linux-kernel - Re: [RFC] staging: media: atomisp: Simplyfy masking bit logic

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Message-ID: <aLbDFcaqzJtZUdCB@stanley.mountain>
Date: Tue, 2 Sep 2025 13:12:37 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Andy Shevchenko <andriy.shevchenko@...el.com>
Cc: Adrian Barnaś <abarnas@...gle.com>,
	Hans de Goede <hansg@...nel.org>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Sakari Ailus <sakari.ailus@...ux.intel.com>,
	Andy Shevchenko <andy@...nel.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-staging@...ts.linux.dev
Subject: Re: [RFC] staging: media: atomisp: Simplyfy masking bit logic

On Tue, Sep 02, 2025 at 12:54:28PM +0300, Andy Shevchenko wrote:
> On Tue, Sep 02, 2025 at 12:02:29PM +0300, Dan Carpenter wrote:
> > On Tue, Sep 02, 2025 at 07:38:40AM +0000, Adrian Barnaś wrote:
> 
> ...
> 
> > >  static inline hive_uedge
> > >  inv_subword(hive_uedge w, unsigned int start, unsigned int end)
> > >  {
> > > -	return w & (~(((1ULL << (end - 1)) - 1) << 1 | 1) | ((1ULL << start) - 1));
> > > +	return w & (~__GENMASK_ULL(end-1, start));
> > >  }
> > 
> > nit: white space.  Add spaces.  Remove parentheses.
> > 
> > These are supposed to be opposites, right?  Subword and inverse Subword.
> > You could dress them up to make them look more opposite.
> > 
> > 	return (w & __GENMASK_ULL(end - 1, start)) >> start;
> > 	return w & ~__GENMASK_ULL(end - 1, start);
> 
> The problem is (and actually with the (end-1, start) above that it might
> generate a really bad code on some CPUs, so I really, really prefer the way
> when _at least_ one of the parameters is constant.

Ah.  Okay.  That makes sense.  Thanks.

regards,
dan carpenter