How to write DSP device drivers.docx

How to write DSP device drivers.docx

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HowtowriteDSPdevicedrivers

HowtowriteDSPdevicedrivers

NickLethabyandDavidFriedland

12/15/20035:

00PMEST

Asdigitalsignalprocessorspickupperipherals,you'llneedtowritenewdevicedrivers.Herearesometime-savingtipsforwritingthemforplatformsbasedonDSPs.

Digitalsignalprocessors（DSPs）arenowoftenintegratedon-chipwithnumerousperipheraldevices,suchasserialports,UARTs,PCI,orUSBports.Asaresult,developingdevicedriversforDSPsrequiressignificantlymoretimeandeffortthaneverbefore.

Inthisarticle,we'llshowyouaDSPdevice-driverarchitecturethatreducesoveralldriverdevelopmenttimebyreusingcodeacrossmultipledevices.We'llalsolookin-depthatanaudiocodecdrivercreatedusingthisarchitecture.ThedesignandcodeexamplesarebasedondriversdevelopedfortheTexasInstruments'DSP/BIOSoperatingsystem,thoughthesameapproachwillworkinanysystem.

HowDSPsdiffer

Whereasmicroprocessorsaremainlyusedforgeneralpurposecontrol,DSPsalmostinvariablydohardreal-timedata-pathprocessing,wheredatasamplesareinputinacontinuousstream.DSPsareoptimizedtomovedataquicklyfromaperipheraltotheDSPcore,leadingtoseveralarchitecturaldifferencesfrommicroprocessors.

Somemicroprocessorsexecuteallcodefromexternalmemoryviaaninstructioncache（I-cache）.I/Operipheralregistersarememory-mappedandaccessedlikeanyotherprogramdata.Incontrast,manyDSPsdon'tprovideI-cache,butdoincludehigh-speed,on-chipmemorythatsupportsefficientprogramexecution.EvenwiththelatestDSPsthatdoprovideI-cache,thememoryisconfigurableaseitheracacheordirectlyaddressablememoryoracombinationofthetwo.It'sacommonpracticetodedicatesomeofthismemorytoreal-timeI/O,criticalcodeloops,anddatatoavoidpotentialnondeterministicbehaviorcausedbycachemisses.

Tomeetreal-timeI/Oneeds,DSPsprovidededicatedserialportsthatconnecttostreamingperipherals,suchascodecsandotherdataconverters.Theinteractionbetweenthecodecandtheserialportissynchronousandhandledentirelyinhardware,thoughinitialconfigurationoftheframesync,transferrate,samplerate,andsamplesizemustbedonebytheDSP.

AlthoughtheserialportscaninteractdirectlywiththeDSPcore,softwaredevelopersavoidthisapproachforreal-timeI/Obecauseofthefrequencyofinterrupts.DSPsgenerallyprovideoneormoredirectmemoryaccess（DMA）controllers,thechannelsofwhichcanbeusedtobuffermultiplesamplesfromtheserialportandthentransferthefullbufferintotheon-chipmemorywithoutDSPinvolvement.TheDSPisonlyinterruptedoncethebufferisfullratherthanoneverydatasample.AfterinitialconfigurationbytheDSP,theDMAandserialportinteractwithoutanyprocessorintervention.Becauseoftheefficiencygains,driversformostDSPperipheralsuseDMA.

WritingacodecdriverforaDSPactuallyinvolvesprogrammingthreedifferentperipherals—thecodecitself,theserialport,andtheDMAcontroller.Figure1showsthedataflowbetweenthedifferentperipherals,theDSP,andtheDSP'sinternalmemory.Later,we'llshowamuchmoredetailedimplementationofaDSPcodecdriver,butfirstlet'sdiscusssomedriver-architecturalissuesthatenablebettercodereusebothattheapplicationanddriverlevels.

Figure1:

ADSPcodecdriverofteninvolvesconfiguringmorethanjustthecodec

Device-driverarchitecture

Adevicedriverperformstwomainfunctions:

∙deviceconfigurationandinitialization

∙datamovement

Deviceconfigurationis,bydefinition,specifictoaparticulardevice.Datamovement,ontheotherhand,ismoregeneric.Inthecaseofastreamingdataperipherallikeacodec,theapplicationultimatelyexpectstosendorreceiveastreamofbuffers.Theapplicationshouldn'thavetoworryabouthowthebuffersaremanagedorwhattypeofcodecisbeingused,beyondissuessuchasdataprecision（numberofbitsinthesample）.

Classdrivers

Byprovidingaclearabstractionbetweenthedriverandtheapplication,youcanessentiallyfreetheapplicationofthespecificsofacertainperipheralandportitmoreeasilytonewhardware.Youcanfurtherapplytheseconceptsofabstractionandreusetothedriveritself.Anumberofdriverfunctionsareindependentoftheunderlyingdevice,suchassynchronizationbetweenthedriverandtheapplication.Youcanprovidetheseservicesinadrivermodulethat'sspecifictoaparticularclassofdevicesbutindependentofanyindividualdevice.Suchamoduleisoftenreferredtoasthe"upperhalf"ofadriver.Forconvenience,we'llusethetermclassdrivertodescribethismodule.

AsFigure2illustrates,anydrivercanbedividedintotwoparts:

aclassdriverthathandlestheapplicationinterfaceandOS-specificsandamini-driverthataddressesthehardwarespecificsofaparticulardevice.

