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

[Christian KÃnig]

Am 21.04.2017 um 13:49 schrieb Ville SyrjÃlÃ:
> On Fri, Apr 21, 2017 at 02:40:18PM +0300, Pekka Paalanen wrote:
> > On Fri, 21 Apr 2017 14:08:04 +0300
> > Ville SyrjÃlà <ville.syrjala@xxxxxxxxxxxxxxx> wrote:
> >
> > > On Fri, Apr 21, 2017 at 11:50:18AM +0200, Gerd Hoffmann wrote:
> > > > On Fr, 2017-04-21 at 12:25 +0300, Ville SyrjÃlà 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.
I don't think your are in the minority. At least I would clearly say 
those formats should be in a fixed byte order and don't care about the 
CPU in the system.

What I need from the driver side is a consistent description of how the 
bytes in memory map to my hardware. What CPU is in use in the system is 
completely irrelevant for that.

Regards,
Christian.