blah()

rti

OS:Scheduleor sys call

sys callor tick

EXECUTETASKQUEU

blah()blah()blah()

add to task queue, and mark execution

DeviceDriver

no critical section hereBOT HALF and Device Driverare mutually exclusive. Not true for TopHalf, so this is a good way to share data between driver and ISR

write()

myapp

otherapp

CSE 466 – Fall 2000 - Introduction - 9

As a Task Diagram

process 2process 1

myappOS/otherOS/driverISR BOTISR TOP

myapp:write(player…)

myapp:blocked by driver onfull queue

ISR BOT and Driver are mutually exclusive, so no problem withshared data structure.

CSE 466 – Fall 2000 - Introduction - 10

The Application

void main() { if (serial = open("/dev/com3", O_RDWR) == -1) exit(); // open serial port if (player = open("/dev/player", O_RDWR) == -1) exit(); // open player device while (1) { read(serial, &byte, 1); // blocking read write(serial, &ack, 1); // send acknowledgement switch(state) case STREAMING:

read(serial,packet,1); // maybe better if non-blocking ? n = getNumTones(*packet);

read(serial, packet+1, n); // read the tone bytesTNE = getTNE(packet); // set new TNEif (isStop(packet)) { close(serial); // close the I/O close(player); // close the player, release the HW resources exit();}getTones(packet, writeBuf); // convert packet to toneswriteBuf[3] = duration; // add durationwrite(player, writeBuf, 4); // write to the playerbreak;

case IDLE: if (byte == 'S') {

state = STREAMING; read(serial,&duration,1); // time slice duration...this is a blocking read}

}}