xorg fbdev support for < 8 bits per pixel

Dushara Jayasinghe dushara.jayasinghe at hydrix.com
Thu May 12 16:31:59 PDT 2011

On 12/05/2011 11:31 PM, Jonathan Morton wrote:
> On Thu, 2011-05-12 at 17:58 +1000, Dushara Jayasinghe wrote:
>> Anyway, just would like to find out, how much effort it would take to
>> add support for devices with 1/2 bits/pixel depth. I noticed that the
>> current xorg-fbdev implementation doesn't support these depths.
>> Just to add context to the question, I'm working on an embedded device
>> with a monochrome LCD. The CPU can generate grayscale images by upping
>> the frame-rate and modulating the pixels but the hardware implementation
>> limits the max FR so I can only support about 2 bits per pixel depth.
> Would it be sufficient to use an 8bpp greyscale framebuffer and simply
> ignore the low-order bits on scanout?
Thanks for you reply Jonathan.

Maybe I'm a bit out of my depth (hehe) here, could you elaborate on this 
please? The CPU (iMX.25) LCD peripheral (LCDC module) uses a chunk of 
memory to scan out and the bit depth is determined by the video mode 
used. My understanding is, the X FB drv writes directly to this chunk of 
memory (please correct me if I'm wrong). What step are you referring to 
as scanout?


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