I spent the last few days tracking down some really weird issues with the OV9640 driver. The major problem was a bright stripe, exactly 90px from the top of the photo. But it only occurred on photos in 640x480 and lower resolution. Also, there were other problems like brightness too dim in certain resolutions, swapped colors, i2c glitches etc. Interestingly enough, I haven't met such issues at the 1280x960 resolution.
The bright stripe was in the end caused by the camera not being ready right after reconfiguration. A little delay pretty much solved it, but it might need some further investigation still. Tinkering some more with the chips' registers, I also managed to eliminate most of the color-swap related problem as well as improve overall image quality.
But then there was that i2c problem. It was pretty hard to even notice it since writing the chip registers worked. The problem popped up when I tried reading the registers back though. It returned total nonsense. Further investigation came up with the chip not being able to communicate in SMBUS-mode so I switched to i2c-mode and it worked like a charm.
Anyway, with the problems listed above solved, the driver reached something that can be called a beta-state. It can now do capture in both RGB565X and VYUY (though it could be better in the RGB565X, the VYUY is fine). Sadly, the RGB555X is still not there, but I wonder if anyone will actually be missing it since there are those two higher-quality formats.
The patch is available here. Actually, I upload a new version of the patch from time to time and the old version of the patch gets overwritten so there might pop up a new version of the driver between blog entries announcing it. Though the changes are often very minor.