Hi Guennadi, On Tuesday 14 August 2012 23:14:18 Guennadi Liakhovetski wrote: > On Tue, 14 Aug 2012, Laurent Pinchart wrote: > > On Tuesday 14 August 2012 13:32:43 Hans Verkuil wrote: > > > On Tue August 14 2012 13:15:21 Laurent Pinchart wrote: > > > > 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. > > > > > > I don't know whether it should be mandated. In the end it doesn't matter > > > to the application: that just wants to get some format that is valid. > > > > > > It's a good recommendation for drivers, but I do not think that there is > > > anything wrong as such with drivers that return the current format. > > > > > > Am I missing something here? Is there any particular advantage of > > > returning a default fallback format from the point of view of an > > > application? > > > > Mostly consistency. I find returning different results for TRY_FMT calls > > with the exact same parameters confusing, both for applications and > > users. > > We've discussed this issue privately with Laurent before, and my opinion > was rather to go with the currently configured format. The advantage of > this would be, that situations, when a user has configured some format and > then is trying to switch to an unsupported format, and instead the driver > switches to a 3rd format, instead of keeping the current one, would be > avoided. > > OTOH, it seems a good idea to whenever possible return the same result in > reply to the same request, in this case to TRY_FMT. And it also seems > logical to have S_FMT do the same thing as TRY_FMT... So, this argument > seems stronger than my original one... Just one request - don't insist on > immediate conversion of existing drivers;-) I'll consider "please submit a patch" as a very valid answer to any conversion request in the near future :-) -- 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
