path: root/Documentation/ia64
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authorLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
committerLinus Torvalds <torvalds@ppc970.osdl.org>2005-04-16 15:20:36 -0700
commit1da177e4c3f41524e886b7f1b8a0c1fc7321cac2 (patch)
tree0bba044c4ce775e45a88a51686b5d9f90697ea9d /Documentation/ia64
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
Diffstat (limited to 'Documentation/ia64')
5 files changed, 670 insertions, 0 deletions
diff --git a/Documentation/ia64/IRQ-redir.txt b/Documentation/ia64/IRQ-redir.txt
new file mode 100644
index 000000000000..f7bd72261283
--- /dev/null
+++ b/Documentation/ia64/IRQ-redir.txt
@@ -0,0 +1,69 @@
+IRQ affinity on IA64 platforms
+ 07.01.2002, Erich Focht <efocht@ess.nec.de>
+By writing to /proc/irq/IRQ#/smp_affinity the interrupt routing can be
+controlled. The behavior on IA64 platforms is slightly different from
+that described in Documentation/IRQ-affinity.txt for i386 systems.
+Because of the usage of SAPIC mode and physical destination mode the
+IRQ target is one particular CPU and cannot be a mask of several
+CPUs. Only the first non-zero bit is taken into account.
+Usage examples:
+The target CPU has to be specified as a hexadecimal CPU mask. The
+first non-zero bit is the selected CPU. This format has been kept for
+compatibility reasons with i386.
+Set the delivery mode of interrupt 41 to fixed and route the
+interrupts to CPU #3 (logical CPU number) (2^3=0x08):
+ echo "8" >/proc/irq/41/smp_affinity
+Set the default route for IRQ number 41 to CPU 6 in lowest priority
+delivery mode (redirectable):
+ echo "r 40" >/proc/irq/41/smp_affinity
+The output of the command
+ cat /proc/irq/IRQ#/smp_affinity
+gives the target CPU mask for the specified interrupt vector. If the CPU
+mask is preceded by the character "r", the interrupt is redirectable
+(i.e. lowest priority mode routing is used), otherwise its route is
+Initialization and default behavior:
+If the platform features IRQ redirection (info provided by SAL) all
+IO-SAPIC interrupts are initialized with CPU#0 as their default target
+and the routing is the so called "lowest priority mode" (actually
+fixed SAPIC mode with hint). The XTP chipset registers are used as hints
+for the IRQ routing. Currently in Linux XTP registers can have three
+ - minimal for an idle task,
+ - normal if any other task runs,
+ - maximal if the CPU is going to be switched off.
+The IRQ is routed to the CPU with lowest XTP register value, the
+search begins at the default CPU. Therefore most of the interrupts
+will be handled by CPU #0.
+If the platform doesn't feature interrupt redirection IOSAPIC fixed
+routing is used. The target CPUs are distributed in a round robin
+manner. IRQs will be routed only to the selected target CPUs. Check
+ cat /proc/interrupts
+On large (multi-node) systems it is recommended to route the IRQs to
+the node to which the corresponding device is connected.
+For systems like the NEC AzusA we get IRQ node-affinity for free. This
+is because usually the chipsets on each node redirect the interrupts
+only to their own CPUs (as they cannot see the XTP registers on the
+other nodes).
diff --git a/Documentation/ia64/README b/Documentation/ia64/README
new file mode 100644
index 000000000000..aa17f2154cba
--- /dev/null
+++ b/Documentation/ia64/README
@@ -0,0 +1,43 @@
+ Linux kernel release 2.4.xx for the IA-64 Platform
+ These are the release notes for Linux version 2.4 for IA-64
+ platform. This document provides information specific to IA-64
+ ONLY, to get additional information about the Linux kernel also
+ read the original Linux README provided with the kernel.
+INSTALLING the kernel:
+ - IA-64 kernel installation is the same as the other platforms, see
+ original README for details.
