Re: [Workshop-2011] RFC: V4L2 API ambiguities

[Laurent Pinchart <laurent.pinchart@ideasonboard.com>]

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