Re: [Qemu-devel] [PATCH 3 of 6] vga shared buffer

[Anthony Liguori <anthony@codemonkey.ws>]

andrzej zaborowski wrote:
> 2008/9/15 Anthony Liguori <anthony@codemonkey.ws>:
>   
>> Stefano Stabellini wrote:
>>     
>>> Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
>>>
>>>       
>> I need to carve out some time to test this series and think about whether it
>> can be done without adding a new DisplayState hook.  I should be able to
>> respond or apply within a few days.
>>     
>
> Any news? I think this change makes sense and it can help remove the
> DIRECT_VRAM hack from vmware_vga.c.
>
>   
>> I'm concerned about the complexity this adds to back-ends.  It's not clear
>> to me whether the performance justifies the added complexity.
>>     
>
> One way to simplify this and everything else is use Stefano's code to
> stop the backends (SDL, VNC) from managing the framebuffer altogether
> and only support what these patches call shared buffer.

This is sort of what I've been thinking about.  If we're going to do 
this at all, we should move to a model where the backends stop managing 
the framebuffers completely.

>   Then if the
> given backed doesn't support given colorspace conversion in hardware,
> it's its call to implement this, it could unify the color conversion
> in qemu more generally.  The drawback is it's hard to support weird
> colorspaces like those supported by OMAP2 display susbsystem, and even
> harder to do features like overlaid framebuffers with different
> colorspaces (but it's not currently implemented anyway).
>   

It's slightly less optimal when you aren't going from a real 
framebuffer.  For instance, right now when in VGA planar mode, we're 
able to basically write pixels in the SDL buffer directly from the mmio 
callbacks.  If we switched to having the VGA devices manage their 
buffers, we would have to write to a buffer and then rely on SDL 
conversion functions to flip the buffer.  I don't know that it's really 
worth optimizing for things like VGA planar mode but I think that's 
really the trade off that we're making.

Regards,

Anthony Liguori

> Regards
>
>
>