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

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Message-ID: <CAATBrPEsea2GfK26W5eg4ZFm=hJLO1pLNax85vvXZ8hb=iGaFQ@mail.gmail.com>
Date: Tue, 2 Sep 2025 15:10:55 +0200
From: Adrian Barnaś <abarnas@...gle.com>
To: Dan Carpenter <dan.carpenter@...aro.org>
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>, 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

> These are supposed to be opposites, right?  Subword and inverse Subword.
> You could dress them up to make them look more opposite.

In fact this name is misleading as well. For me it should be named
`clear_subword()`. It is zero-ing the "subword" provided with low and
high bit boundary (start, end), to be then easily replaced with binary
or operation.
And this operation is not an inverse of extracting a subword using the
subword function.

> 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.

It would be  easier to create a bitmask sing GENMASK_ULL and just
reverse it but if it is not safe, this is what looks fine for me and
works the same as previous function:

// Added a helper to create a mask
static inline unsigned long long subword_bitmask(unsigned int start,
unsigned int end)
{
        return GENMASK_ULL(end - 1 , 0) & ~(GENMASK_ULL(start, 0) >> 1);
}

/* inverse subword bits move like this: MSB[xxxx____xxxx]LSB ->
MSB[xxxx0000xxxx]LSB */
static inline unsigned long long
clear_subword(unsigned long long w, unsigned int start, unsigned int end)
{
        return w & ~subword_bitmask(start, end);
}

Best regards
Adrian Barnaś