Hi Hans, On Tuesday 14 August 2012 12:54:34 Hans Verkuil wrote: > On Tue August 14 2012 01:54:16 Laurent Pinchart wrote: > > On Monday 13 August 2012 14:27:56 Hans Verkuil wrote: > > > Hi all! > > > > > > As part of the 2012 Kernel Summit V4L2 workshop I will be discussing a > > > bunch of V4L2 ambiguities/improvements. > > > > > > I've made a list of all the V4L2 issues and put them in two categories: > > > issues that I think are easy to resolve (within a few minutes at most), > > > and those that are harder. > > > > > > If you think I put something in the easy category that you believe is > > > actually hard, then please let me know. > > > > > > If you attend the workshop, then please read through this and think > > > about it a bit, particularly for the second category. > > > > > > If something is unclear, or you think another topic should be added, > > > then let me know as well. > > > > > Easy: > > [snip] > > > > > 4) What should a driver return in TRY_FMT/S_FMT if the requested format > > > is not supported (possible behaviours include returning the currently > > > selected format or a default format). > > > > > > The spec says this: "Drivers should not return an error code unless > > > the input is ambiguous", but it does not explain what constitutes an > > > ambiguous input. Frankly, I can't think of any and in my opinion > > > TRY/S_FMT should never return an error other than EINVAL (if the buffer > > > type is unsupported)or EBUSY (for S_FMT if streaming is in progress). > > > > > > Returning an error for any other reason doesn't help the application > > > since the app will have no way of knowing what to do next. > > > > That wasn't my point. Drivers should obviously not return an error. Let's > > consider the case of a driver supporting YUYV and MJPEG. If the user calls > > TRY_FMT or S_FMT with the pixel format set to RGB565, should the driver > > return a hardcoded default format (one of YUYV or MJPEG), or the > > currently selected format ? In other words, should the pixel format > > returned by TRY_FMT or S_FMT when the requested pixel format is not valid > > be a fixed default pixel format, or should it depend on the currently > > selected pixel format ? > > Actually, in this case I would probably choose a YUYV format that is closest > to the requested size. If a driver supports both compressed and > uncompressed formats, then it should only select a compressed format if the > application explicitly asked for it. Handling compressed formats is more > complex than uncompressed formats, so that seems a sensible rule. That wasn't my point either :-) YUYV/MJPEG was just an example. You can replace MJPEG with UYVY or NV12 above. What I want to know is whether TRY_FMT and S_FMT must, when given a non-supported format, return a fixed supported format or return a supported format that can depend on the currently selected format. > The next heuristic I would apply is to choose a format that is closest to > the requested size. > > So I guess my guidelines would be: > > 1) If the pixelformat is not supported, then choose an uncompressed format > (if possible) instead. > 2) Next choose a format closest to, but smaller than (if possible) the > requested size. > > But this would be a guideline only, and in the end it should be up to the > driver. Just as long TRY/S_FMT always returns a format. -- Regards, Laurent Pinchart -- 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
