Re: Driver for Sharp LH155BA

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

Richard Wolf wrote:
> Hi,
> 
> I'm writing a driver for the Sharp LH155BA LCD driver.  This is a driver
> designed for controlling small/medium lcd panels (up to 128 x 64 pixels).  
> 
> I'm trying to decide whether it is sensible to write a frame buffer driver
> for a device of this nature.  Does the frame buffer abstraction make sense
> for a display this small?  

The dimensions of the display does not matter.

> 
> On our target hardware it is not possible to access the display memory
> directly.  Rather, all accesses to display memory and other control
> registers are through a small number of io ports.  Therefore it is not
> possible to map display memory.  Is this relevant?  

Yes, if the display memory is not mappable, then it won't be usable as
a driver for X, for example.

You can however allocate a shadow buffer in system RAM which can be mmapp'ed.
Then on intervals, perhaps by interrupt or timer, update the physical screen.
Unfortunately, this is inefficient, as you may be updating the screen even if
the shadow buffer haven't changed.  But that is the best you can do without
hacking the kernel.

Another option is to just forget about mmap, and create your own fb_read and
fb_write hooks.  This is more efficient, but not all apps use this method to
update the framebuffer. The driver will be usable as a console though.

Tony


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