linux-kernel - Re: [PATCH] video: fbdev: via_aux

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Date:   Thu, 8 Mar 2018 11:58:50 +0000
From:   Emil Velikov <emil.l.velikov@...il.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc:     Florian Tobias Schandinat <FlorianSchandinat@....de>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        linux-fbdev <linux-fbdev@...r.kernel.org>,
        "Gustavo A. R. Silva" <garsilva@...eddedor.com>,
        "Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>,
        ML dri-devel <dri-devel@...ts.freedesktop.org>
Subject: Re: [PATCH] video: fbdev: via_aux_vt1632: remove VLA usage

Hi Gustavo,

On 7 March 2018 at 19:54, Gustavo A. R. Silva <gustavo@...eddedor.com> wrote:
> In preparation to enabling -Wvla, remove VLA and replace it
> with a fixed-length array instead. Also, remove variable 'len'.
>
What is the reason behind adding -Wvla? Is there a thread some that I
can read up on?

> Notice that no new IDs have been added in seven years.
>
> Signed-off-by: Gustavo A. R. Silva <gustavo@...eddedor.com>
> ---
>  drivers/video/fbdev/via/via_aux_vt1632.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/video/fbdev/via/via_aux_vt1632.c b/drivers/video/fbdev/via/via_aux_vt1632.c
> index d24f4cd..0cd0d2a 100644
> --- a/drivers/video/fbdev/via/via_aux_vt1632.c
> +++ b/drivers/video/fbdev/via/via_aux_vt1632.c
> @@ -35,10 +35,10 @@ static void probe(struct via_aux_bus *bus, u8 addr)
>                 .addr   =       addr,
>                 .name   =       name};
>         /* check vendor id and device id */
> -       const u8 id[] = {0x06, 0x11, 0x92, 0x31}, len = ARRAY_SIZE(id);
> -       u8 tmp[len];
> +       const u8 id[4] = {0x06, 0x11, 0x92, 0x31};
> +       u8 tmp[4];
>
> -       if (!via_aux_read(&drv, 0x00, tmp, len) || memcmp(id, tmp, len))
> +       if (!via_aux_read(&drv, 0x00, tmp, 4) || memcmp(id, tmp, 4))

Generally, hard coding a bunch of numbers like that makes for
confusing and fragile code.

A lot simpler and more obvious solution is like the following.
It silences the compiler warning (-Wvla - pedantic) you while keeping
the original clarity.

        const u8 id[] = {0x06, 0x11, 0x92, 0x31}, len = ARRAY_SIZE(id);
-       u8 tmp[len];
+       u8 tmp[ARRAY_SIZE(id)]; // Using len causes a Wvla warning

HTH
Emil