Re: [Qemu-devel] [PATCH 4 of 5] sdl shared buffer support

[Anthony Liguori <anthony@codemonkey.ws>]

Stefano Stabellini wrote:
> Anthony Liguori wrote:
>
>   
>> Stefano Stabellini wrote:
>>     
>>> This patch implements shared buffer support in sdl.c.
>>> It also supports paletted 8 bit colour depths using the
>>> palette functions provided by the SDL library.
>>>   
>>>       
>> So the theory goes that it's possible with this patch that both surfaces
>> are hardware surfaces so blitting is fast?
>>     
>
>
> The theory goes that SDL blitting and color conversion functions should
> be as optimized as you can get, and hardware accelerated if your card
> supports it.
> Besides I think that offloading blitting and color conversion to a
> graphic library is a good idea.
>   

It requires two paths in the code to handle the case where the backend 
can't handle conversion itself.  If we're not getting any sort of 
benefit from offloading, then I don't see the need for increased code 
complexity.

>> In practice, when using X11
>> both surfaces are going to be XShmImages so I don't think this is going
>> to perform any better than the current code.
>>     
>
> Only the guest framebuffer derived surface is always a software surface,
> the host window can be an hardware surface (as far as I know).
>   

Not when using SDL on X11.  It will use an XShmImage if possible, 
otherwise falling back to a normal XImage.  There's really no other way 
to do surfaces in X11 without using one of the extensions (like DGA).

Regards,

Anthony Liguori