Figure2:

Anydrivercanbedividedintotwoparts:

aclassdriverandamini-driver

BecauseoflargedifferencesbetweendeviceslikecodecsandUARTs,you'lltypicallyneedtoimplementseveralclassdriverstosupportalltheperipheralsusedwithaDSP.WhendesigningthedrivermodelforDSP/BIOS,twooftheclassdriversweimplementedwere:

∙AnSIOclassdriverforframe-orientedstreamingdevicessuchascodecsanddataconvertersthattransmitfixed-sizedframesofdata.

∙AGIOclassdriverprovidesbasicread/writeoperationsfordevicessuchasUARTs.

Ascanbeseenfromthesedefinitions,theclassdriversdefinetheI/Omodelsusedbytheapplication.Sincetheclassdriverisresponsibleforsynchronizationbetweentheapplicationanddriver,itwilldeterminewhethertheI/Oissynchronous（wheretheapplicationthreadblocksonanI/Otransactionsothatanotherthreadcanrun）orasynchronous（wheretheapplicationthreadcontinuestorunandreliesonanotificationmechanismthatwillinformtheapplicationwhentheI/Otransactioniscomplete）.Althoughclassdriversaredevice-independent,they'reintimatelyassociatedwiththeoperatingsystemsincetheyuseoperatingsystemservices,suchassemaphores.

Mini-drivers

Thespecificsofaperipheralareaddressedinthelowerhalfofthedriver,forwhichwe'llusethetermmini-driver.Themini-driverisresponsibleforalldevice-specificinitializationandcontrolandforpassingabufferofdatato（orreceivingabufferfrom）theclassdriver.Themini-drivermustdefineastandardinterfacetotheclassdriversinceitenablesaclassdrivertoworkwithmultiplemini-driversorviceversa.Forexample,inasystemwithmultiplecodecsofdifferenttypes,youcansavecodespacebyhavingjustoneinstanceoftheclassdrivercodeworkwithallthedifferentcodecmini-drivers.

Themostdevice-specificroutinesarethosethatinitializedevicecontrolregisters.Theseoperationsrequirethecalculationofspecificbitpatternstosettheappropriateflagvaluesineachcontrolregister.Althoughthedeviceinitializationroutineswillneverbeportable,implementationandmaintenancecanbemademuchsimplerbythedevelopmentofabasichardwareabstractionlayer（HAL）forthedeviceregisters.

Example:

Codecdriver

ToillustratethisDSPdevice-driverarchitecture,let'slookatanexamplethatrevealsthedesigndecisionsandimplementationoftheclassdriverandthemini-driver.We'llusethecodecdevicedriverforTexasInstruments'TMS320C5402DSPstarterkitboard.ThecodeissimilartootherDSP/codeccombinations,soyoucanadaptitasnecessary.

ToavoidgettinglostintheDMAsetupcode,we'lldemonstratesomeofthedriverconceptsusingasimplesample-by-sampleaudiocodecdriverthatprocessessamplesonceperinterrupt.Thistypeofdriverissomewhatsimplisticbecauseyou'dalmostalwaysuseaDMAtoenabletheDSPtoprocessonframesofdataratherthanhavingtoserviceaninterruptforeverydatapoint.

Thecodec'sclassdriver

OurexampleusestheSIOclassdriver.Likemostsoftwaremodules,thebestwaytounderstandtheSIOclassdriveristolookatsomeactualcode.Theexample,showninListing1,simplyreadssounddatafromtheaudiocodecdevicedriver,copiesthedatatoanotherbuffer,andthentransmitsitbackouttothecodecsothatitcanbeheardthroughaspeaker.

Listing1:

Applicationstartup

voidmain（）

{

    void*buf0,buf1,buf2,buf3;

    /*

     *AllocatebuffersfortheSIObufferexchange

     */

    buf0=（void*）MEM_calloc（0,BUFSIZE,BUFALIGN）;

    buf1=（void*）MEM_calloc（0,BUFSIZE,BUFALIGN）;

    buf2=（void*）MEM_calloc（0, BUFSIZE,BUFALIGN）;

    buf3 = （void*） MEM_calloc（0, BUFSIZE,BUFALIGN）;

    /*

     *CreatethetaskandopentheI/Ostreams

     */

    TSK_create（echo）;

    inStream = SIO_create（"/codec",SIO_INPUT,BUFSIZE）;

    outStream = SIO_create（"/codec",SIO_OUTPUT,BUFSIZE）;

    /* 

     * Start the DSP/BIOS scheduler when main （） exits 

     */

}

Thecodestarts,asallCprogramsdo,inmain（）.TheapplicationusestheDSP/BIOSmemorymanagertoallocatefourbuffersfromthesystem'sheapspaceandthentwoSIOobjectsarecreatedtostreamdatatoandfromthedevicedriver.

TheapplicationusestheSIO_create（）calltocreatethechannels.TheSIO_create（）functionargumentsindicatesomeofthedesigndecisionstobemadewhenimplementingthisclassdriver.Forinstance,we'lldecideherethatanSIOstreamcanbeopenedforeitherreadingorwriting,butnotboth.Ifbidirectionalcommunicationisrequired,theapplicationsimplyopenstwochannels（asinthisexample）.Thisunidirectionalchannelimplementationismoreefficient,andmanydataconvertersoperateinonlyonedirection.

Inaddition,becausemostcodecsoperateonfixed-sized