Hi Hans, On Tuesday 14 August 2012 13:11:49 Hans Verkuil wrote: > On Tue August 14 2012 13:06:46 Laurent Pinchart wrote: > > 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: > > > > [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. > > Well, the currently selected format is irrelevant. The user is obviously > requesting something else and the driver should attempt to return something > that is at least somewhat close to what it requested. If that's impossible, > then falling back to some default format is a good choice. > > Does that answer the question? Yes it does, and I agree with that. Some drivers return the currently selected format when a non-supported format is requested. I think the spec should be clarified to make it mandatory to return a fixed default format independent of the currently selected format, and non-compliant drivers should be fixed. -- 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
