Re: Should I use FBINFO_VIRTFB?

[Timur Tabi <timur@freescale.com>]

Geert Uytterhoeven wrote:

> This depends on which of these two is fastest for scrolling the console:
>   - panning the virtual screen and redrawing the missing part: set
> FBINFO_PARTIAL_PAN_OK
>   - copying or redrawing the screen: don't set FBINFO_PARTIAL_PAN_OK

I'm a little confused about panning support in my driver (which I didn't write -- I'm just cleaning it up).  There is .fb_pan_display function, but it only gets called early in the boot process, and only with xoffset=0 and yoffset=0.  After that, it never seems to get called again.  Under what circumstances is panning really used?

> 
>> I have the same problem with FBINFO_READS_FAST.
> 
> You should set this flag if reading from frame buffer memory is a fast
> operation.
> On many graphics devices, reading from frame buffer memory is much slower
> than writing. As you use system RAM, you probably want to set it.

I see a lot of drivers that set FBINFO_VIRTFB but don't set FBINFO_READS_FAST.  Why?  

> If this flag is set, scrolling is implemented by copying memory around.
> If not set, scrolling is implemented by redrawing the whole screen.
> 
> A simple way to find the optimal settings of both flags (all 4 combinations) is
> running "clear; time cat big_text_file" and comparing the timing results

That's the funny thing -- I've been trying various combinations of FBINFO_VIRTFB, FBINFO_PARTIAL_PAN_OK, and FBINFO_READS_FAST, and I always get the same timing result.  It seems to have no affect on performance.

-- 
Timur Tabi
Linux kernel developer at Freescale