Re: [Linaro-mm-sig] [RFC] ARM DMA mapping TODO, v1

[Thomas Hellstrom]

On 04/29/2011 09:35 AM, Benjamin Herrenschmidt wrote:
> We have problems with AGP and macs, we chose to mostly ignore them and
> things have been working so-so ... with the old DRM. With DRI2 being
> much more aggressive at mapping/unmapping things, things became a lot
> less stable and it could be in part related to that. IE. Aliases are
> similarily forbidden but we create them anyways.
>
>    

Do you have any idea how other OS's solve this AGP issue on Macs?
Using a fixed pool of write-combined pages?

> > c)  If neither of the above applies, we might be able to either use
> > explicit cache flushes (which will require a TTM cache sync API), or
> > require the device to use snooping mode. The architecture may also
> > perhaps have a pool of write-combined pages that we can use. This should
> > be indicated by defines in the api header.
> >      
> Right. We should still shoot HW designers who give up coherency for the
> sake of 3D benchmarks. It's insanely stupid.
>    

I agree. From a driver writer's perspective having the GPU always 
snooping the system pages would be a dream. On the GPUs that do support 
snooping that I have looked at, its internal MMU usually support both 
modes, but the snooping mode is way slower (we're talking 50-70% or so 
slower texturing operations), and often buggy causing crashes or scanout 
timing issues since system designers apparently don't really count on it 
being used. I've found it usable for device-to-system memory blits.

In addition memcpy to device is usually way faster if the destination is 
write-combined. Probably due to cache thrashing effects.

/Thomas

> Cheers,
> Ben.
>
>    
> > /Thomas
> >
> >
> >
> >
> >      
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> > >
> > >        
>
>    

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