Hi Florian, On Wednesday 22 June 2011 07:45:45 Florian Tobias Schandinat wrote: > On 06/21/2011 10:31 PM, Laurent Pinchart wrote: > > On Tuesday 21 June 2011 22:49:14 Geert Uytterhoeven wrote: > >> On Tue, Jun 21, 2011 at 17:36, Laurent Pinchart wrote: > >>> +The FOURCC-based API replaces format descriptions by four character > >>> codes +(FOURCC). FOURCCs are abstract identifiers that uniquely define > >>> a format +without explicitly describing it. This is the only API that > >>> supports YUV +formats. Drivers are also encouraged to implement the > >>> FOURCC-based API for RGB +and grayscale formats. > >>> + > >>> +Drivers that support the FOURCC-based API report this capability by > >>> setting +the FB_CAP_FOURCC bit in the fb_fix_screeninfo capabilities > >>> field. + > >>> +FOURCC definitions are located in the linux/videodev2.h header. > >>> However, and +despite starting with the V4L2_PIX_FMT_prefix, they are > >>> not restricted to V4L2 +and don't require usage of the V4L2 subsystem. > >>> FOURCC documentation is +available in > >>> Documentation/DocBook/v4l/pixfmt.xml. > >>> + > >>> +To select a format, applications set the FB_VMODE_FOURCC bit in the > >>> +fb_var_screeninfo vmode field, and set the fourcc field to the desired > >>> FOURCC. +The bits_per_pixel, red, green, blue, transp and nonstd fields > >>> must be set to +0 by applications and ignored by drivers. Note that the > >>> grayscale and fourcc +fields share the same memory location. > >>> Application must thus not set the +grayscale field to 0. > >> > >> These are the only parts I don't like: (ab)using the vmode field (this > >> isn't really a vmode flag), and the union of grayscale and fourcc (avoid > >> unions where possible). > > > > I've proposed adding a FB_NONSTD_FORMAT bit to the nonstd field as a > > FOURCC mode indicator in my initial RFC. Florian Tobias Schandinat > > wasn't very happy with that, and proposed using the vmode field instead. > > > > Given that there's virtually no fbdev documentation, whether the vmode > > field and/or nonstd field are good fit for a FOURCC mode indicator is > > subject to interpretation. > > The reason for my suggestion is that the vmode field is accepted to contain > only flags and at least to me there is no hard line what is part of the > video mode and what is not.
Lacks of documentation indeed makes that line fuzzy. I really hope that api.txt will be extended to cover the full fbdev API :-) > In contrast the nonstd field is already used in a lot of different > (incompatible) ways. I think if we only use the nonstd field for handling > FOURCC it is likely that some problems will appear. > > >> What about storing the FOURCC value in nonstd instead? > > > > Wouldn't that be a union of nonstd and fourcc ? :-) FOURCC-based format > > setting will be a standard fbdev API, I'm not very keen on storing it in > > the nonstd field without a union. > > > >> As FOURCC values are always 4 ASCII characters (hence all 4 bytes must > >> be non-zero), I don't think there are any conflicts with existing values > >> of nonstd. To make it even safer and easier to parse, you could set bit > >> 31 of nonstd as a FOURCC indicator. > > > > I would then create a union between nonstd and fourcc, and document > > nonstd as being used for the legacy API only. Most existing drivers use > > a couple of nonstd bits only. The driver that (ab)uses nonstd the most > > is pxafb and uses bits 22:0. Bits 31:24 are never used as far as I can > > tell, so nonstd& 0xff000000 != 0 could be used as a FOURCC mode test. > > > > This assumes that FOURCCs will never have their last character set to > > '\0'. Is that a safe assumption for the future ? > > Yes, I think. The information I found indicates that space should be used > for padding, so a \0 shouldn't exist. > I think using only the nonstd field and requiring applications to check the > capabilities would be possible, although not fool proof ;) > > Great work, Laurent, do you have plans to modify fbset to allow using this > format API from the command line? Once we agree on an API, I will implement it in a driver and update fbset. -- Regards, Laurent Pinchart
