[PATCH/RFC] fbdev: Add FOURCC-based format configuration API

[Laurent Pinchart]

Hi Florian,

Thanks for the feedback.

On Monday 01 August 2011 00:54:48 Florian Tobias Schandinat wrote:
> On 07/31/2011 08:32 PM, Geert Uytterhoeven wrote:
> > On Thu, Jul 28, 2011 at 12:51, Laurent Pinchart wrote:
> >>> As for struct fb_var_screeninfo fields to support switching to a FOURCC
> >>> mode, I also prefer an explicit dedicated flag to specify switching to
> >>> it. Even though using FOURCC doesn't fit under the notion of a
> >>> videomode, using one of .vmode bits is too tempting, so, I would
> >>> actually take the plunge and use FB_VMODE_FOURCC.
> >> 
> >> Another option would be to consider any grayscale>  1 value as a FOURCC.
> >> I've briefly checked the in-tree drivers: they only assign grayscale
> >> with 0 or 1, and check whether grayscale is 0 or different than 0. If a
> >> userspace application only sets grayscale>  1 when talking to a driver
> >> that supports the FOURCC-based API, we could get rid of the flag.
> >> 
> >> What can't be easily found out is whether existing applications set
> >> grayscale to a>  1 value. They would break when used with FOURCC-aware
> >> drivers if we consider any grayscale>  1 value as a FOURCC. Is that a
> >> risk we can take ?
> > 
> > I think we can. I'd expect applications to use either 1 or -1 (i.e.
> > all ones), both are
> > invalid FOURCC values.
> > 
> > Still, I prefer the nonstd way.
> > And limiting traditional nonstd values to the lowest 24 bits (there
> > are no in-tree
> > drivers using the highest 8 bits, right?).
> 
> Okay, it would be okay for me to
> - write raw FOURCC values in nonstd, enable FOURCC mode if upper byte != 0
> - not having an explicit flag to enable FOURCC
> - in FOURCC mode drivers must set visual to FB_VISUAL_FOURCC
> - making support of FOURCC visible to userspace by capabilites |=
> FB_CAP_FOURCC
> 
> The capabilities is not strictly necessary but I think it's very useful as
> - it allows applications to make sure the extension is supported (for
> example to adjust the UI)
> - it allows applications to distinguish whether a particular format is not
> supported or FOURCC at all
> - it allows signaling further extensions of the API
> - it does not hurt, one line per driver and still some bytes in fixinfo
> free

Without a FOURCC capability applications will need to try FOURCCs blindly. 
Drivers that are not FOURCC aware would then risk interpreting the FOURCC as 
something else. As you mention below applications will need that check that 
visual == FB_VISUAL_FOURCC, so it's less of an issue than I initially thought, 
but it doesn't become a non-issue. The display might still show glitches.

> So using it would look like this:
> - the driver must have capabilities |= FB_CAP_FOURCC
> - the application may check capabilities to know whether FOURCC is
> supported - the application may write a raw FOURCC value in nonstd to
> request changing to FOURCC mode with this format
> - when the driver switches to a FOURCC mode it must have visual =
> FB_VISUAL_FOURCC and the current FOURCC format in nonstd
> - the application should check visual and nonstd to make sure it gets what
> it wanted
> 
> 
> So if there are no strong objections against this I think we should
> implement it. I do not really care whether we use a union or not but I
> think if we decide to have one it should cover all fields that are
> undefined/unused in FOURCC mode.
> 
> 
> Hope we can find anything that everyone considers acceptable,

This sounds good to me, except that I would use the grayscale field instead of 
the nonstd field. nonstd has pretty weird usecases, while grayscale is better 
defined. nonstd might also make sense combined with FOURCC-based modes, while 
grayscale would be completely redundant.

What's your opinion on that ?

-- 
Regards,

Laurent Pinchart