+ Compiling and running this kernel requires an IA-64 compliant GCC
+ compiler. And various software packages also compiled with an
+ IA-64 compliant GCC compiler.
+CONFIGURING the kernel:
+ Configuration is the same, see original README for details.
+COMPILING the kernel:
+ - Compiling this kernel doesn't differ from other platform so read
+ the original README for details BUT make sure you have an IA-64
+ compliant GCC compiler.
+ - General issues:
+ o Hardly any performance tuning has been done. Obvious targets
+ include the library routines (IP checksum, etc.). Less
+ obvious targets include making sure we don't flush the TLB
+ needlessly, etc.
+ o SMP locks cleanup/optimization
+ o IA32 support. Currently experimental. It mostly works.
diff --git a/Documentation/ia64/efirtc.txt b/Documentation/ia64/efirtc.txt
new file mode 100644
index 000000000000..ede2c1e51cd7
--- /dev/null
+++ b/Documentation/ia64/efirtc.txt
@@ -0,0 +1,128 @@
+EFI Real Time Clock driver
+S. Eranian <eranian@hpl.hp.com>
+March 2000
+I/ Introduction
+This document describes the efirtc.c driver has provided for
+the IA-64 platform.
+The purpose of this driver is to supply an API for kernel and user applications
+to get access to the Time Service offered by EFI version 0.92.
+EFI provides 4 calls one can make once the OS is booted: GetTime(),
+SetTime(), GetWakeupTime(), SetWakeupTime() which are all supported by this
+driver. We describe those calls as well the design of the driver in the
+following sections.
+II/ Design Decisions
+The original ideas was to provide a very simple driver to get access to,
+at first, the time of day service. This is required in order to access, in a
+portable way, the CMOS clock. A program like /sbin/hwclock uses such a clock
+to initialize the system view of the time during boot.
+Because we wanted to minimize the impact on existing user-level apps using
+the CMOS clock, we decided to expose an API that was very similar to the one
+used today with the legacy RTC driver (driver/char/rtc.c). However, because
+EFI provides a simpler services, not all all ioctl() are available. Also
+new ioctl()s have been introduced for things that EFI provides but not the
+EFI uses a slightly different way of representing the time, noticeably
+the reference date is different. Year is the using the full 4-digit format.
+The Epoch is January 1st 1998. For backward compatibility reasons we don't
+expose this new way of representing time. Instead we use something very
+similar to the struct tm, i.e. struct rtc_time, as used by hwclock.
+One of the reasons for doing it this way is to allow for EFI to still evolve
+without necessarily impacting any of the user applications. The decoupling
+enables flexibility and permits writing wrapper code is ncase things change.
+The driver exposes two interfaces, one via the device file and a set of
+ioctl()s. The other is read-only via the /proc filesystem.
+As of today we don't offer a /proc/sys interface.
+To allow for a uniform interface between the legacy RTC and EFI time service,
+we have created the include/linux/rtc.h header file to contain only the
+"public" API of the two drivers. The specifics of the legacy RTC are still
+in include/linux/mc146818rtc.h.
+III/ Time of day service
+The part of the driver gives access to the time of day service of EFI.
+Two ioctl()s, compatible with the legacy RTC calls:
+ Read the CMOS clock: ioctl(d, RTC_RD_TIME, &rtc);
+ Write the CMOS clock: ioctl(d, RTC_SET_TIME, &rtc);
+The rtc is a pointer to a data structure defined in rtc.h which is close
+to a struct tm:
+struct rtc_time {
+ int tm_sec;
+ int tm_min;
+ int tm_hour;
+ int tm_mday;
+ int tm_mon;
+ int tm_year;
+ int tm_wday;
+ int tm_yday;
+ int tm_isdst;
+The driver takes care of converting back an forth between the EFI time and
+this format.
+Those two ioctl()s can be exercised with the hwclock command:
+For reading:
+# /sbin/hwclock --show
+Mon Mar 6 15:32:32 2000 -0.910248 seconds
+For setting:
+# /sbin/hwclock --systohc
+Root privileges are required to be able to set the time of day.
