Linux USB驱动程序基础.docx

Linux USB驱动程序基础.docx

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LinuxUSB驱动程序基础

LinuxUSB驱动程序基础

来源:

ChinaUnix博客　日期：

2008.04.1023:

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LinuxUSBDriverBasics

Introduction

Driversaresoftwarecomponentsthatoperatingsystemsusetoprovide

hardwarespecificservicestoapplications.Thiswebpageattemptstodocument

thebasicsofUSBdriversonLinux.Thegoalhereistoprovideyouwithabasic

understandingofhowUSBdevicedriversonLinuxwork.

TheFileAbstraction

Linux,likeotherUnixderivedoperatingsystems,triestomakeapplications

simplerbyprovidingacommonhardwareabstraction,theFile.Essentially,

interactionswithalmostallhardwarecanbeabstractedintothesameinterface

thattheoperatingsystemprovidesformanipulatingfiles.Hence,youcan"Open"

adriver,"Read"adriver,"Write"toadriverand"Close"adriver.This

abstractionextendsallthewayintotheapplication,whocanusethesame

systemcallsthatitusestoopenandmanipulatefilestotalktohardware.

Thewaythisworksisthatthekernelcreatesnodesinthefilesystem,

typicallylocatedin/devthatrepresentaparticularinterfacetoadriver.To

talktoaparticulardriveranapplicationwillopenthe/deventryassociated

withthatdriver.Thefiledescriptorreturnedbythatopenispassedtoall

futuresystemcalls（read,write,select）,andiseventuallypassedto

close.

Besidesdrivers,Unixalsousesthisfileparadigmforvariousdifferent

kindsofIPC,andevenforsocketcommunicationsoveranetwork.

Itisquitesurprisinghowmanycompletelydifferentkindsofhardwarecanbe

modeledwithjust4operations（open,close,readandwrite）.Thatsaid,their

isanotherveryimportantsystemcallthatunixapplicationsdeveloperscommonly

use,select（）.Theselect（）systemcallallowsapplicationstopolland

determinewhetherdatacouldbereadfrom,orwrittentoafiledescriptor

withoutblocking.

Sometimesapieceofhardwareprovidessomefunctionalitythatdoesn'tfit

wellintothisfilecentricparadigm.Toallowforthis,unixapplications

typicallymakeuseoftheioctl（）systemcall.Thiscalltakesanumericvalue

thatisessentiallyanidentifierforaspecificpieceoffunctionalityinthe

driver.

TheJoboftheDeviceDriver

Simplystated,itisthejobofthedrivertoprovidefunctionsthatthe

kernelcanusetoimplementthisfileprogrammingparadigm.Applicationsdonot

directlycallfunctionsinthedriver,insteadtheycallfunctionsinlibcthat

eventuallycallintothekernel（viathesystemcallinterface）.Thekernel

implementationsofthesesystemcallscallintoyourdriver.

Asyoumightexpect,thismeansthatyou'regoingtobeimplementatingopen,

close,readandwriteinsideyourdriver.Toimplementselect（）,thekernel

needsyoutoimplementamethodinyourdrivercalledpoll（）.Otherthanthese,

theirareonlyahandfullofotherfunctionsthatneedtobeimplemented,and

youhaveadriver!

LoadingaDriver

Linuxsupportsessentiallytwokindsofdriverdevelopmentmodels:

1）

Compiledintothekernel,2）Dynamicallyloadabledrivermodules.Ifconfigured

properly,option2hastheadvantagethatdriverscanbeloadedautomaticallyby

thekernelonlywhentheyareactuallyneededtoserviceanapplicationrequest.

Option1presentsalogicallycleaner,andslightlyfasterscenario.

Thechoiceisuptoyou.Thatsaid,wewillnowcoverthebasicsofloading

andunloadingakernelmoduleintoarunningkernel.

First,understandthataLinuxkerneldrivercannotbedynamicallylinked

againstanylibrarys（notevenlibc,whichmanyprogrammersdontevenknowthey

arelinkingagainst）.Theonlycodeexternaltoyourdriverthatyourallowedto

makeuseofarefunctionsthatareimplementedwithinthekernel.Infact,

kernelmodulesarenever"Linked",insteadtheyarerelocatableobjectfiles,

.o'sorsometimes.ko's.

Thefollowingcommandisusedtoinstalladriverintoarunningkernel:

[td@objective~]$insmoddriver.ko

Thefollowingcommandisusedtoshowwhatdriversarecurrentlyloadedinto

thekernel:

[td@objective~]$lsmod

ModuleSizeUsedby

md540331

ipv626310522

ipt_REJECT54411

ipt_state18575

ip_conntrack413691ipt_state

iptable_filter28811

ip_tables195213ipt_REJECT,ipt_state,iptable_filter

video156850

button40330

battery92850

ac48050

uhci_hcd350250

ehci_hcd400130

hw_random59730

i2c_i80186530

i2c_core213131i2c_i801

snd_intel8x0341770

snd_ac97_codec749371snd_intel8x0

snd_pcm_oss506730

snd_mixer_oss177291snd_pcm_oss

snd_pcm988893snd_intel8x0,snd_ac97_codec

snd_timer328371snd_pcm

snd572856snd_intel8x0,snd_ac97_codec

soundcore107851snd

snd_page_alloc96692snd_intel8x0,snd_pcm

e10001025730

dm_snapshot174130

dm_zero21130

dm_mirror256450

ext31306332

jbd831611ext3

dm_mod573336dm_snapshot,dm_zero,dm_mirror

ata_piix94130

libata469171ata_piix

sd_mod202890

scsi_mod1463132libata,sd_mod

Thefollowingcommandisusedtoremoveadriverfromarunningkernel:

[td@objective~]$rmmoddriver.ko

Sinceyouneverlinkyourdriver,youwillnotknowaboutunresolvedsymbols

untilyouactuallyloadyourdriverintothekernel.Itisduringthe"insmod"

processthatthekernelbindsallofthecallstokernelfunctionsinyourcode

totheiractualcurrentlocationsinmemory.Thisprocessiscalled"resolving

symboldependancies".

