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

On Friday 24 June 2011 21:45:57 Florian Tobias Schandinat wrote:

On 06/24/2011 06:55 PM, Geert Uytterhoeven wrote:

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On Fri, Jun 24, 2011 at 08:19, Paul Mundt wrote:

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On Thu, Jun 23, 2011 at 06:08:03PM +0200, Geert Uytterhoeven wrote:

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On Wed, Jun 22, 2011 at 07:45, Florian Tobias Schandinat wrote:

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On 06/21/2011 10:31 PM, Laurent Pinchart wrote:

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On Tuesday 21 June 2011 22:49:14 Geert Uytterhoeven wrote:

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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 ;)

So we can declare the 8 msb bits of nonstd reserved, and assume FOURCC
if any of them is set.

Nicely backwards compatible, as sane drivers should reject nonstd
values they don't support (apps _will_ start filling in FOURCC values
ignoring capabilities, won't they?).

That seems like a reasonable case, but if we're going to do that then
certainly the nonstd bit encoding needs to be documented and treated as
a hard ABI.

I'm not so sure about the if any bit in the upper byte is set assume
FOURCC case though, there will presumably be other users in the future
that will want bits for themselves, too. What exactly was the issue with
having a FOURCC capability bit in the upper byte?

That indeed gives less issues (as long as you don't stuff 8-bit ASCII
in the MSB) and more bits for tradiditional nonstd users.

The only disadvantage I can see is that it requires adding this bit in the
application and stripping it when interpreting it. But I think the 24 lower
bits should be enough for driver specific behavior (as the current values
really can only be interpreted per driver).

I'm also not keen on adding/stripping the MSB. It would be easier for 
applications to use FOURCCs directly.

quoted

BTW, after giving it some more thought: what does the FB_CAP_FOURCC buys
us? It's not like all drivers will support whatever random FOURCC mode
you give them, so you have to check whether it's supported by doing a
set_var first.

Because any solution purely based on the nonstd field is insane. The abuse
of that field is just too widespread. Drivers that use nonstd usually only
check whether it is zero or nonzero and others will just interpret parts
of nonstd and ignore the rest. They will happily accept FOURCC values in
the nonstd and just doing the wrong thing. Even if we would fix those the
problems applications will run into with older kernels will remain.

I agree with Florian here. Many drivers currently check whether nonstd != 0. 
Who knows what kind of values applications feed them ?

I'd like to reach an agreement on the API, and implement it. I'm fine with 
either grayscale or nonstd to store the FOURCC (with a slight preference for 
grayscale), and with either a vmode flag or using the most significant byte of 
the grayscale/nonstd field to detect FOURCC mode. I believe FB_CAP_FOURCC (or 
something similar) is needed.

-- 
Regards,

Laurent Pinchart