Re: [PATCH 0/6] perf/ftrace: Introduce hexadecimal type casting

[Arnaldo Carvalho de Melo]

Em Thu, Aug 18, 2016 at 05:57:32PM +0900, Masami Hiramatsu escreveu:
> Hi Arnaldo and Steven,
> 
> Here is an RFC series of hexadecimal type casting and
> changing default type casting of perf and ftrace.
> 
> I've introduced x8,x16,x32,x64 according to previous
> discussion on LKML.
>   https://lkml.org/lkml/2016/8/10/339
> 
> This series includes not only adding hexadecimal types
> (x8,x16,x32,x64), but also checking it is supported by
> running kernel and keeping the backward compativility.
> 
> [1/6] Add hexadecimal type casting, but does not touch
>    existing types like 'u8'.
> [2/6] Show the supported types on README of ftrace so
>    that user application (e.g. perf) can check that.
> [3/6] Add a type availability check to perf-probe.
> [4/6] Add hexadecimal prefix support to perf-probe if
>    it is supported by the kernel. 
> [5/6] Change the perf-probe default type casting for
>    unsigned type to hexadecimal (for backward compatibility)
> [6/6] Change ftrace's 'uNN' to show value in decimal
>    and use 'xNN' by default (for backward compatibility)
> 
> This way, we can also add "octal" type, pointer type,
> and "character" type etc. and perf can check whether
> the kernel supports it or not. :)

But this requires a kernel update... If we do it all in the tooling
side, no kernel changes are required _and_ newer tools will work with
older kernels, as this is just a formatting issue, the value is there
and from its format one can infer its value, it is not even necessary to
look at its "type".

I understand this is necessary for ftrace, because the pretty printer is
in the kernel, but I don't see why we would prevent tooling from doing
this pretty printing work and make it support any kernel.

I.e. no need at all for checking if the kernel supports anything, just
pretty print it.

- Arnaldo