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

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Message-ID: <aLa78SILWdVG9AJj@smile.fi.intel.com>
Date: Tue, 2 Sep 2025 12:42:09 +0300
From: Andy Shevchenko <andriy.shevchenko@...el.com>
To: Adrian Barnaś <abarnas@...gle.com>
Cc: 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>,
	Dan Carpenter <dan.carpenter@...aro.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 07:38:40AM +0000, Adrian Barnaś wrote:
> Simplified masking logic in pci/hive_isp_css_common/host/vmem.c.

...

> ---
> 
> I have tested this change on whole range of *valid* inputs, and it gives
> the same results as before, but this function seems to be little
> counter-intuitive as far as start is a (bit index) but end is
> (bit index + 1).

We can't change it without changing the callers at the same time.
So, better to change just helpers and later on, if working, change
the semantics of the parameter.

> ---

...

>  subword(hive_uedge w, unsigned int start, unsigned int end)
>  {
> -	return (w & (((1ULL << (end - 1)) - 1) << 1 | 1)) >> start;
> +	return (w & __GENMASK_ULL(end-1, 0)) >> start;

Why __ variant?

>  }

...

>  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));
>  }

Ditto.

-- 
With Best Regards,
Andy Shevchenko