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    Subject[PATCH] x86/doc: doc the using earlyprintk=dbgp

    Impact: doc

    simple using howto for earlyprintk=dbgp

    Signed-off-by: Yinghai Lu <yinghai@kernel.org>

    ---
    Documentation/x86/earlyprintk.txt | 41 ++++++++++++++++++++++++++++++++++++++
    1 file changed, 41 insertions(+)

    Index: linux-2.6/Documentation/x86/earlyprintk.txt
    ===================================================================
    --- /dev/null
    +++ linux-2.6/Documentation/x86/earlyprintk.txt
    @@ -0,0 +1,41 @@
    +
    +Using earlyprintk with USB2 Debug port and debug cable
    +
    +1. HW:
    +a. target system need to have debug port
    +
    +# lspci -vvv
    +...
    +00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI])
    + Subsystem: Lenovo ThinkPad T61
    + Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
    + Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
    + Latency: 0
    + Interrupt: pin D routed to IRQ 19
    + Region 0: Memory at fe227000 (32-bit, non-prefetchable) [size=1K]
    + Capabilities: [50] Power Management version 2
    + Flags: PMEClk- DSI- D1- D2- AuxCurrent=375mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
    + Status: D0 PME-Enable- DSel=0 DScale=0 PME+
    + Capabilities: [58] Debug port: BAR=1 offset=00a0
    + Kernel driver in use: ehci_hcd
    + Kernel modules: ehci-hcd
    +...
    +b. netchip USB debug cable
    + http://www.plxtech.com/products/NET2000/NET20DC/default.asp
    +
    +c. console system: have USB2 port
    +
    +2. SW setting
    +a. console system: need to have kernel config
    + CONFIG_USB_SERIAL_DEBUG
    + you should get /dev/ttyUSBx
    +
    + # cat /dev/ttyUSBx could get output
    +
    +b. target system: need to have kernel config
    + CONFIG_EARLY_PRINTK_DBGP
    +
    + boot command line: earlyprintk=dbgp
    +
    +c. for Nvidia Southbridge based system: kernel will try to probe and find out which port has debug device connneted.
    +

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