Re: fixing fbdev for various framebuffer configs

["Antonino A. Daplas" <adaplas@gmail.com>]

Geert Uytterhoeven wrote:
> On Fri, 29 Jul 2005, Jon Smirl wrote:
>> On 7/29/05, Antonino A. Daplas <adaplas@gmail.com> wrote:
>>> Jon Smirl wrote:
>>>> It has been determined that bits per pixel is insuffucient to
>>>> enumerate all the needed fbconfigs. Here are the possible fgconfigs
>>>> from the OpenGL headers:
>>>>
>>>> total bits
>>>> 8     GL_R3_G3_B2
>>>> 8     GL_RGBA2
>>>> 12   GL_RGB4
>>>> 15   GL_RGB5
>>>> 16   GL_R5_G6_B5
>>>> 16   GL_RGBA4
>>>> 16   GL_RGB5_A1
>>>> 24   GL_RGB8
>>>> 30   GL_RGB10
>>>> 32   GL_RGBA8
>>>> 32   GL_RGB10_A2
>>>> 36   GL_RGB12
>>>> 48   GL_RGB16
>>>> 48   GL_RGBA12
>>>> 48   GL_FLOAT_RGB16
>>>> 64   GL_RGBA16
>>>> 64   GL_FLOAT_RGBA16
>>>> 96   GL_FLOAT_RGB32
>>>> 128 GL_FLOAT_RGBA32
>>>>
>>>> We need to be able to set any of these through the fbdev interface. We
>>>> also need to allow for possible future expansion.
>>> To set for example, RGBA16, set {transp|green|red|blue}.len to 16, the
>>> offsets to 0, 16, 32, 48, and bits_per_pixel to 64.  Then let the driver
>>> sort it out.  I don't know what we can do about the FLOAT formats...
>>>
>>>> Ideas on how to adjust the fbdev interface?
>>> Well, no need to adjust the interface.  But it is limited though to
>>> 16 bits per channel.  Redefining them to 32 will break a lot of apps.
> 
> It's only the colormap entries that are limited to 16 bits per color component,
> right? The color components in pixel values can be much larger (up to 2^31 bits
> :-).

Yes, but struct fb_cmap has entries of u16 and this is used by struct
fb_var_screeninfo and by the cmap functions. fb_setcolreg is also passed
with u16 values.  ioctls affected will be FBIO{GET|PUT}_VSCREENINFO ,
FBIO{GET|PUT}CMAP, all used in userspace.

> 
>> My current idea is to get rid of the bit_per_pixel attribute and
>> replace it with config. Config would take strings like 8,8,8 or
>> 5,5,5,1 or 32.,32.,32.
> 
> That's not acceptable. How do you differentiate between RGB888 (without alpha)
> using 24 or 32 bits per pixel?
> 
> Or more general, what with hardware that has unused bits in its pixel values,
> e.g. hardware that has 8 bits per pixel but uses only 1 bit per byte in
> monochrome mode and 4 bits in 16-color mode?

Agree,

Tony


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