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

[Laurent Pinchart]

Ping ?

As stated in my previous mail, I'd like to reach an agreement on the API, and 
implement it. I'm fine with either grayscale or nonstd to store the FOURCC 
(with a slight preference for grayscale), and with either a vmode flag or 
using the most significant byte of the grayscale/nonstd field to detect FOURCC 
mode. I believe FB_CAP_FOURCC (or something similar) is needed.

Paul, Geert, Florian, what are your opinions ?

On Monday 11 July 2011 17:32:51 Laurent Pinchart wrote:
> On Friday 24 June 2011 21:45:57 Florian Tobias Schandinat wrote:
> > On 06/24/2011 06:55 PM, Geert Uytterhoeven wrote:
> > > On Fri, Jun 24, 2011 at 08:19, Paul Mundt wrote:
> > >> On Thu, Jun 23, 2011 at 06:08:03PM +0200, Geert Uytterhoeven wrote:
> > >>> On Wed, Jun 22, 2011 at 07:45, Florian Tobias Schandinat 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?
> > > 
> > > That indeed gives less issues (as long as you don't stuff 8-bit ASCII
> > > in the MSB) and more bits for tradiditional nonstd users.
> > 
> > The only disadvantage I can see is that it requires adding this bit in
> > the application and stripping it when interpreting it. But I think the
> > 24 lower bits should be enough for driver specific behavior (as the
> > current values really can only be interpreted per driver).
> 
> I'm also not keen on adding/stripping the MSB. It would be easier for
> applications to use FOURCCs directly.
> 
> > > BTW, after giving it some more thought: what does the FB_CAP_FOURCC
> > > buys us? It's not like all drivers will support whatever random FOURCC
> > > mode you give them, so you have to check whether it's supported by
> > > doing a set_var first.
> > 
> > Because any solution purely based on the nonstd field is insane. The
> > abuse of that field is just too widespread. Drivers that use nonstd
> > usually only check whether it is zero or nonzero and others will just
> > interpret parts of nonstd and ignore the rest. They will happily accept
> > FOURCC values in the nonstd and just doing the wrong thing. Even if we
> > would fix those the problems applications will run into with older
> > kernels will remain.
> 
> I agree with Florian here. Many drivers currently check whether nonstd !=
> 0. Who knows what kind of values applications feed them ?
> 
> I'd like to reach an agreement on the API, and implement it. I'm fine with
> either grayscale or nonstd to store the FOURCC (with a slight preference
> for grayscale), and with either a vmode flag or using the most significant
> byte of the grayscale/nonstd field to detect FOURCC mode. I believe
> FB_CAP_FOURCC (or something similar) is needed.

-- 
Regards,

Laurent Pinchart