From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Message-ID: <41D01309.3000304@free.fr> Date: Mon, 27 Dec 2004 14:50:01 +0100 From: Jerome Glisse MIME-Version: 1.0 To: Benjamin Herrenschmidt References: <41CEC6B0.5020106@free.fr> <1104137527.5615.20.camel@gaston> <41D00564.6010507@free.fr> <1104154320.23891.27.camel@gaston> In-Reply-To: <1104154320.23891.27.camel@gaston> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Cc: linuxppc-dev list , linuxppc64-dev Subject: Re: PATCH uninorth3 (G5) agp support List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , >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