linux-kernel - Re: [PATCH v3 02/31] media: v4l2-common: Add helper function v4l_get_required_align_by

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Message-ID: <aP+nf1hpvkafxHrF@lizhi-Precision-Tower-5810>
Date: Mon, 27 Oct 2025 13:10:23 -0400
From: Frank Li <Frank.li@....com>
To: Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc: Rui Miguel Silva <rmfrfs@...il.com>,
	Martin Kepplinger <martink@...teo.de>,
	Purism Kernel Team <kernel@...i.sm>,
	Mauro Carvalho Chehab <mchehab@...nel.org>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>,
	Eugen Hristev <eugen.hristev@...aro.org>,
	Shawn Guo <shawnguo@...nel.org>,
	Sascha Hauer <s.hauer@...gutronix.de>,
	Pengutronix Kernel Team <kernel@...gutronix.de>,
	Fabio Estevam <festevam@...il.com>, Peng Fan <peng.fan@....com>,
	Alice Yuan <alice.yuan@....com>, Vinod Koul <vkoul@...nel.org>,
	Kishon Vijay Abraham I <kishon@...nel.org>,
	Philipp Zabel <p.zabel@...gutronix.de>,
	Steve Longerbeam <slongerbeam@...il.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-media@...r.kernel.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, imx@...ts.linux.dev,
	linux-arm-kernel@...ts.infradead.org, linux-phy@...ts.infradead.org,
	linux-staging@...ts.linux.dev
Subject: Re: [PATCH v3 02/31] media: v4l2-common: Add helper function
 v4l_get_required_align_by_bpp()

On Mon, Oct 27, 2025 at 03:19:27AM +0200, Laurent Pinchart wrote:
> 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@....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.

drivers/media/platform/nxp/imx-mipi-csis.c
imx_cpi_set_fmt() in
https://lore.kernel.org/imx/20250729-imx8qxp_pcam-v4-3-4dfca4ed2f87@nxp.com/

I am not sure if userspace to pass down a old width to driver, like 1033.

Frank

>
> > + * @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