Re: [Qemu-devel] [PATCH] [RFC] Variable video ram size option

[Gerd Hoffmann]

Jamie Lokier wrote:
> Gerd Hoffmann wrote:
>> Jamie Lokier wrote:
>>> I don't know why "ypan" is off by default;
>> Performance.  When reaching the end of video memory ypan does a memcpy
>> on the framebuffer.  ywrap doesn't need the memcpy, but doesn't work on
>> all hardware.  The memcpy is painful slow because framebuffer *read*
>> access isn't something gfx cards are optimized for.
> 
> Why read the framebuffer at all?

Has been quite a while that I looked closer at the fb code and followed
fb development.

IIRC it has  to do something with the scrollback (Shift-PgUp/Down)
implementation:  The video memory is used as scrollback buffer.  Thus
some information is kept in the video memory *only*.

> My experience with real hardware is it makes scrolling noticably faster.

Well, maybe it has been changed then.  I remember having seen some
discussion on changing the scrollback implementation to not depend on
video memory, which could have fixed this too.

cheers,
  Gerd