On Wednesday 16 February 2005 16:18, Martijn Sipkema wrote: > [...] > > > Being able to read huge textures from system memory would be > > > a nice feature anyway. > > > > Reading from system memory in this way was dropped as a simplification > > and security feature. > > I think most graphics hardware is able to read/write directly to system > memory and with large textures and offscreen windows using a lot of memory > it would be inconvenient to not be able to store them in system memory > without losing hardware rendering.
Of course there is no hard data regarding this, but considering that what we require *lots* of bandwidth on the PCI bus for geometry data, I don't believe texturing from system memory is feasible anyway. The only case where being unable to render directly from system memory is likely a drawback is when you want to use a texture only once or twice. Also remember that the *size* of video memory really isn't going to be a bottleneck in this design. > The security part I don't really understand. Do you mean to not provide > any way for the graphics hardware to write to system memory? I don't > see how reading from system memory can be avoided... Well, if I understand Timothy correctly, the CPU can tell the graphics card to start DMA transfers, but those work just like a memcpy() does (but without locking up the CPU during the transfer). However, texturing can only be done from video memory. The security aspect is that the kernel doesn't necessarily have to verify the base address for textures any more, because a userspace application cannot program the texture base address in a way that allows to read (or write) arbitrary system memory. I personally believe that this is negligible, but I don't have any data to back it up either way. > It may be a simplication, but it is also, IMHO, a serious drawback. > > One might even want to have local memory act as a cache for system > memory, but you'd need the graphics hardware to be able to read/write > to system memory to be able to do so also.. It is still possible to do this (implemented in the driver software) if I understand correctly, because the graphics card can still control DMA transfers. It just can't access system memory directly for texturing. cu, Nicolai > > --ms
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