Pancho  wrote:
> On 19/06/2020 03:07, John Carter wrote:
> > T M Smith  wrote in
> > news:5df3fded54.Broadband@tmsmith36.plus.com:
> >
> >> Using Rasbian on a Pi is there a log kept of the data that scolls
> >> past as the system boots?
> >>
> >> Malcolm Smith
> >>
> >
> > I was told early on that there was no log. It has something to do with
> > the BOOT process starting in the GPU and there is a point where the
> > filesystem is mounted such that it is too late to do any BOOT logging.
> >
> > Pleae correct me here if this is wrong.
> >
>
> I can't comment on whether you are right or wrong.

Mostly wrong.

The bootloader (the thing that does the coloured square) doesn't write to
flash and doesn't log TTBOMK (apart from on the Pi 4 where using the serial
port is an option).

But as soon as the Linux kernel is loaded it starts logging.  To begin with
it logs to a memory buffer.  When the filesystem/init system comes up that
buffer is written to disc and subsequent logging gets appended.

The memory buffer can be dumped via the 'dmesg' command or the /proc/kmsg
device.  The location of the log can vary by distro but /var/log/messages,
kern.log and boot.log are good places to start.

> I understand the BIOS boot sequence might not be able to write to disk
> but surely the boot process needs access to the file system before
> booting Linux from the file system.

The two stages of bootloader do know how to read SD card and more recently
USB.  However it's a lot simpler and less dangerous to read something than
it is to write it.

Theo

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