+IV/ Wakeup Alarm service
+EFI provides an API by which one can program when a machine should wakeup,
+i.e. reboot. This is very different from the alarm provided by the legacy
+RTC which is some kind of interval timer alarm. For this reason we don't use
+the same ioctl()s to get access to the service. Instead we have
+introduced 2 news ioctl()s to the interface of an RTC.
+We have added 2 new ioctl()s that are specific to the EFI driver:
+ Read the current state of the alarm
+ ioctl(d, RTC_WKLAM_RD, &wkt)
+ Set the alarm or change its status
+ ioctl(d, RTC_WKALM_SET, &wkt)
+The wkt structure encapsulates a struct rtc_time + 2 extra fields to get
+status information:
+struct rtc_wkalrm {
+ unsigned char enabled; /* =1 if alarm is enabled */
+ unsigned char pending; /* =1 if alarm is pending */
+ struct rtc_time time;
+As of today, none of the existing user-level apps supports this feature.
+However writing such a program should be hard by simply using those two
+Root privileges are required to be able to set the alarm.
+V/ References.
+Checkout the following Web site for more information on EFI:
diff --git a/Documentation/ia64/fsys.txt b/Documentation/ia64/fsys.txt
new file mode 100644
index 000000000000..28da181f9966
--- /dev/null
+++ b/Documentation/ia64/fsys.txt
@@ -0,0 +1,286 @@
+-*-Mode: outline-*-
+ Light-weight System Calls for IA-64
+ -----------------------------------
+ Started: 13-Jan-2003
+ Last update: 27-Sep-2003
+ David Mosberger-Tang
+ <davidm@hpl.hp.com>
+Using the "epc" instruction effectively introduces a new mode of
+execution to the ia64 linux kernel. We call this mode the
+"fsys-mode". To recap, the normal states of execution are:
+ - kernel mode:
+ Both the register stack and the memory stack have been
+ switched over to kernel memory. The user-level state is saved
+ in a pt-regs structure at the top of the kernel memory stack.
+ - user mode:
+ Both the register stack and the kernel stack are in
+ user memory. The user-level state is contained in the
+ CPU registers.
+ - bank 0 interruption-handling mode:
+ This is the non-interruptible state which all
+ interruption-handlers start execution in. The user-level
+ state remains in the CPU registers and some kernel state may
+ be stored in bank 0 of registers r16-r31.
+In contrast, fsys-mode has the following special properties:
+ - execution is at privilege level 0 (most-privileged)
+ - CPU registers may contain a mixture of user-level and kernel-level
+ state (it is the responsibility of the kernel to ensure that no
+ security-sensitive kernel-level state is leaked back to
+ user-level)
+ - execution is interruptible and preemptible (an fsys-mode handler
+ can disable interrupts and avoid all other interruption-sources
+ to avoid preemption)
+ - neither the memory-stack nor the register-stack can be trusted while
+ in fsys-mode (they point to the user-level stacks, which may
+ be invalid, or completely bogus addresses)
+In summary, fsys-mode is much more similar to running in user-mode
+than it is to running in kernel-mode. Of course, given that the
+privilege level is at level 0, this means that fsys-mode requires some
+care (see below).
+* How to tell fsys-mode
+Linux operates in fsys-mode when (a) the privilege level is 0 (most
+privileged) and (b) the stacks have NOT been switched to kernel memory
+yet. For convenience, the header file <asm-ia64/ptrace.h> provides
+three macros:
+ user_mode(regs)
+ user_stack(task,regs)
+ fsys_mode(task,regs)
+The "regs" argument is a pointer to a pt_regs structure. The "task"
+argument is a pointer to the task structure to which the "regs"
+pointer belongs to. user_mode() returns TRUE if the CPU state pointed
+to by "regs" was executing in user mode (privilege level 3).
+user_stack() returns TRUE if the state pointed to by "regs" was
+executing on the user-level stack(s). Finally, fsys_mode() returns
+TRUE if the CPU state pointed to by "regs" was executing in fsys-mode.
