On Mon, 11 Jul 2011, 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 ? FWIW, I prefer the original Laurent's proposal, with a slight uncertainty about whether we need the .capability field, or whether the try-and-check approach is sufficient. As for struct fb_var_screeninfo fields to support switching to a FOURCC mode, I also prefer an explicit dedicated flag to specify switching to it. Even though using FOURCC doesn't fit under the notion of a videomode, using one of .vmode bits is too tempting, so, I would actually take the plunge and use FB_VMODE_FOURCC. As for the actual location of the fourcc code, I would leave .nonstd alone: who knows, maybe drivers will need both, whereas using grayscale and fourcc certainly doesn't make any sense. And I personally don't see a problem with using a union: buggy applications are, well, buggy... Actually, since in FOURCC mode we don't need any of __u32 bits_per_pixel; /* guess what */ __u32 grayscale; /* != 0 Graylevels instead of colors */ struct fb_bitfield red; /* bitfield in fb mem if true color, */ struct fb_bitfield green; /* else only length is significant */ struct fb_bitfield blue; struct fb_bitfield transp; /* transparency */ so, we could put them all in the union for the case, if we need anything else for the fourcc configuration in the future. > 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. Thanks Guennadi --- Guennadi Liakhovetski, Ph.D. Freelance Open-Source Software Developer http://www.open-technology.de/ -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
