Linux Device Driversoverview

Jeff Foster

Linux Device Drivers overview, September 12, 2000 2

Introduction

Goals of Linux Device Drivers Teach people how to write drivers Teach people some programming tricks Serve as reference

Target audience: Linux user with no kernel knowledge ...but with a deep understanding of their

device

Linux Device Drivers overview, September 12, 2000 3

(Some of) Our Goals

Find bugs in existing driversShow existing drivers don’t have

bugsHelp people write new drivers

Mismatch with book

Linux Device Drivers overview, September 12, 2000 4

What to Look For

Discussion of common mistakesAPI requirements (KPI?)

driver functions return positive ints on success, negative ints on failure (pp50-51)

kernel won’t call non-blocking I/O functions if previous request still pending (p264)

Invariants “every kernel function that calls kmalloc

(GFP_KERNEL) should be reentrant” (p153)

Linux Device Drivers overview, September 12, 2000 5

What to Ignore

Warning: Out of date! Written for 2.0.x, little bit on 2.1.x (Chap. 17) 2.4.x will be released soon

Locking got a lot finerSymbol table interface changedMany fast/slow call distinctions gone/dev changing in the future (currently awful)

Linux Device Drivers overview, September 12, 2000 6

Other Places to Look

The source code! linux/include/blk.h: “All functions called

within end_request() must be atomic.”Patches to the source

(2.2.14) linux/drivers/char/acquirewdt.c

-unregister_reboot_notifier(&acq_notifier);

+register_reboot_notifier(&acq_notifier);

Linux kernel mailing list (high volume)The web

Linux Device Drivers overview, September 12, 2000 7

Partial Directory Layout

/usr/src/linux/arch architecture-specific stuff

/i386 sometimes you have to look here

/drivers device drivers/char character devices -- most modular

/block block devices -- more integrated

...

/init boot up code

/kernel the core OS (e.g., scheduler)

/mem black magic

Also /fs, /net, /lib, ...

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Kernel Architecture

It looks like only /drivers interesting ...but the kernel is more monolithic than it

looks ...especially block devices (buffer

management)

char devices better behaved Start here

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Topic 1: Locking

Four kinds of locks in kernel Spin locks and read-write locks Interrupt enable/disable

Sometimes combined, e.g., spin_lock_irq

Whole kernel lock

Locks are used all over the placeAre they used correctly? consistently?

Linux Device Drivers overview, September 12, 2000 10

Topic 2: Interrupt Time

When in_interrupt is true, code cannot Access current Call the scheduler (may sleep) Call kmalloc(GFP_KERNEL) (may sleep) Copy to/from user-space (may sleep) more?

Also see Dawson Engler’s work

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Topic 3: Resource Allocation

Drivers get and release system resources Memory IRQs module numbers (maybe) space for their code (mod usage count)

Are the resources handled correctly? Leaks lead to instability -- reboot to reclaim

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What We’re Missing

Many errors in drivers are with device interface, not kernel interface see patch files

No device-specific info in book

How do we find these bugs?