linux-kernel - Re: [PATCH v3 1/2] vsprintf: Add %p[mM]U for uppercase MAC address

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Date:   Fri, 28 Apr 2023 08:56:59 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Konrad Gräfe <k.graefe@...eware.de>,
        Quentin Schulz <quentin.schulz@...obroma-systems.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Kyungmin Park <kyungmin.park@...sung.com>,
        Andrzej Pietrasiewicz <andrzej.p@...labora.com>,
        Felipe Balbi <balbi@...com>
Cc:     stable@...r.kernel.org
Subject: Re: [PATCH v3 1/2] vsprintf: Add %p[mM]U for uppercase MAC address

On 27/04/2023 13.51, Konrad Gräfe wrote:
> The CDC-ECM specification requires an USB gadget to send the host MAC
> address as uppercase hex string. This change adds the appropriate
> modifier.

Thinking more about it, I'm not sure this is appropriate, not for a
single user like this. vsprintf() should not and cannot satisfy all
possible string formatting requirements for the whole kernel. The %pX
extensions are convenient for use with printk() and friends where one
needs what in other languages would be "string interpolation" (because
then the caller doesn't need to deal with temporary stack buffers and
pass them as %s arguments), but for single items like this, snprintf()
is not necessarily the right tool for the job.

In this case, the caller can just as well call string_upper() on the
result, or not use sprintf() at all and do a tiny loop with
hex_byte_pack_upper().

Rasmus