+The fsys_mode() macro is equivalent to the expression:
+ !user_mode(regs) && user_stack(task,regs)
+* How to write an fsyscall handler
+The file arch/ia64/kernel/fsys.S contains a table of fsyscall-handlers
+(fsyscall_table). This table contains one entry for each system call.
+By default, a system call is handled by fsys_fallback_syscall(). This
+routine takes care of entering (full) kernel mode and calling the
+normal Linux system call handler. For performance-critical system
+calls, it is possible to write a hand-tuned fsyscall_handler. For
+example, fsys.S contains fsys_getpid(), which is a hand-tuned version
+of the getpid() system call.
+The entry and exit-state of an fsyscall handler is as follows:
+** Machine state on entry to fsyscall handler:
+ - r10 = 0
+ - r11 = saved ar.pfs (a user-level value)
+ - r15 = system call number
+ - r16 = "current" task pointer (in normal kernel-mode, this is in r13)
+ - r32-r39 = system call arguments
+ - b6 = return address (a user-level value)
+ - ar.pfs = previous frame-state (a user-level value)
+ - PSR.be = cleared to zero (i.e., little-endian byte order is in effect)
+ - all other registers may contain values passed in from user-mode
+** Required machine state on exit to fsyscall handler:
+ - r11 = saved ar.pfs (as passed into the fsyscall handler)
+ - r15 = system call number (as passed into the fsyscall handler)
+ - r32-r39 = system call arguments (as passed into the fsyscall handler)
+ - b6 = return address (as passed into the fsyscall handler)
+ - ar.pfs = previous frame-state (as passed into the fsyscall handler)
+Fsyscall handlers can execute with very little overhead, but with that
+speed comes a set of restrictions:
+ o Fsyscall-handlers MUST check for any pending work in the flags
+ member of the thread-info structure and if any of the
+ TIF_ALLWORK_MASK flags are set, the handler needs to fall back on
+ doing a full system call (by calling fsys_fallback_syscall).
+ o Fsyscall-handlers MUST preserve incoming arguments (r32-r39, r11,
+ r15, b6, and ar.pfs) because they will be needed in case of a
+ system call restart. Of course, all "preserved" registers also
+ must be preserved, in accordance to the normal calling conventions.
+ o Fsyscall-handlers MUST check argument registers for containing a
+ NaT value before using them in any way that could trigger a
+ NaT-consumption fault. If a system call argument is found to
+ contain a NaT value, an fsyscall-handler may return immediately
+ with r8=EINVAL, r10=-1.
+ o Fsyscall-handlers MUST NOT use the "alloc" instruction or perform
+ any other operation that would trigger mandatory RSE
+ (register-stack engine) traffic.
+ o Fsyscall-handlers MUST NOT write to any stacked registers because
+ it is not safe to assume that user-level called a handler with the
+ proper number of arguments.
+ o Fsyscall-handlers need to be careful when accessing per-CPU variables:
+ unless proper safe-guards are taken (e.g., interruptions are avoided),
+ execution may be pre-empted and resumed on another CPU at any given
+ time.
+ o Fsyscall-handlers must be careful not to leak sensitive kernel'
+ information back to user-level. In particular, before returning to
+ user-level, care needs to be taken to clear any scratch registers
+ that could contain sensitive information (note that the current
+ task pointer is not considered sensitive: it's already exposed
+ through ar.k6).
+ o Fsyscall-handlers MUST NOT access user-memory without first
+ validating access-permission (this can be done typically via
+ probe.r.fault and/or probe.w.fault) and without guarding against
+ memory access exceptions (this can be done with the EX() macros
+ defined by asmmacro.h).
