On 02/12/2018 09:56 AM, Peter Zijlstra wrote:
> On Mon, Feb 12, 2018 at 08:47:57AM -0500, Prarit Bhargava wrote:
>>
>>
>> On 02/12/2018 08:34 AM, Peter Zijlstra wrote:
>>> On Thu, Jan 18, 2018 at 10:09:51AM -0500, Prarit Bhargava wrote:
>>>>  config ACPI_SPCR_TABLE
>>>> +  bool "ACPI Serial Port Console Redirection Support"
>>>> +  default y if X86
>>>> +  help
>>>> +    Enable support for Serial Port Console Redirection (SPCR) Table.
>>>> +    This table provides information about the configuration of the
>>>> +    earlycon console.
>>>
>>> I just got asked this by oldconfig, which left me rather puzzled, WTF
>>> does this do?
>>
>> Odd -- I thought I had taken care of that :(  My apologies Peter.
>>
>> ACPI SPCR is used by a vendor to define the serial console for a system.  If
>> SPCR exists a user can add kernel parameter "earlycon" (no extra kernel
>> parameters) and the console will work out-of-the-box.
>>
>> The serial console configuration varies from vendor to vendor.  This takes 
>> the
>> guess-work out of defining a serial console.
> 
> But when I specify "earlyprintk=serial,ttyS0,115200" this SPCR crud will
> not interfere?
> 

I tested "earlyprintk=serial,ttyS0,115200" on a system which is known to have a
functional console with "console=ttyS0,1152008N1" both with and without
CONFIG_ACPI_SPCR_TABLE enabled.

I do not see any significant difference in output.

diff of boot logs:

--- earlyprintk_no_spcr_enable  2018-02-12 10:11:00.477288423 -0500
+++ earlyprintk_spcr_enable     2018-02-12 10:16:02.282574797 -0500
@@ -1,5 +1,5 @@
 microcode: microcode updated early to revision 0x2000035, date = 2017-10-17
-Linux version 4.15.0+ (r...@intel-purley-lr-02.khw.lab.eng.bos.redhat.com) (gcc
version 4.8.5 20150623 (Red Hat 4.8.5-28) (GCC)) #5 SMP Mon Feb 12 10:06:08 EST 
2018
+Linux version 4.15.0+ (r...@intel-purley-lr-02.khw.lab.eng.bos.redhat.com) (gcc
version 4.8.5 20150623 (Red Hat 4.8.5-28) (GCC)) #6 SMP Mon Feb 12 10:11:42 EST 
2018
 Command line: BOOT_IMAGE=/vmlinuz-4.15.0+
root=/dev/mapper/rhel_intel--purley--lr--02-root ro crashkernel=auto
rd.lvm.lv=rhel_intel-purley-lr-02/root rd.lvm.lv=rhel_intel-purley-lr-02/swap
earlyprintk=serial,ttyS0,115200 LANG=en_US.UTF-8
 x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers'
 x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
@@ -205,7 +205,7 @@
 e820: last_pfn = 0x6f800 max_arch_pfn = 0x400000000
 Using GB pages for direct mapping
 Secure boot disabled
-RAMDISK: [mem 0x3a563000-0x3bba2fff]
+RAMDISK: [mem 0x3a5da000-0x3bba2fff]
 ACPI: Early table checksum verification disabled
 ACPI: RSDP 0x000000006922A014 000024 (v02 INTEL )
 ACPI: XSDT 0x00000000690DD188 000114 (v01 INTEL  EDK2     00000000      
01000013)
@@ -519,6 +519,8 @@
 ACPI: INT_SRC_OVR (bus 0 bus_irq 9 global_irq 9 high level)
 Using ACPI (MADT) for SMP configuration information
 ACPI: HPET id: 0x8086a701 base: 0xfed00000
+ACPI: SPCR: SPCR table version 1
+ACPI: SPCR: console: uart,io,0x3f8,115200
 smpboot: Allowing 432 CPUs, 224 hotplug CPUs
 PM: Registered nosave memory: [mem 0x00000000-0x00000fff]
 PM: Registered nosave memory: [mem 0x000a0000-0x000fffff]
@@ -555,10 +557,10 @@
 log_buf_len total cpu_extra contributions: 1765376 bytes
 log_buf_len min size: 524288 bytes
 log_buf_len: 4194304 bytes
-early log buf free: 477984(91%)
-Memory: 394593504K/401282564K available (10252K kernel code, 2147K rwdata,
3664K rodata, 2504K init, 1800K bss, 6689060K reserved, 0K cma-reserved)
+early log buf free: 477876(91%)
+Memory: 394593976K/401282564K available (10252K kernel code, 2147K rwdata,
3664K rodata, 2504K init, 1800K bss, 6688588K reserved, 0K cma-reserved)
 Kernel/User page tables isolation: enabled
-ftrace: allocating 35205 entries in 138 pages
+ftrace: allocating 35206 entries in 138 pages
 Hierarchical RCU implementation.
        RCU restricting CPUs from NR_CPUS=8192 to nr_cpu_ids=432.
        Tasks RCU enabled.

P.

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