Re: PATCH uninorth3 (G5) agp support

[Jerome Glisse <j.glisse@free.fr>]

>Sure. There is still a pending issue though with AGP on the G5. The
>problem is that we create a non-cacheable mapping for the RAM pages of
>the AGP aperture (both in-kernel and for userland) while they already
>have a cacheable mapping via the normal kernel linear mapping of main
>memory.
>
>The result is that there is a potential cache aliasing issue, aggravated
>by the fact that the G5 is quite aggressive on pre-fetching and thuis,
>may end up prefetching some of the AGP cache lines (via the linear
>mapping) even if no actual access is ever done to these pages.
>
>Unfortunately, if a collision occurs (a non-cacheable access to some
>space that do exist in the cache at the same time), the result is
>undefined, and is likely to result in a checkstop (the CPU just stops).
>
>I really don't know of a simple remedy at this point. The problem is
>partially due to the fact that we do the linear mapping using large
>pages, so we can't simply undo the cacheable mapping for the pages that
>ended up beeing allocated for AGP... An option would be to eventually
>reserve the AGP memory early during boot and not include it in the
>linear mapping at all. Another thing to test is that maybe U3 is smart
>enough to snoop AGP accesses, and thus we could have the AGP mappings be
>cacheable as well (though that may require some stronger synchronisation
>directives in the DRM code).
>
>Ben.
>  
>

I got some lockup after adding agp support but i don't know if
it came from my playing with the r300 (most probable
source :)) or from the cache collision you are talking about.

Anyway it's not time to think to that :) Have good hollydays.

I will look a bit further in this to see if i can find anythings
that may helps.

best,
Jerome Glisse