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

[Geert Uytterhoeven]

On Mon, Aug 1, 2011 at 00:54, Florian Tobias Schandinat
<FlorianSchandinat at gmx.de> wrote:
> On 07/31/2011 08:32 PM, Geert Uytterhoeven wrote:
>> On Thu, Jul 28, 2011 at 12:51, Laurent Pinchart
>> <laurent.pinchart at ideasonboard.com>  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
>
>
> 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

As several of the FOURCC formats duplicate formats you can already
specify in some
other way (e.g. the RGB and greyscale formats), and as FOURCC makes life easier
for the application writer, I'm wondering whether it makes sense to add FOURCC
support in the generic layer for drivers that don't support it? I.e.
the generic layer would
fill in fb_var_screeninfo depending on the requested FOURCC mode, if possible.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds