Re: [PATCH/RFC] fbdev: Add FOURCC-based format configuration API

[Paul Mundt <lethal@linux-sh.org>]

On Thu, Jun 23, 2011 at 06:08:03PM +0200, Geert Uytterhoeven wrote:
> On Wed, Jun 22, 2011 at 07:45, Florian Tobias Schandinat
> <FlorianSchandinat@gmx.de> wrote:
> > On 06/21/2011 10:31 PM, Laurent Pinchart wrote:
> >> On Tuesday 21 June 2011 22:49:14 Geert Uytterhoeven wrote:
> >>> As FOURCC values are always 4 ASCII characters (hence all 4 bytes must
> >>> be non-zero), I don't think there are any conflicts with existing values
> >>> of
> >>> nonstd. To make it even safer and easier to parse, you could set bit 31
> >>> of
> >>> nonstd as a FOURCC indicator.
> >>
> >> I would then create a union between nonstd and fourcc, and document nonstd
> >> as
> >> being used for the legacy API only. Most existing drivers use a couple of
> >> nonstd bits only. The driver that (ab)uses nonstd the most is pxafb and
> >> uses
> >> bits 22:0. Bits 31:24 are never used as far as I can tell, so nonstd&
> >> 0xff000000 != 0 could be used as a FOURCC mode test.
> >>
> >> This assumes that FOURCCs will never have their last character set to
> >> '\0'. Is
> >> that a safe assumption for the future ?
> >
> > Yes, I think. The information I found indicates that space should be used
> > for padding, so a \0 shouldn't exist.
> > I think using only the nonstd field and requiring applications to check the
> > capabilities would be possible, although not fool proof ;)
> 
> So we can declare the 8 msb bits of nonstd reserved, and assume FOURCC if
> any of them is set.
> 
> Nicely backwards compatible, as sane drivers should reject nonstd values they
> don't support (apps _will_ start filling in FOURCC values ignoring capabilities,
> won't they?).
> 
That seems like a reasonable case, but if we're going to do that then
certainly the nonstd bit encoding needs to be documented and treated as a
hard ABI.

I'm not so sure about the if any bit in the upper byte is set assume
FOURCC case though, there will presumably be other users in the future
that will want bits for themselves, too. What exactly was the issue with
having a FOURCC capability bit in the upper byte?