Hi Geert,
On Monday 01 August 2011 11:49:46 Geert Uytterhoeven wrote:
On Mon, Aug 1, 2011 at 00:54, 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
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.
That's a good idea, but I'd like to add that in a second step. I'm working on a proof-of-concept by porting a driver to the FOURCC-based API first.