Linux内核配置文件之解读Makefile

vivi与Linuxkernel的参数传递情景分析收藏在上一部分提到过了，vivi作为bootloader，向内核传递启动参数是其本职工作之一。要把这个情景分析清楚，不仅仅需要分析vivi的参数机制，而且要分析Linuxkernel的接收机制。因为这是一个简单的通信过程，比起本科所学习的TCP/IP来简单的多，但是因为简单，所以在协议上并不规范，理解上反而不如TCP/IP协议。下面就分为两个方面对此情景分析。一、综述内核参数传递机制现在内核参数传递机制有两种：一种是基于structparam_struct，这种已经比较老了。缺点是该结构每个成员的位置是固定的，受限比较大。另外一种就是新的structtagway。说新是相对的，Linuxkernel2.4.x都希望采用这种tag的方式。关于这方面的资料，可以有如下参考（所给出的目录是基于linux-2.4.18的内核，以顶层Makefile所在目录为当前目录。这里基于ARM架构的S3C2410，其他的SoC可以类比很容易得到）：1、关于bootloader的理解--【Documentation/arm/booting】此文档详细的讲述了bootloader的作用，具体内容如下：[armlinux@lqmarm]$catBootingBootingARMLinux=================Author:RussellKingDate:18May2002Thefollowingdocumentationisrelevantto2.4.18-rmk6andbeyond.InordertobootARMLinux,yourequireabootloader,whichisasmallprogramthatrunsbeforethemainkernel.Thebootloaderisexpectedtoinitialisevariousdevices,andeventuallycalltheLinuxkernel,passinginformationtothekernel.Essentially,thebootloadershouldprovide(asaminimum)thefollowing:1.SetupandinitialisetheRAM.2.Initialiseoneserialport.3.Detectthemachinetype.4.Setupthekerneltaggedlist.5.Callthekernelimage.1.SetupandinitialiseRAM---------------------------Existingbootloaders:MANDATORYNewbootloaders:MANDATORYThebootloaderisexpectedtofindandinitialiseallRAMthatthekernelwilluseforvolatiledatastorageinthesystem.Itperformsthisinamachinedependentmanner.(ItmayuseinternalalgorithmstoautomaticallylocateandsizeallRAM,oritmayuseknowledgeoftheRAMinthemachine,oranyothermethodthebootloaderdesignerseesfit.)2.Initialiseoneserialport-----------------------------Existingbootloaders:OPTIONAL,RECOMMENDEDNewbootloaders:OPTIONAL,RECOMMENDEDThebootloadershouldinitialiseandenableoneserialportonthetarget.Thisallowsthekernelserialdrivertoautomaticallydetectwhichserialportitshoulduseforthekernelconsole(generallyusedfordebuggingpurposes,orcommunicationwiththetarget.)Asanalternative,thebootloadercanpasstherelevant'console='optiontothekernelviathetaggedlistsspecifingtheport,andserialformatoptionsasdescribedinlinux/Documentation/kernel-parameters.txt.3.Detectthemachinetype--------------------------Existingbootloaders:OPTIONALNewbootloaders:MANDATORYThebootloadershoulddetectthemachinetypeitsrunningonbysomemethod.Whetherthisisahardcodedvalueorsomealgorithmthatlooksattheconnectedhardwareisbeyondthescopeofthisdocument.ThebootloadermustultimatelybeabletoprovideaMACH_TYPE_xxxvaluetothekernel.(seelinux/arch/arm/tools/mach-types).4.Setupthekerneltaggedlist-------------------------------Existingbootloaders:OPTIONAL,HIGHLYRECOMMENDEDNewbootloaders:MANDATORYThebootloadermustcreateandinitialisethekerneltaggedlist.AvalidtaggedliststartswithATAG_COREandendswithATAG_NONE.TheATAG_COREtagmayormaynotbeempty.AnemptyATAG_COREtaghasthesizefieldsetto'2'(0x00000002).TheATAG_NONEmustsetthesizefieldtozero.Anynumberoftagscanbeplacedinthelist.Itisundefinedwhetherarepeatedtagappendstotheinformationcarriedbytheprevioustag,orwhetheritreplacestheinformationinitsentirety;sometagsbehaveastheformer,othersthelatter.Thebootloadermustpassataminimumthesizeandlocationofthesystemmemory,androotfilesystemlocation.Therefore,theminimumtaggedlistshouldlook:+-----------+base-|ATAG_CORE||+-----------+||ATAG_MEM||increasingaddress+-----------+||ATAG_NONE||+-----------+vThetaggedlistshouldbestoredinsystemRAM.Thetaggedlistmustbeplacedinaregionofmemorywhereneitherthekerneldecompressornorinitrd'bootp'programwilloverwriteit.Therecommendedplacementisinthefirst16KiBofRAM.5.Callingthekernelimage---------------------------Existingbootloaders:MANDATORYNewbootloaders:MANDATORYTherearetwooptionsforcallingthekernelzImage.IfthezImageisstoredinflash,andislinkedcorrectlytoberunfromflash,thenitislegalforthebootloadertocallthezImageinflashdirectly.ThezImagemayalsobeplacedinsystemRAM(atanylocation)andcalledthere.Notethatthekerneluses16KofRAMbelowtheimagetostorepagetables.Therecommendedplacementis32KiBintoRAM.Ineithercase,thefollowingconditionsmustbemet:-CPUregistersettingsr0=0,r1=machinetypenumberdiscoveredin(3)above.r2=physicaladdressoftaggedlistinsystemRAM.-CPUmodeAllformsofinterruptsmustbedisabled(IRQsandFIQs)TheCPUmustbeinSVCmode.(AspecialexceptionexistsforAngel)-Caches,MMUsTheMMUmustbeoff.Instructioncachemaybeonoroff.Datacachemustbeoff.-Thebootloaderisexpectedtocallthekernelimagebyjumpingdirectlytothefirstinstructionofthekernelimage.可以看出bootloader最少具备5项功能，上面比较清晰。可以看出，现在2.4的内核都是希望采用taggedlist的方式来进行传递的，这里没有提到比较老的方式。这里要特别注意的是，r2=physicaladdressoftaggedlistinsystemRAM.，这里的“必须”是针对于taggedlist而言的，如果采用param_struct，则并没有这个限制。这在后面将会详细分析，而这正是可能导致疑惑的地方。2、参数传递数据结构的定义位置【include/asm/setup.h】，在这里就可以看到两种参数传递方式了。可以说，现在bootloader和Linuxkernel约定的参数传递机制就是这两种，必须严格按照这两种机制进行传输，否则的话，kernel可能因为无法识别bootloader传递过来的参数而导致无法启动。关于这两种方式，在这里还有说明：/**linux/include/asm/setup.h**Copyright(C)1997-1999RussellKing**Thisprogramisfreesoftware;youcanredistributeitand/ormodify*itunderthetermsoftheGNUGeneralPublicLicenseversion2as*publishedbytheFreeSoftwareFoundation.**Structurepassedtokerneltotellitaboutthe*hardwareit'srunningon.Seelinux/Documentation/arm/Setup*formoreinfo.**NOTE:*Thisfilecontainstwoways