I've started working to get PCI MGA cards, the PCI G450 specifically, working with DRI. My initial goal is to just get it working with crummy performance, then improve it by adding support for IOMMUs (to simulate AGP texturing) on systems like pSeries and AMD64 that have them.
I've started by digging through the DRI init process in the X.org MGA DDX. As near as I can tell, the driver uses AGP memory for four things. To make the PCI cards work, I'll need to make it do without AGP for these things.
1. WARP microcode. This seems *really* odd to me. The DDX carves off a 32KiB chunk of AGP space and gives it to the kernel to use to store the WARP microcode. Why is the DDX involved in this *at all*? The microcode exists only in the kernel module. It seems that the DRM could just as easily drm_pci_alloc a chunk of memory large enough to hold the microcode for the card (which is different for G400-class cards and G200-class cards).
2. Primary DMA buffer. The DDX carves of 1MB for the primary DMA buffer. I don't think that's outside the reasonable realm for drm_pci_alloc. If it is, can this work with a smaller buffer?
3. Secondary DMA buffers. The DDX carves off room for 128 64KiB DMA buffers. I haven't dug that deeply, but I seem to recall that the DRI driver uses these buffers as non-contiguous. That is, it treats them as 128 separate buffers and not a big 8MB buffer that it cards 64KiB chunks from. If that's the case, then it should be easy enough to modify the driver the drm_pci_alloc (upto) 128 64KiB chunks for PCI cards. Is there any actual performance benefit to having this be in AGP space at all or do they just have to be in the same "address space" as the primary DMA buffer?
4. AGP textures. Without an IOMMU, we pretty much have to punt here. Performance will be bad, but I can live with that.
I think this is all pretty much correct. I don't think the primary dma buffer needs to be anything like that large, secondary buffers can be non-contiguous from a hardware interaction point of view, but who knows what assumptions are coded into the drm dma code, etc.
Keith
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