linux-kernel - Re: [PATCH v3] media: si2168: Refactor command setup code

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Date:   Sun, 14 Jul 2019 20:31:18 +0200
From:   Matthias Schwarzott <zzam@...too.org>
To:     Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Marc Gonzalez <marc.w.gonzalez@...e.fr>
Cc:     Brad Love <brad@...tdimension.cc>, Antti Palosaari <crope@....fi>,
        Jonathan Neuschäfer <j.neuschaefer@....net>,
        linux-media <linux-media@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        MSM <linux-arm-msm@...r.kernel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>
Subject: Re: [PATCH v3] media: si2168: Refactor command setup code

Am 13.07.19 um 12:02 schrieb Mauro Carvalho Chehab:
> Em Sat, 13 Jul 2019 00:11:12 +0200
> Marc Gonzalez <marc.w.gonzalez@...e.fr> escreveu:
> 
>> On 12/07/2019 19:45, Mauro Carvalho Chehab wrote:
>>
>>> Brad Love <brad@...tdimension.cc> escreveu:
>>>   
>>> IMHO, using sizeof() here is a very bad idea.  
>>
>> You may have a point...
>> (Though I'm not proposing a kernel API function, merely code
>> refactoring for a single file that's unlikely to change going
>> forward.)
> 
> Yes, I know, but we had already some bugs due to the usage of
> sizeof() on similar macros at drivers in the past.
> 
>> It's also bad form to repeat the cmd size (twice) when the compiler
>> can figure it out automatically for string literals (which is 95%
>> of the use-cases).
>>
>> I can drop the macro, and just use the helper...
> 
> The helper function sounds fine.
> 
>>
>> Or maybe there's a GCC extension to test that an argument is a
>> string literal...
> 
> If this could be evaluated by some advanced macro logic that
> would work not only with gcc but also with clang, then a
> macro that does what you proposed could be useful.
> 
> There are some ways to check the type of a macro argument, but I'm
> not sure if are there any way for it to distinguish between a
> string constant from a char * array.
> 
Maybe something like this will prevent compilation if the argument is no
string literal:

#define CMD_SETUP(cmd, args, rlen) \
	cmd_setup(cmd, args "", sizeof(args) - 1, rlen)

Another idea is a check like:

#define CMD_SETUP(cmd, args, rlen) \
	do { \
		BUILD_BUG_ON(#args[0] != "\""); \
		cmd_setup(cmd, args "", sizeof(args) - 1, rlen) \
	} while(0)

Regards
Matthias