Re: XFree86 5.0 TODO

On 25 Mar 2003, Juliusz Chroboczek wrote:

> L> So in it's current form DGA is pretty useless even when we ignore
> L> it's sequrity problems.
> 
> There's at least one legitimate use of DGA that I can see.  I believe
> it's the only one.
> 
> Consider a client that is doing its rendering client-side, but cannot
> render to a shared-memory image, for example because it's using a
> library that insists on rendering in buffers it allocates itself, or
> because it's generating images in a non-X format.
> 
> Such a client will need to do a copy (possibly changing formats on the
> fly) from system memory to the framebuffer.  Because it didn't render
> to shm, it needs to do two copies -- either client buffer to shm to
> framebuffer (ShmPutImage), or else client buffer to socket buffer to
> framebuffer (PutImage).  With DGA, it can do a single copy, and thus
> limit both memory access and cache pollution.
> 
> As I've said, I believe this to be the only legitimate application of
> DGA, and I only know of it being actually required in video players
> that use third-party codecs and that don't enjoy the benefits of Xv.
> Whether this justifies the continuing existence of DGA (which highly
> competent people tend to strongly dislike) is not something I can judge.
> 

   This is only a legitimate usage when in DGA mode.  Many of the
players I've seen use DGA to get the framebuffer address and then
use that outside of the relatively safe full-screen mode to render
to the root window, which is not safe at all.  


			Mark.