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

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Message-ID: <aLaypTjukJJloGuL@stanley.mountain>
Date: Tue, 2 Sep 2025 12:02:29 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
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>,
	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).

Yeah.  The "end - 1" is unfortunate.  I guess we accidentally started
counting from 1...  I looked at how to remove it but got lost.

> 
> It is follow up to: https://lore.kernel.org/linux-staging/20250901091050.1935505-1-abarnas@google.com/
> ---
>  .../staging/media/atomisp/pci/hive_isp_css_common/host/vmem.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/staging/media/atomisp/pci/hive_isp_css_common/host/vmem.c b/drivers/staging/media/atomisp/pci/hive_isp_css_common/host/vmem.c
> index 722b684fbc37..9703e39b7497 100644
> --- a/drivers/staging/media/atomisp/pci/hive_isp_css_common/host/vmem.c
> +++ b/drivers/staging/media/atomisp/pci/hive_isp_css_common/host/vmem.c
> @@ -22,14 +22,14 @@ typedef hive_uedge *hive_wide;
>  static inline hive_uedge
>  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;
>  }
>  
>  /* inverse subword bits move like this: MSB[xxxx____xxxx]LSB -> MSB[xxxx0000xxxx]LSB */
>  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);

regards,
dan carpenter