linux-kernel - Re: [PATCH v7 7/8] media: imagination: Round to closest multiple for cropping region

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Date: Tue, 14 May 2024 16:22:28 -0400
From: Nicolas Dufresne <nicolas@...fresne.ca>
To: Devarsh Thakkar <devarsht@...com>, Andy Shevchenko
	 <andriy.shevchenko@...ux.intel.com>
Cc: mchehab@...nel.org, hverkuil-cisco@...all.nl,
 linux-media@...r.kernel.org,  linux-kernel@...r.kernel.org,
 benjamin.gaignard@...labora.com,  sebastian.fricke@...labora.com,
 dri-devel@...ts.freedesktop.org,  laurent.pinchart@...asonboard.com,
 praneeth@...com, nm@...com, vigneshr@...com,  a-bhatia1@...com,
 j-luthra@...com, b-brnich@...com, detheridge@...com,  p-mantena@...com,
 vijayp@...com, andrzej.p@...labora.com, p.zabel@...gutronix.de, 
 airlied@...il.com, daniel@...ll.ch, akpm@...ux-foundation.org, 
 gregkh@...uxfoundation.org, adobriyan@...il.com, jani.nikula@...el.com
Subject: Re: [PATCH v7 7/8] media: imagination: Round to closest multiple
 for cropping region

Le samedi 11 mai 2024 à 22:38 +0530, Devarsh Thakkar a écrit :
> Hi Andy,
> 
> Thanks for the quick review.
> On 10/05/24 20:40, Andy Shevchenko wrote:
> > On Fri, May 10, 2024 at 12:10:01AM +0530, Devarsh Thakkar wrote:
> > > If neither of the flags to round down (V4L2_SEL_FLAG_LE) or round up
> > > (V4L2_SEL_FLAG_GE) are specified by the user, then round to nearest
> > > multiple of requested value while updating the crop rectangle coordinates.
> > > 
> > > Use the rounding macro which gives preference to rounding down in case two
> > > nearest values (high and low) are possible to raise the probability of
> > > cropping rectangle falling inside the bound region.
> > 
> > This is arguable. How do we know that the bigger range is supported?
> > The safest side is to go smaller than bigger.
> > 
> 
> Yes and that's what the driver does when do when application passes
> V4L2_SEL_FLAG_LE while doing the selection. If application does not
> specify explicitly whether to round down or round up the cropping
> parameters requested by it (i.e app is neither passing V4L2_SEL_FLAG_LE
> nor V4L2_SEL_FLAG_GE flags), then it is preferred by driver to round the
> cropping parameters to nearest possible value by either rounding down or
> rounding up to align with hardware requirements.
> 
> For e.g. If requested width for cropping region is 127 and HW requires
> width to be multiple of 64 then we would prefer to round it up to 128
> rather than rounding down to a more distant value (i.e. 64), but if
> requested cropping width is 129 then we would prefer to instead round it
> down to 128. But if requested cropping width is 160 then there are two
> nearest possible values 160 - 32 = 128 and 160 + 32 = 192 and in which
> case we prefer the smaller value as you suggested and that's why the
> driver uses round_closest_down.
> 
> For any reason, if still the cropping rectangle falls beyond the bound
> region, then driver will return out of range error (-ERANGE) to
> application.

I would appreciate if this change was based on specification text, meaning
improving the next if that behaviour is undefined. We might not be able to fix
it everywhere, but we can recommend something.

Nicolas

> 
> Regards
> Devarsh
> 
>