linux-kernel - Re: [PATCH v1 19/24] media: rkvdec-h264: Add field decoding support

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Message-ID: <ce1cbeab5e4a8a8338f2567f05af3226305410c7.camel@collabora.com>
Date:   Thu, 31 Mar 2022 09:40:18 -0400
From:   Nicolas Dufresne <nicolas.dufresne@...labora.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
Cc:     Ezequiel Garcia <ezequiel@...guardiasur.com.ar>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        kernel@...labora.com, linux-media@...r.kernel.org,
        linux-rockchip@...ts.infradead.org, linux-staging@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 19/24] media: rkvdec-h264: Add field decoding support

Le mercredi 30 mars 2022 à 18:16 +0300, Dan Carpenter a écrit :
> Yeah.  I'm aboslutely fine with whatever you do.  Some of the questions
> you're asking occurred to me too but I don't have the answers.
> 
> > > > > > +		for (i = 0; i < builder->num_valid; i++) {
> > > > > > +			struct v4l2_h264_reference *ref;
> > > > > > +			u8 dpb_valid;
> > > > > > +			u8 bottom;
> > > > > 
> > > > > These would be better as type bool.
> > > > 
> > > > I never used a bool for bit operations before, but I guess that can work, thanks
> > > > for the suggestion. As this deviates from the original code, I suppose I should
> > > > make this a separate patch ?
> > > 
> > > I just saw the name and wondered why it was a u8.  bool does make more
> > > sense and works fine for the bitwise stuff.  But I don't really care at
> > > all.
> > 
> > I'll do that in v2, in same patch, looks minor enough. I think if using bool
> > could guaranty that only 1 or 0 is  possible, it would be even better, but don't
> > think C works like this.
> 
> I'm not sure I understand.  If you assign "bool x = <any non-zero>;"
> then x is set to true.  Do you want a static checker warning for if
> <any non-zero> can be something other than one or zero?  The problem is
> that people sometimes deliberately do stuff like "bool x = var & 0xf0;".
> Smatch will complain if you assign a negative value to x.
> 
> test.c:8 test() warn: assigning (-3) to unsigned variable 'x'
> 
> It's supposed to print a warning if you used it to save error codes like:
> 
> 	x = some_kernel_function();
> 
> But it does not.  :/  Something to investigate.

That would be an amazing catch, you might have seen a lot of:

  x = !!(var & 0xf0)

For branches, it does no matter, but if you use x it like this dpb_valid
variable is used, not having 0 or 1 can lead to very surprising results. In the
end its used like this

  set_reg(reg0, val | (x << N))

So using bool type can hint the analyzer that 0 or 1 was likely expected, while
currently an u8 would be ambiguous and lead to false positive if we were to
warn.

> 
> regards,
> dan carpenter
>