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ff61f0791c
Move the x86 documentation under Documentation/arch/ as a way of cleaning up the top-level directory and making the structure of our docs more closely match the structure of the source directories it describes. All in-kernel references to the old paths have been updated. Acked-by: Dave Hansen <dave.hansen@linux.intel.com> Cc: linux-arch@vger.kernel.org Cc: x86@kernel.org Cc: Borislav Petkov <bp@alien8.de> Cc: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/lkml/20230315211523.108836-1-corbet@lwn.net/ Signed-off-by: Jonathan Corbet <corbet@lwn.net>
152 lines
6.2 KiB
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152 lines
6.2 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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============
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Early Printk
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============
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Mini-HOWTO for using the earlyprintk=dbgp boot option with a
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USB2 Debug port key and a debug cable, on x86 systems.
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You need two computers, the 'USB debug key' special gadget and
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two USB cables, connected like this::
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[host/target] <-------> [USB debug key] <-------> [client/console]
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Hardware requirements
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=====================
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a) Host/target system needs to have USB debug port capability.
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You can check this capability by looking at a 'Debug port' bit in
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the lspci -vvv output::
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# lspci -vvv
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...
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00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI])
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Subsystem: Lenovo ThinkPad T61
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Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
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Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
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Latency: 0
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Interrupt: pin D routed to IRQ 19
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Region 0: Memory at fe227000 (32-bit, non-prefetchable) [size=1K]
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Capabilities: [50] Power Management version 2
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Flags: PMEClk- DSI- D1- D2- AuxCurrent=375mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
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Status: D0 PME-Enable- DSel=0 DScale=0 PME+
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Capabilities: [58] Debug port: BAR=1 offset=00a0
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^^^^^^^^^^^ <==================== [ HERE ]
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Kernel driver in use: ehci_hcd
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Kernel modules: ehci-hcd
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...
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.. note::
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If your system does not list a debug port capability then you probably
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won't be able to use the USB debug key.
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b) You also need a NetChip USB debug cable/key:
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http://www.plxtech.com/products/NET2000/NET20DC/default.asp
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This is a small blue plastic connector with two USB connections;
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it draws power from its USB connections.
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c) You need a second client/console system with a high speed USB 2.0 port.
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d) The NetChip device must be plugged directly into the physical
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debug port on the "host/target" system. You cannot use a USB hub in
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between the physical debug port and the "host/target" system.
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The EHCI debug controller is bound to a specific physical USB
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port and the NetChip device will only work as an early printk
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device in this port. The EHCI host controllers are electrically
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wired such that the EHCI debug controller is hooked up to the
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first physical port and there is no way to change this via software.
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You can find the physical port through experimentation by trying
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each physical port on the system and rebooting. Or you can try
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and use lsusb or look at the kernel info messages emitted by the
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usb stack when you plug a usb device into various ports on the
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"host/target" system.
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Some hardware vendors do not expose the usb debug port with a
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physical connector and if you find such a device send a complaint
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to the hardware vendor, because there is no reason not to wire
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this port into one of the physically accessible ports.
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e) It is also important to note, that many versions of the NetChip
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device require the "client/console" system to be plugged into the
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right hand side of the device (with the product logo facing up and
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readable left to right). The reason being is that the 5 volt
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power supply is taken from only one side of the device and it
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must be the side that does not get rebooted.
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Software requirements
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=====================
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a) On the host/target system:
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You need to enable the following kernel config option::
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CONFIG_EARLY_PRINTK_DBGP=y
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And you need to add the boot command line: "earlyprintk=dbgp".
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.. note::
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If you are using Grub, append it to the 'kernel' line in
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/etc/grub.conf. If you are using Grub2 on a BIOS firmware system,
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append it to the 'linux' line in /boot/grub2/grub.cfg. If you are
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using Grub2 on an EFI firmware system, append it to the 'linux'
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or 'linuxefi' line in /boot/grub2/grub.cfg or
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/boot/efi/EFI/<distro>/grub.cfg.
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On systems with more than one EHCI debug controller you must
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specify the correct EHCI debug controller number. The ordering
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comes from the PCI bus enumeration of the EHCI controllers. The
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default with no number argument is "0" or the first EHCI debug
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controller. To use the second EHCI debug controller, you would
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use the command line: "earlyprintk=dbgp1"
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.. note::
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normally earlyprintk console gets turned off once the
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regular console is alive - use "earlyprintk=dbgp,keep" to keep
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this channel open beyond early bootup. This can be useful for
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debugging crashes under Xorg, etc.
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b) On the client/console system:
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You should enable the following kernel config option::
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CONFIG_USB_SERIAL_DEBUG=y
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On the next bootup with the modified kernel you should
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get a /dev/ttyUSBx device(s).
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Now this channel of kernel messages is ready to be used: start
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your favorite terminal emulator (minicom, etc.) and set
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it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to
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see the raw output.
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c) On Nvidia Southbridge based systems: the kernel will try to probe
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and find out which port has a debug device connected.
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Testing
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=======
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You can test the output by using earlyprintk=dbgp,keep and provoking
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kernel messages on the host/target system. You can provoke a harmless
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kernel message by for example doing::
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echo h > /proc/sysrq-trigger
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On the host/target system you should see this help line in "dmesg" output::
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SysRq : HELP : loglevel(0-9) reBoot Crashdump terminate-all-tasks(E) memory-full-oom-kill(F) kill-all-tasks(I) saK show-backtrace-all-active-cpus(L) show-memory-usage(M) nice-all-RT-tasks(N) powerOff show-registers(P) show-all-timers(Q) unRaw Sync show-task-states(T) Unmount show-blocked-tasks(W) dump-ftrace-buffer(Z)
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On the client/console system do::
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cat /dev/ttyUSB0
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And you should see the help line above displayed shortly after you've
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provoked it on the host system.
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If it does not work then please ask about it on the linux-kernel@vger.kernel.org
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mailing list or contact the x86 maintainers.
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