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

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Message-ID: <20170421114913.GK30290@intel.com>
Date:   Fri, 21 Apr 2017 14:49:13 +0300
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     Pekka Paalanen <ppaalanen@...il.com>
Cc:     Gerd Hoffmann <kraxel@...hat.com>, dri-devel@...ts.freedesktop.org,
        Daniel Vetter <daniel.vetter@...el.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, Apr 21, 2017 at 02:40:18PM +0300, Pekka Paalanen wrote:
> On Fri, 21 Apr 2017 14:08:04 +0300
> Ville Syrjälä <ville.syrjala@...ux.intel.com> 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.
> 
> Hi,
> 
> yeah, one should really be explicit on which component's endianess does
> "native" refer to. I just can't imagine "GPU native" to ever be an
> option, because then userspace needs a way to discover what the
> GPU endianess is,

It has to know that. How else would it know how to write the bytes into
memory in the right order for the GPU to consume, or read the stuff the
GPU produced?

> and I believe that would only deepen the swamp, not
> drain it, because suddenly you need two enums to describe one format.
> 
> Ville, wording aside, what do think about changing the endianess
> definition? Is it going in the right direction?

I don't think so, but I guess I'm in the minority.

-- 
Ville Syrjälä
Intel OTC