Re: basic FB development questions

[Geert Uytterhoeven <geert@linux-m68k.org>]

On Wed, 23 Apr 2008, Christian Schoenebeck wrote:
> First of all, AFAIK most FB drivers are based upon memory mapped IO, right? 
> But in my case the graphics are forwarded via calls of Linux's USB subsystem. 
> Which FB approach would suggest in this case? Something like the virtual 
> framebuffer driver with some kind of static allocated buffer? If yes, what 
> kind of function would I have to implement to trigger the screen sync between 
> that modified buffer and the hardware (that is sending the buffer's data via 
> USB to the device)? Or should I implement mmap? I mean: I know everything 
> about the USB side to recognize the device and sending data etc., I just 
> don't know how to establish a FB callback or something to "flush" the 
> suggested graphics buffer.

Documentation/fb/deferred_io.txt

> In the "fb_var_screeninfo" struct there are the following members which I'm 
> not sure about their precise purpose:
> 
> 	* "pixclock": is that the average speed (in ps) with which one pixel of the
> 	  screen is changed?

It's the inverse of the dot clock (in Hz).

> 	* "left_margin", "right_margin", "upper_margin", "lower_margin":
> 	  I'm confused about these members, I first thought that would be some kind
> 	  of reference margin (in pixels), but the header file says those are timing
> 	  values?
> 
> 	* "hsync_len", "vsync_len":
> 	  what's that exactly and is it a timing or pixel value?
> 
> If there are basic docs about FB terms etc, please let me know!

Documentation/fb/framebuffer.txt, section 5. Video Mode Timings.

But if your screen is 100% digital, they may not matter at all, and
zeroes are OK for the margins and sync lengths.

IIRC, fbset doesn't like pixclock being zero, so you should fill in
something.

Gr{oetje,eeting}s,

						Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
							    -- Linus Torvalds

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