On 07/31/2011 08:32 PM, Geert Uytterhoeven wrote:
On Thu, Jul 28, 2011 at 12:51, Laurent Pinchart laurent.pinchart@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
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,
Florian Tobias Schandinat