Re: [RFC PATCH 00/11] tracing: of: Boot time tracing using devicetree

[Masami Hiramatsu]

Hello,

On Thu, 27 Jun 2019 19:58:17 +0900
Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:

> Hi Rob,
> 
> On Wed, 26 Jun 2019 15:58:50 -0600
> Rob Herring <robh+dt@xxxxxxxxxx> wrote:
> 
> > On Fri, Jun 21, 2019 at 10:18 AM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> > >
> > > Hi,
> > >
> > > Here is an RFC series of patches to add boot-time tracing using
> > > devicetree.
> > >
> > > Currently, kernel support boot-time tracing using kernel command-line
> > > parameters. But that is very limited because of limited expressions
> > > and limited length of command line. Recently, useful features like
> > > histogram, synthetic events, etc. are being added to ftrace, but it is
> > > clear that we can not expand command-line options to support these
> > > features.
> > >
> > > Hoever, I've found that there is a devicetree which can pass more
> > > structured commands to kernel at boot time :) The devicetree is usually
> > > used for dscribing hardware configuration, but I think we can expand it
> > > for software configuration too (e.g. AOSP and OPTEE already introduced
> > > firmware node.) Also, grub and qemu already supports loading devicetree,
> > > so we can use it not only on embedded devices but also on x86 PC too.
> > 
> > Do the x86 versions of grub, qemu, EFI, any other bootloader actually
> > enable DT support? I didn't think so. Certainly, an x86 kernel doesn't
> > normally (other than OLPC and ce4100) have a defined way to even pass
> > a dtb from the bootloader to the kernel and the kernel doesn't
> > unflatten the dtb.
> 
> Sorry, the grub part, I just found this entry. I need to check this
> can work on x86 too.

I've confirmed that grub-x86 doesn't support devicetree option. I tried
to add it, and tested it.

https://github.com/mhiramat/grub/commit/644c35bfd2d18c772cc353b74215344f8264923a

This works if there is ACPI, if it includes /chosen/linux,ftrace node only.
(Anyway, we don't need other nodes on x86)

At this moment, grub doesn't support DT overlay, so on arm/arm64 user must
decompile DTB, add linux,ftrace node for tracing and compile it again.
But if it supports overlay, I think we can give an overlay for tracer setting
on boot up, that will be handy on arm/arm64 too. :)

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>