Re: [PATCH v3 02/31] media: v4l2-common: Add helper function v4l_get_required_align_by_bpp()

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

On Thu, Aug 21, 2025 at 04:15:37PM -0400, Frank Li wrote:
> Add helper v4l_get_required_align_by_bpp() to help get width alignment
> requirement. Basic replace below logic and enhance to any 2^[0..31] in
> drivers/media/platform/nxp/imx-mipi-csis.c
> 
> mipi_csis_set_fmt(
> {
>         ...
> 
>         switch (csis_fmt->width % 8) {
>         case 0:
>                 align = 0;
>                 break;
>         case 4:
>                 align = 1;
>                 break;
>         case 2:
>         case 6:
>                 align = 2;
>                 break;
>         default:
>                 /* 1, 3, 5, 7 */
>                 align = 3;
>                 break;
>         }
> 	...
> }
> 
> Signed-off-by: Frank Li <Frank.Li@nxp.com>
> ---
> The same patch will be see at https://lore.kernel.org/imx/20250729-imx8qxp_pcam-v4-2-4dfca4ed2f87@nxp.com/
> dw csi2 patch also this
> ---
>  include/media/v4l2-common.h | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
> 
> diff --git a/include/media/v4l2-common.h b/include/media/v4l2-common.h
> index 39dd0c78d70f7b935c2e10f9767646d2cedd3079..0a9da5e8daaddf26903d9ff4bed08b283dcd38b2 100644
> --- a/include/media/v4l2-common.h
> +++ b/include/media/v4l2-common.h
> @@ -704,4 +704,34 @@ static inline bool v4l2_is_quant_valid(__u8 quantization)
>  	       quantization == V4L2_QUANTIZATION_LIM_RANGE;
>  }
>  
> +/**
> + * v4l_get_required_align_by_bpp - get align requirement for
> + *	v4l_bound_align_image(). (bpp * width) % (1 << align) have to be 0.
> + *	given number bpp, get width's alignment requirement. For example,
> + *	if align is 3, means require bpp * width must be multiples of 8.
> + *	    bpp     return  width's requirememnt
> + *	    0       0       none
> + *	    1       3       8 (need 3 zero bits)
> + *	    2       2       4
> + *	    3       3       8
> + *	    4       1       2
> + *	    5       3       8
> + *	    6       2       4
> + *	    7       3       8

This is supposed to be a brief one-line description. The rest of the
documentation goes after the argument.

Also, have checked the formatting of the generated documentation ?

> + * @bpp: input bpp

I have no idea if this is a bits per pixel or bytes per pixel value. I'm
actually not sure what the function is even supposed to do. It feels
it's really a ad-hoc helper, I would like to see it being used in
multiple drivers to see if it makes sense.

> + * @align: expected alignment, 2^(align).
> + *
> + * return: required alignment.
> + */
> +static inline u32 v4l_get_required_align_by_bpp(u32 bpp, u32 align)
> +{
> +	int pos;
> +
> +	if (bpp == 0)
> +		return 0;
> +
> +	pos = ffs(bpp) - 1;
> +	return pos > align ? 0 : align - pos;
> +}
> +
>  #endif /* V4L2_COMMON_H_ */
> 

-- 
Regards,

Laurent Pinchart