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