+The above restrictions may seem draconian, but remember that it's
+possible to trade off some of the restrictions by paying a slightly
+higher overhead. For example, if an fsyscall-handler could benefit
+from the shadow register bank, it could temporarily disable PSR.i and
+PSR.ic, switch to bank 0 (bsw.0) and then use the shadow registers as
+needed. In other words, following the above rules yields extremely
+fast system call execution (while fully preserving system call
+semantics), but there is also a lot of flexibility in handling more
+complicated cases.
+* Signal handling
+The delivery of (asynchronous) signals must be delayed until fsys-mode
+is exited. This is acomplished with the help of the lower-privilege
+transfer trap: arch/ia64/kernel/process.c:do_notify_resume_user()
+checks whether the interrupted task was in fsys-mode and, if so, sets
+PSR.lp and returns immediately. When fsys-mode is exited via the
+"br.ret" instruction that lowers the privilege level, a trap will
+occur. The trap handler clears PSR.lp again and returns immediately.
+The kernel exit path then checks for and delivers any pending signals.
+* PSR Handling
+The "epc" instruction doesn't change the contents of PSR at all. This
+is in contrast to a regular interruption, which clears almost all
+bits. Because of that, some care needs to be taken to ensure things
+work as expected. The following discussion describes how each PSR bit
+is handled.
+PSR.be Cleared when entering fsys-mode. A srlz.d instruction is used
+ to ensure the CPU is in little-endian mode before the first
+ load/store instruction is executed. PSR.be is normally NOT
+ restored upon return from an fsys-mode handler. In other
+ words, user-level code must not rely on PSR.be being preserved
+ across a system call.
+PSR.up Unchanged.
+PSR.ac Unchanged.
+PSR.mfl Unchanged. Note: fsys-mode handlers must not write-registers!
+PSR.mfh Unchanged. Note: fsys-mode handlers must not write-registers!
+PSR.ic Unchanged. Note: fsys-mode handlers can clear the bit, if needed.
+PSR.i Unchanged. Note: fsys-mode handlers can clear the bit, if needed.
+PSR.pk Unchanged.
+PSR.dt Unchanged.
+PSR.dfl Unchanged. Note: fsys-mode handlers must not write-registers!
+PSR.dfh Unchanged. Note: fsys-mode handlers must not write-registers!
+PSR.sp Unchanged.
+PSR.pp Unchanged.
+PSR.di Unchanged.
+PSR.si Unchanged.
+PSR.db Unchanged. The kernel prevents user-level from setting a hardware
+ breakpoint that triggers at any privilege level other than 3 (user-mode).
+PSR.lp Unchanged.
+PSR.tb Lazy redirect. If a taken-branch trap occurs while in
+ fsys-mode, the trap-handler modifies the saved machine state
+ such that execution resumes in the gate page at
+ syscall_via_break(), with privilege level 3. Note: the
+ taken branch would occur on the branch invoking the
+ fsyscall-handler, at which point, by definition, a syscall
+ restart is still safe. If the system call number is invalid,
+ the fsys-mode handler will return directly to user-level. This
+ return will trigger a taken-branch trap, but since the trap is
+ taken _after_ restoring the privilege level, the CPU has already
+ left fsys-mode, so no special treatment is needed.
+PSR.rt Unchanged.
+PSR.cpl Cleared to 0.
+PSR.is Unchanged (guaranteed to be 0 on entry to the gate page).
+PSR.mc Unchanged.
+PSR.it Unchanged (guaranteed to be 1).
+PSR.id Unchanged. Note: the ia64 linux kernel never sets this bit.
+PSR.da Unchanged. Note: the ia64 linux kernel never sets this bit.
+PSR.dd Unchanged. Note: the ia64 linux kernel never sets this bit.
+PSR.ss Lazy redirect. If set, "epc" will cause a Single Step Trap to
+ be taken. The trap handler then modifies the saved machine
+ state such that execution resumes in the gate page at
+ syscall_via_break(), with privilege level 3.
+PSR.ri Unchanged.
+PSR.ed Unchanged. Note: This bit could only have an effect if an fsys-mode
+ handler performed a speculative load that gets NaTted. If so, this
+ would be the normal & expected behavior, so no special treatment is
+ needed.
