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

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Message-ID: <aRyzYoCTNJQrlsou@lizhi-Precision-Tower-5810>
Date: Tue, 18 Nov 2025 12:56:50 -0500
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 01:10:23PM -0400, Frank Li wrote:
> 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.

Do you agree this change, I can post this patch + imx-mipi-csis.c as
sperate patches?

dwc csi2 need split more small serise to reduce review efforts.

Frank

>
> 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