Re: [PATCH v4 02/13] media: media/test_drivers: Replace open-coded parity calculation with parity_odd()

On Wed, Apr 09, 2025 at 02:41:03PM -0400, Yury Norov wrote:
> On Thu, Apr 10, 2025 at 02:23:09AM +0800, Kuan-Wei Chiu wrote:
> > On Wed, Apr 09, 2025 at 01:03:42PM -0400, Yury Norov wrote:
> > > On Wed, Apr 09, 2025 at 11:43:45PM +0800, Kuan-Wei Chiu wrote:
> > > > Refactor parity calculations to use the standard parity_odd() helper.
> > > > This change eliminates redundant implementations.
> > > > 
> > > > Co-developed-by: Yu-Chun Lin <[email protected]>
> > > > Signed-off-by: Yu-Chun Lin <[email protected]>
> > > > Signed-off-by: Kuan-Wei Chiu <[email protected]>
> > > > ---
> > > >  drivers/media/test-drivers/vivid/vivid-vbi-gen.c | 8 ++------
> > > >  1 file changed, 2 insertions(+), 6 deletions(-)
> > > > 
> > > > diff --git a/drivers/media/test-drivers/vivid/vivid-vbi-gen.c b/drivers/media/test-drivers/vivid/vivid-vbi-gen.c
> > > > index 70a4024d461e..5e1b7b1742e4 100644
> > > > --- a/drivers/media/test-drivers/vivid/vivid-vbi-gen.c
> > > > +++ b/drivers/media/test-drivers/vivid/vivid-vbi-gen.c
> > > > @@ -5,6 +5,7 @@
> > > >   * Copyright 2014 Cisco Systems, Inc. and/or its affiliates. All rights reserved.
> > > >   */
> > > >  
> > > > +#include <linux/bitops.h>
> > > >  #include <linux/errno.h>
> > > >  #include <linux/kernel.h>
> > > >  #include <linux/ktime.h>
> > > > @@ -165,12 +166,7 @@ static const u8 vivid_cc_sequence2[30] = {
> > > >  
> > > >  static u8 calc_parity(u8 val)
> > > >  {
> > > > -	unsigned i;
> > > > -	unsigned tot = 0;
> > > > -
> > > > -	for (i = 0; i < 7; i++)
> > > > -		tot += (val & (1 << i)) ? 1 : 0;
> > > > -	return val | ((tot & 1) ? 0 : 0x80);
> > > > +	return val | (parity_odd(val) ? 0 : 0x80);
> > > 
> > > So, if val == 0 than parity_odd(val) is also 0, and this can be
> > > simplified just to:
> > >         return parity(val) ? 0 : 0x80;
> > > Or I miss something?
> > >
> > If val == 0x01, the return value of calc_parity() will remain 0x01.
> > If changed to return parity_odd(val) ? 0 : 0x80;, the return value will
> > be changed to 0x00.
> 
> Sorry, I meant
>         return val ? 0 : 0x80;
> 
> This 'val | (parity_odd(val)' is only false when val == 0, right?
> When val != 0, compiler will return true immediately, not even calling
> parity().
>
I'm still confused.

Maybe you're interpreting the code as:

	(val | parity(val)) ? 0 : 0x80

But what we're trying to do is:

	val | (parity(val) ? 0 : 0x80)

So, for example, if val == 0x06, the return value will be 0x86.
Only returning 0 or 0x80 seems wrong to me.
Or did I misunderstand something?

Regards,
Kuan-Wei

> I think we need a comment from authors.