+PSR.bn Unchanged. Note: fsys-mode handlers may clear the bit, if needed.
+ Doing so requires clearing PSR.i and PSR.ic as well.
+PSR.ia Unchanged. Note: the ia64 linux kernel never sets this bit.
+* Using fast system calls
+To use fast system calls, userspace applications need simply call
+__kernel_syscall_via_epc(). For example
+-- example fgettimeofday() call --
+-- fgettimeofday.S --
+#include <asm/asmmacro.h>
+.save ar.pfs, r11
+mov r11 = ar.pfs
+mov r2 = 0xa000000000020660;; // gate address
+ // found by inspection of System.map for the
+ // __kernel_syscall_via_epc() function. See
+ // below for how to do this for real.
+mov b7 = r2
+mov r15 = 1087 // gettimeofday syscall
+br.call.sptk.many b6 = b7
+.restore sp
+mov ar.pfs = r11
+br.ret.sptk.many rp;; // return to caller
+-- end fgettimeofday.S --
+In reality, getting the gate address is accomplished by two extra
+values passed via the ELF auxiliary vector (include/asm-ia64/elf.h)
+ o AT_SYSINFO : is the address of __kernel_syscall_via_epc()
+ o AT_SYSINFO_EHDR : is the address of the kernel gate ELF DSO
+The ELF DSO is a pre-linked library that is mapped in by the kernel at
+the gate page. It is a proper ELF shared object so, with a dynamic
+loader that recognises the library, you should be able to make calls to
+the exported functions within it as with any other shared library.
+AT_SYSINFO points into the kernel DSO at the
+__kernel_syscall_via_epc() function for historical reasons (it was
+used before the kernel DSO) and as a convenience.
diff --git a/Documentation/ia64/serial.txt b/Documentation/ia64/serial.txt
new file mode 100644
index 000000000000..f51eb4bc2ff1
--- /dev/null
+++ b/Documentation/ia64/serial.txt
@@ -0,0 +1,144 @@
+ As of 2.6.10, serial devices on ia64 are named based on the
+ order of ACPI and PCI enumeration. The first device in the
+ ACPI namespace (if any) becomes /dev/ttyS0, the second becomes
+ /dev/ttyS1, etc., and PCI devices are named sequentially
+ starting after the ACPI devices.
+ Prior to 2.6.10, there were confusing exceptions to this:
+ - Firmware on some machines (mostly from HP) provides an HCDP
+ table[1] that tells the kernel about devices that can be used
+ as a serial console. If the user specified "console=ttyS0"
+ or the EFI ConOut path contained only UART devices, the
+ kernel registered the device described by the HCDP as
+ /dev/ttyS0.
+ - If there was no HCDP, we assumed there were UARTs at the
+ legacy COM port addresses (I/O ports 0x3f8 and 0x2f8), so
+ the kernel registered those as /dev/ttyS0 and /dev/ttyS1.
+ Any additional ACPI or PCI devices were registered sequentially
+ after /dev/ttyS0 as they were discovered.
+ With an HCDP, device names changed depending on EFI configuration
+ and "console=" arguments. Without an HCDP, device names didn't
+ change, but we registered devices that might not really exist.
+ For example, an HP rx1600 with a single built-in serial port
+ (described in the ACPI namespace) plus an MP[2] (a PCI device) has
+ these ports:
+ pre-2.6.10 pre-2.6.10
+ MMIO (EFI console (EFI console
+ address on builtin) on MP port) 2.6.10
+ ========== ========== ========== ======
+ builtin 0xff5e0000 ttyS0 ttyS1 ttyS0
+ MP UPS 0xf8031000 ttyS1 ttyS2 ttyS1
+ MP Console 0xf8030000 ttyS2 ttyS0 ttyS2
+ MP 2 0xf8030010 ttyS3 ttyS3 ttyS3
+ MP 3 0xf8030038 ttyS4 ttyS4 ttyS4
+ EFI knows what your console devices are, but it doesn't tell the
+ kernel quite enough to actually locate them. The DIG64 HCDP
+ table[1] does tell the kernel where potential serial console
+ devices are, but not all firmware supplies it. Also, EFI supports
+ multiple simultaneous consoles and doesn't tell the kernel which
+ should be the "primary" one.
