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

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Message-ID: <469f6c0d-186e-aaf7-469f-3d5b416b2b3f@embeddedor.com>
Date:   Thu, 8 Mar 2018 11:27:25 -0600
From:   "Gustavo A. R. Silva" <garsilva@...eddedor.com>
To:     Emil Velikov <emil.l.velikov@...il.com>,
        "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>,
        "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 Emil,

On 03/08/2018 05:58 AM, Emil Velikov wrote:
> 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?
> 

Sure. This is the thread: https://lkml.org/lkml/2018/3/7/621

>> 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.
> 

You are correct.

> 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
> 

Yeah, this works just fine.

There are a total of four instances of this same issue in other files 
for the VIA component. I'll fix them and send a single patch shortly.

Thanks for the feedback.
I appreciate it.
--
Gustavo