[PATCH v3 0/3] Add command to display or save Linux PStore dumps

[Heiko Schocher]

Hello Wolfgang, Heinrich,

Am 20.03.2020 um 09:08 schrieb Wolfgang Denk:
> Dear Heinrich,
> 
> In message <a1b7a877-e950-b09a-a0f6-2c9c9cb7e1fb at gmx.de> you wrote:
>>
>>> To me this sounds very much like what you are adding here (plus a
>>> few features more).  Does it make sense to unify such code?
>>
>> It seems you are relating to
>> https://lore.kernel.org/lkml/844oyrqvvb.fsf@sauna.l.org/t/
> 
> No, I'm not.  I was talking of my own code from many, many years
> ago.
> 
>> ramoops in Linux is exactly doing what was suggested in 2009. You can
>> find the Documentation/admin-guide/ramoops.rst
> 
> We had this in U-Boot long before that time.  It was a key
> requirement when we added POST support in 2002.
> 
>> git grep -GHrn 'shared log' finds nothing in U-Boot. So if any part of
>> the old implementation in U-Boot exists, could you, please, point us to
>> the coding.
> 
> The shared log buffer support was added by commit 56f94be3ef63:
> 
> commit 56f94be3ef63732384063e110277ed89701b6471 (tag: LABEL_2002_11_05_1735)
> Author: Wolfgang Denk <wdenk>
> Date:   Tue Nov 5 16:35:14 2002 +0000
> 
>      * Add support for log buffer which can be passed to Linux kernel's
>        syslog mechanism; used especially for POST results.
> 
>      * Patch by Klaus Heydeck, 31 Oct 2002:
>        Add initial support for kup4k board
> 
> The code was mainly in common/cmd_log.c, but this got heahily
> rewritten and is now in cmd/log.c ; apparently this got lost (like
> the original copyright, sic!) when Simon modified / rewrote this
> driver.
> 
> For history, try: git log --follow -- common/cmd_log.c
> 
> 
>> If the original design never made it into Linux and there is an
>> established Linux interface since 2011, I would plead to eliminate any
>> remaining non-compliant coding from U-Boot should it exist.
> 
> I understand what Linus has is one-way, only focussing on crash
> dump, i. e. it does not allow to pass information from U-Boot to Linux?
> 
> Also, my understanding is that the changes needed in Linux are
> pretty small.
> 
> Maybe Heiko can comment on that...

Since 2002 the logbuffer support in linux changed, so the old
patch does not work anymore.

First there is now the struct "printk_log" [1]

Unfortunately the size of it is dependend on linux configuration.

Each log entry is prepended from such a struct, just missing
the possibility to detect if entry is valid (as linux was the only
user of it, this was not needed).

There is already a possibilty to set a log buffer length through
kernel commandline parameter "log_buf_len" [2].
If this is passed, kernel already uses a new area (currently
memblock_alloced) for log buffer storage!

So added now the following to linux logbuffer handling:

Pass the persistent logbuffer address and length through a
kernel commandline parameter and use it instead memblock_alloc()

Add a magic field in [1] which I use to detect if there are valid
entries and where is the end of logbuffer entries in the new logbuffer
area (Not that easy as there was may a wrap around, but also this is
detectable)

-> start and end in the new log area is now known.

Now copy the new logbuffer messages from __log_buf with the
already existing functions in linux kernel to the end of the
new logbuffer area ... and you have old linux bootlogs and
the new ones after a reboot.

aditionally changes in U-Boot:

- enable PRAM to reserve RAM at the end of RAM for logbuffer
   so U-Boot does not use this memory space. Already in U-Boot.

- add common/log_buffer.c log backend, which uses the functions
   from kernel for analysing/writing content of the logbuffer area.

   "log rec" U-Boot command appends now also U-Boot log messages to
   the logbuffer area.

- add "log show" command, with which you can show current content
   of logbuffer area

And now U-Boot log messages also shown in linux bootlog ...

bye
Heiko
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/printk/printk.c#n368

[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/printk/printk.c#n1114

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