+ So how do you tell Linux which console device to use?
+ - If your firmware supplies the HCDP, it is simplest to
+ configure EFI with a single device (either a UART or a VGA
+ card) as the console. Then you don't need to tell Linux
+ anything; the kernel will automatically use the EFI console.
+ (This works only in 2.6.6 or later; prior to that you had
+ to specify "console=ttyS0" to get a serial console.)
+ - Without an HCDP, Linux defaults to a VGA console unless you
+ specify a "console=" argument.
+ NOTE: Don't assume that a serial console device will be /dev/ttyS0.
+ It might be ttyS1, ttyS2, etc. Make sure you have the appropriate
+ entries in /etc/inittab (for getty) and /etc/securetty (to allow
+ root login).
+ The kernel can't start using a serial console until it knows where
+ the device lives. Normally this happens when the driver enumerates
+ all the serial devices, which can happen a minute or more after the
+ kernel starts booting.
+ 2.6.10 and later kernels have an "early uart" driver that works
+ very early in the boot process. The kernel will automatically use
+ this if the user supplies an argument like "console=uart,io,0x3f8",
+ or if the EFI console path contains only a UART device and the
+ firmware supplies an HCDP.
+ No kernel output after elilo prints "Uncompressing Linux... done":
+ - You specified "console=ttyS0" but Linux changed the device
+ to which ttyS0 refers. Configure exactly one EFI console
+ device[3] and remove the "console=" option.
+ - The EFI console path contains both a VGA device and a UART.
+ EFI and elilo use both, but Linux defaults to VGA. Remove
+ the VGA device from the EFI console path[3].
+ - Multiple UARTs selected as EFI console devices. EFI and
+ elilo use all selected devices, but Linux uses only one.
+ Make sure only one UART is selected in the EFI console
+ path[3].
+ - You're connected to an HP MP port[2] but have a non-MP UART
+ selected as EFI console device. EFI uses the MP as a
+ console device even when it isn't explicitly selected.
+ Either move the console cable to the non-MP UART, or change
+ the EFI console path[3] to the MP UART.
+ Long pause (60+ seconds) between "Uncompressing Linux... done" and
+ start of kernel output:
+ - No early console because you used "console=ttyS<n>". Remove
+ the "console=" option if your firmware supplies an HCDP.
+ - If you don't have an HCDP, the kernel doesn't know where
+ your console lives until the driver discovers serial
+ devices. Use "console=uart, io,0x3f8" (or appropriate
+ address for your machine).
+ Kernel and init script output works fine, but no "login:" prompt:
+ - Add getty entry to /etc/inittab for console tty. Look for
+ the "Adding console on ttyS<n>" message that tells you which
+ device is the console.
+ "login:" prompt, but can't login as root:
+ - Add entry to /etc/securetty for console tty.
+[1] http://www.dig64.org/specifications/DIG64_PCDPv20.pdf
+ The table was originally defined as the "HCDP" for "Headless
+ Console/Debug Port." The current version is the "PCDP" for
+ "Primary Console and Debug Port Devices."
+[2] The HP MP (management processor) is a PCI device that provides
+ several UARTs. One of the UARTs is often used as a console; the
+ EFI Boot Manager identifies it as "Acpi(HWP0002,700)/Pci(...)/Uart".
+ The external connection is usually a 25-pin connector, and a
+ special dongle converts that to three 9-pin connectors, one of
+ which is labelled "Console."
+[3] EFI console devices are configured using the EFI Boot Manager
+ "Boot option maintenance" menu. You may have to interrupt the
+ boot sequence to use this menu, and you will have to reset the
+ box after changing console configuration.

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