linux-kernel - Re: [PATCH v5 2/2] tracing: Add documentation for funcgraph-retval and graph_retval

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Date:   Tue, 21 Mar 2023 10:31:21 +0800
From:   Ding Hui <dinghui@...gfor.com.cn>
To:     Donglin Peng <pengdonglin@...gfor.com.cn>, mhiramat@...nel.org,
        rostedt@...dmis.org, linux@...linux.org.uk, mark.rutland@....com,
        will@...nel.org, catalin.marinas@....com, palmer@...belt.com,
        paul.walmsley@...ive.com, tglx@...utronix.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, mingo@...hat.com,
        xiehuan09@...il.com, huangcun@...gfor.com.cn,
        dolinux.peng@...il.com
Cc:     linux-trace-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/2] tracing: Add documentation for funcgraph-retval
 and graph_retval_hex

On 2023/3/20 21:16, Donglin Peng wrote:

> +There are some limitations when using the funcgraph-retval currently:
> +
> +- Even if the function return type is void, a return value will still
> +  be printed, and you can just ignore it.
> +
> +- Even if the return value is not an error code actually, it may be
> +  displayed as an error code. You should read the code to check.
> +  For example, both 0xfe and 0xfffe are be interpreted as -2.

For char and short types, displaying as signed decimal may be not 
appropriate, because they are rarely used to store error code.

So in "smart" mode (graph_retval_hex=0), I suggest just smart convert 
error value stored in int or pointer to signed decimal.

> +- Only the value of the first return register will be recorded and
> +  printed even if the return values may be stored in two registers
> +  actually. For example, both the eax and edx are used to store a
> +  64 bit return value in the x86 architecture, and the eax stores
> +  the low 32 bit, the edx stores the high 32 bit, however only the
> +  value stored in eax will be recorded and printed.
> +
>   You can put some comments on specific functions by using
>   trace_printk() For example, if you want to put a comment inside
>   the __might_sleep() function, you just have to include

-- 
Thanks,
- Ding Hui