linux-kernel - Re: [PATCH] drm: fourcc byteorder: brings header file comments in line with reality.

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Date:   Thu, 11 May 2017 23:23:11 +0200
From:   Pavel Machek <pavel@....cz>
To:     Ville Syrjälä <ville.syrjala@...ux.intel.com>
Cc:     Gerd Hoffmann <kraxel@...hat.com>, dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...el.com>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Ilia Mirkin <imirkin@...m.mit.edu>,
        Michel Dänzer <michel@...nzer.net>,
        Alex Deucher <alexdeucher@...il.com>,
        amd-gfx@...ts.freedesktop.org,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Sean Paul <seanpaul@...omium.org>,
        David Airlie <airlied@...ux.ie>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm: fourcc byteorder: brings header file comments in
 line with reality.

On Fri 2017-04-21 14:08:04, Ville Syrjälä wrote:
> On Fri, Apr 21, 2017 at 11:50:18AM +0200, Gerd Hoffmann wrote:
> > On Fr, 2017-04-21 at 12:25 +0300, Ville Syrjälä wrote:
> > > On Fri, Apr 21, 2017 at 09:58:24AM +0200, Gerd Hoffmann wrote:
> > > > While working on graphics support for virtual machines on ppc64 (which
> > > > exists in both little and big endian variants) I've figured the comments
> > > > for various drm fourcc formats in the header file don't match reality.
> > > > 
> > > > Comments says the RGB formats are little endian, but in practice they
> > > > are native endian.  Look at the drm_mode_legacy_fb_format() helper.  It
> > > > maps -- for example -- bpp/depth 32/24 to DRM_FORMAT_XRGB8888, no matter
> > > > whenever the machine is little endian or big endian.  The users of this
> > > > function (fbdev emulation, DRM_IOCTL_MODE_ADDFB) expect the framebuffer
> > > > is native endian, not little endian.  Most userspace also operates on
> > > > native endian only.
> > > 
> > > I'm not a fan of "native". Native to what? "CPU" or "host" is what I'd
> > > call it.
> > 
> > native == whatever the cpu is using.
> > 
> > I personally find "native" more intuitive, but at the end of the day I
> > don't mind much.  If people prefer "host" over "native" I'll change it.
> 
> "native" to me feels more like "native to the GPU" since these things
> really are tied to the GPU not the CPU. That's also why I went with the
> explicit endianness originally so that the driver could properly declare
> what the GPU supports.

You can easily have more than one GPU in the system. Plus these are
used by cameras / frame grabbers, too. So anything else than CPU
endianness is badly defined.

(And I agree with the rest of the thread -- we should really be
explicit; fourcc should specify what format the image data are in, and
it should be possible to write fourcc + raw data into file and
transfer it between machines.)


									Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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