linux-kernel - Re: [RFC PATCH 18/20] lib: image-formats: Add v4l2 formats support

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Message-ID: <20190322144210.GB3888@intel.com>
Date:   Fri, 22 Mar 2019 16:42:10 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Nicolas Dufresne <nicolas@...fresne.ca>
Cc:     Paul Kocialkowski <paul.kocialkowski@...tlin.com>,
        Maxime Ripard <maxime.ripard@...tlin.com>,
        Daniel Vetter <daniel.vetter@...el.com>,
        David Airlie <airlied@...ux.ie>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        Mauro Carvalho Chehab <mchehab@...nel.org>,
        Sakari Ailus <sakari.ailus@...ux.intel.com>,
        linux-kernel@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        Hans Verkuil <hans.verkuil@...co.com>,
        Laurent Pinchart <laurent.pinchart@...asonboard.com>,
        Thomas Petazzoni <thomas.petazzoni@...tlin.com>,
        linux-media@...r.kernel.org, Daniel Stone <daniels@...labora.com>
Subject: Re: [RFC PATCH 18/20] lib: image-formats: Add v4l2 formats support

On Thu, Mar 21, 2019 at 03:14:06PM -0400, Nicolas Dufresne wrote:
> Le jeudi 21 mars 2019 à 18:35 +0200, Ville Syrjälä a écrit :
> > > I'm not sure what it's worth, but there is a "pixel format guide"
> > > project that is all about matching formats from one API to another: 
> > > https://afrantzis.com/pixel-format-guide/ (and it has an associated
> > > tool too).
> > > 
> > > On the page about DRM, it seems that they got the word that DRM formats
> > > are the native pixel order in little-endian systems:
> > > https://afrantzis.com/pixel-format-guide/drm.html
> > 
> > Looks consistent with the official word in drm_fourcc.h.
> > 
> > $ python3 -m pfg find-compatible V4L2_PIX_FMT_XBGR32 drm
> > Format: V4L2_PIX_FMT_XBGR32
> > Is compatible on all systems with:
> >         DRM_FORMAT_XRGB8888
> > Is compatible on little-endian systems with:
> > Is compatible on big-endian systems with:
> > 
> > $ python3 -m pfg find-compatible DRM_FORMAT_XRGB8888 v4l2
> > Format: DRM_FORMAT_XRGB8888
> > Is compatible on all systems with:
> >         V4L2_PIX_FMT_XBGR32
> > Is compatible on little-endian systems with:
> > Is compatible on big-endian systems with:
> > 
> > Even works both ways :)
> > 
> > > Perhaps some drivers have been abusing the format definitions, leading
> > > to the inconsistencies that Nicolas could witness?
> > 
> > That is quite possible, perhaps even likely. No one really
> > seems interested in making sure big endian systems actually
> > work properly. I believe the usual approach is to hack
> > around semi-rnadomly until the correct colors accidentally
> > appear on the screen.
> 
> We did not hack around randomly.

BTW I didn't mean to imply it was you who hacked around randomly.
Sorry if you got that impression.

What I was trying to convey is the following sequence of events:
1. random person X gets their hand on a big endian machine for
   a while
2. colors are wrong
3. they hack stuff until the colors are correct in their
   current use case
4. they move on to more interesting things

-- 
Ville Syrjälä
Intel