UnderstandingtheUniversalSerialBus

Fromthehighestlevel

AsI'msureyouknowfromyourownuseofUSB,theUSBbus,besidesoffering

communicationsalsoofferspowertodevicesconnectedtoit.TheUSBcanoffer

upto500mAforthedevicesconnectedtoit.Devicesthatneedmorethanthis

canbeselfpowered.

Besidesthis,yourprobablyawareofthefactthattheUSBcanbeexpanded

withhubs.

Howhardwareisabouttomakeyourlifeeasier

Thankfully,yourdriverwillnothavetocommunicatedirectlyonthe

UniversalSerialBus.Instead,yourdriverwillcommunicatetotheUSBhost

controller,whichwillcommunicateonthebusonyourbehalf.TheUSBhost

controllerisapieceofhardwarethatactsasafocalpointforallofthe

CPU'sinteractionwiththeUSBhardware.Ithidesmostofthecomplexityof

dealingwithUSBandalsoprotectstheUSBhardwarefrompotentiallybadly

writtendriversoftwarethatmightotherwiseaffectthewholebus.

Theiraretwokindsofhostcontrollerhardwarecommonlyinuse,UHCI

（UniversalHostControllerInterface）andOHCI（OpenHostControllerInterface）.

Thegoodnewsisthatbothofthesehostcontrollerspresentthesameinterface

toyourdriver,soyoureallydonthavetocareaboutwhatkindofhost

controllerhardwareispresent.Asyoumightexpect,UHCI&OHCIhardware

hasitsowndriver,thatthankfullyyouwillnotneedtotouch.Itwillbethe

jobofyourdrivertocommunicate（indirectly）withthehostcontrollerdriver,

toconfigure,readfromorwritetodevicesontheUSB.

TypesofCommunicationsontheUSB

USBdeviceshavethefullrangeofdifferentspeed,latencyandreliability

requirements.Thingslikemiceandkeyboardtransferonlysmallamountsofdata,

relativelyrarely,buttheycarealotaboutlatency.Noonelikeaslow

keyboard.

Ontheotherhand,USBwebcamsoftentransfercompressedMPEGvideo,whichis

extremelyhighbandwidth,butbecauseMPEGwasdesignedtobetransmittedover

lossycommunicationschannels,itisOKiftheoccasionalpacketislost.

Tosupportthesedifferingrequirements,USBsupportsanumberof

communicationstypes.

ControlTransfers-Controltransfersareusedwhenyouneed

reliable（thedataMUSTgettheir）communicationsandyouaresendingavery

smallamountofdata.Essentially,thesetransfersarecommands,andtheirarea

fewcommandsthateverydeviceisrequiredtosupport（GET_STATUS,

CLEAR_FEATURE,SET_FEATURE,SET_ADDRESS,GET_DESCRIPTOR,SET_DESCRIPTOR,

GET_CONFIGURATION,SET_CONFIGURATION,GET_INTERFACE,SET_INTERFACE,

SYNCH_FRAME）.

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BulkTransfers-Bulktransfersareusedwhenyouneed

reliable（thedataMUSTgettheir）communicationsandyouaresendinglarge

amountsofdata.

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InterruptTransfers-Interrupttransfersaresimmilarto

bulktransfersexcepttheyareconfiguredtooccurautomaticallyatsome

interval.Thesetypesoftransfersareusefulfordevicesthatstreamconstant

dataoftheUSB.

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IsochronousTransfers-Isochronoustransfersareveryfast,

andhaveguaranteedbusbandwidth,buttheyhavenoreliability.Thistransfer

typeseemstohavebeendesignedwithMPEGinmind.

Thecodethatyouwriteisgoingtohavetospecifytothehostcontroller

whattypesofcommuncationsyouwanttouse.Thankfully,anextremelywiderange

ofdevicetypesfitnicelyintotheabovetypesofdatatransfers.

TheheirarchicalviewoftheUSB

IfyouthinkaboutwhatIhavedescribedsofar,itshouldbefairlyapparent

thatsinceyouwillbedealingonlyindirectlywithactualUSBdevicesand

insteadbecommunicatingthroughahostcontroller,thathostcontrollermust

havesomekindofdatamodelforthedevicesthatareconnectedtoitontheUSB

bus.Itmustpresentsomedescriptionofthedevicesconnected,andtheir

capabilities.

ThecapabilitesofaUSBdevicearemodeledheirarchicallybythehost

controller.YourdriverwillcommunicatetoaUSBdevicebymakingrequeststo

nodesofthisheirarchy.Tofullyunderstandthereasoningforallofthelayers

oftheheirarchyIamabouttotellyouaboutpleaseconsiderhowgenericahost

controllermustbe,givenhowwiderangingthefunctionalityofUSBhardwarecan

be.

WhenaUSBisfirstpluggedintothebusitisdetectedbythehost

controller.ThehostcontrollersendsittheGET_DESCRIPTORcommandand

retrieveswhatiscalledthedevicedescr