debuggingbyprinting.docx

debuggingbyprinting.docx

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debuggingbyprinting

debuggingbyprinting

Probablythesimplestwaytogetsomedebuginformationfromyourkernelcodeisbyprintingoutvariousinformationwiththekernel'sequivalentofprintf-theprintkfunctionanditsderivatives.

Thekinprintkisusedtospecificallyremindkerneldevelopersthattheenvironmentisdifferent.

Contents[hide]

1Usage

2LogLevels

3Ratelimitingandonetimemessages

4Printkfromuserspace

5Internals/Changingthesizeoftheprintkbuffer

6ProsandCons

7Debuggingearlybootproblems

7.1Accessingtheprintkbufferafterasilenthangonboot

7.1.1RedbootexampleonaFreescaleADSboard

7.1.2U-bootexampleonanOMAPOSKboard

7.1.3Grub

7.2UsingCONFIG_DEBUG_LLandprintascii（）（ARMonly）

8NetConsole

8.1Netconsoleresources

9Misc

9.1Dmesg/Clearingthebuffer

9.2PrintkTimestamps

9.3Printingbuffersashex

9.4DynamicDebugging

10Disablingprintkmessagesatcompiletime

11Referencesandexternallinks

Usage

printkworksmoreorlessthesamewayasprintfinuserspace,soifyoueverdebuggedyouruserspaceprogramusingprintf,youarereadytodothesamewithyourkernelcode,e.g.byadding:

printk（"MyDebuggerisPrintk\n"）;

Thiswasn'tthatdifficult,wasit?

Usuallyyouwouldprintoutsomemoreinterestinginformationlike

printk（"Var1%dvar2%d\n",var1,var2）;

justlikeinuserspace.

Inordertoseethekernelmessages,justusethe

$dmesg

commandinoneofyourshells-thisonewillprintoutthewholekernellog

buffertoyou.Mostoftheconversionspecifierssupportedbytheuser-spacelibraryroutineprintf（）arealsoavailableinthekernel;therearesomenotableadditions,including"%pf",whichwillprintthesymbolnameinplaceofthenumericpointervalue,ifavailable.

Thesupportedformatstringsarequiteextensivelydocumentedin

Documentation/printk-formats.txt

Howeverpleasenote:

alwaysuse%zu,%zdor%zxforprinting

size_tandssize_tvalues.ssize_tandsize_tarequitecommonvaluesinthekernel,so

pleaseusethe%ztoavoidannoyingcompilewarnings.Author'spracticaladvice:

Ifyouwanttodebuganoops（e.gcausedbyreleasingaresourcetwice）inyourdriverandyoudon'thaveacluewheretheoopshappens,simplyaddthisline

printk（KERN_ALERT"DEBUG:

Passed%s%d\n",__FUNCTION__,__LINE__）;

aftereachpossiblyoffendingstatement.

Recompileand（re-）loadthemoduleandtriggertheerrorcondition-yourlognowshowsyouthelastlinethatwassuccessfullyexecutedbeforetheoopshappened.

Ofcourseyoushouldremovethese'rude'statementsbeforeshippingyourmodule;）LogLevels

Ifyoulookintorealkernelcodeyouwillalwaysseesomethinglike:

printk（KERN_ERR"somethingwentwrong,returncode:

%d\n",ret）;

where"KERN_ERR"isoneoftheeightdifferentloglevelsdefinedin

include/linux/kern_levels.handspecifiestheseverityoftheerrormessage.

NotethatthereisNOcommabetweentheKERN_ERRandtheformatstring（asthepreprocessorconcatenatesbothstrings）

Theloglevelsare:

Name

String

Meaning

aliasfunction

KERN_EMERG

"0"

Emergencymessages,systemisabouttocrashorisunstable

pr_emerg

KERN_ALERT

"1"

Somethingbadhappenedandactionmustbetakenimmediately

pr_alert

KERN_CRIT

"2"

Acriticalconditionoccurredlikeaserioushardware/softwarefailure

pr_crit

KERN_ERR

"3"

Anerrorcondition,oftenusedbydriverstoindicatedifficultieswiththehardware

pr_err

KERN_WARNING

"4"

Awarning,meaningnothingseriousbyitselfbutmightindicateproblems

pr_warning

KERN_NOTICE

"5"

Nothingserious,butnotablynevertheless.Oftenusedtoreportsecurityevents.

pr_notice

KERN_INFO

"6"

Informationalmessagee.g.startupinformationatdriverinitialization

pr_info

KERN_DEBUG

"7"

Debugmessages

pr_debug,pr_develifDEBUGisdefined

KERN_DEFAULT

"d"

ThedefaultkernelloglevelKERN_CONT

""

"continued"lineoflogprintout（onlydoneafteralinethathadnoenclosing\n）[1]

pr_cont