linux-kernel - Re: [PATCH] of: Split up name & type in modalias generation

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Date:   Fri, 7 Sep 2018 13:49:22 -0500
From:   Rob Herring <robh+dt@...nel.org>
To:     Thierry Reding <thierry.reding@...il.com>
Cc:     Frank Rowand <frowand.list@...il.com>, devicetree@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] of: Split up name & type in modalias generation

On Fri, Sep 7, 2018 at 9:22 AM Thierry Reding <thierry.reding@...il.com> wrote:
>
> From: Thierry Reding <treding@...dia.com>
>
> The kernel's vsnprintf() implementation discards all alpha-numeric
> characters following a %p conversion specifier. This is done in order to
> generically skip any of the various modifiers that the kernel supports.
> Unfortunately, the OF modalias is generated with a format string that
> violates the assumption made by vsnprintf():
>
>         of:N%pOFnT%s
>
> While processing the above format string, vsnprintf() will eat the 'T'
> character, assuming that it belongs to the preceeding %p specifier. This
> results in a modalias with an incompatible format, which in turn causes
> the automatic loading of drivers based on modalias to no longer work.
>
> To fix this, split up the generation of the name & type fields into two
> separate snprintf() calls to avoid confusing the parser.
>
> Fixes: 73813f8483b1 ("of: Convert to using %pOFn instead of device_node.name")
> Signed-off-by: Thierry Reding <treding@...dia.com>
> ---
> Note that a more elegant fix would be to make the %p format specifier
> parser report back the exact number of characters consumed. I briefly
> tried to implement it, but quickly ran into numerous special cases
> that make this solution rather involved.
>
> I can spend some more time to improve this in general if that's what we
> ultimately want, but I think this patch is a better short-term fix to
> workaround the issue.

See my reply on the original patch. I've updated the patch in my
dt/next branch with the fix to use %c.

Rob