On 01/11/2019 21:59, Sven Schnelle wrote: > On Sat, Oct 26, 2019 at 07:54:40PM +0200, Sven Schnelle wrote: >> Hi Mark, >> >> On Sat, Oct 26, 2019 at 10:35:20AM +0100, Mark Cave-Ayland wrote: >> >>>> However, the VRAM in Artist is not really exposed to the Host. Instead, >>>> there's the Chipset inbetween that can do byte swapping (Colormap is LE, >>>> VRAM is BE) and Bit-to-Byte/Word/Dword conversion. For example you could >>>> write 0x55 into that VRAM region, and the chipset would expand that to >>>> VRAM Bytes: 00 01 00 01 00 01 00 01. And to make it even worse emulation >>>> wise it can also do different encodings for Read or Write accesses, and >>>> mask out certain bits of the data. So after trying to convert it to the >>>> "dirty bitmap" API i decided to just leave it as it is. The CPU load >>>> used by the display update code is usually < 1%, so it's ok for me. >>> >>> Wow that sounds that some interesting hardware(!). Does it make sense to >>> model the >>> behaviour of the chipset separately using a proxy MemoryRegion similar to >>> virtio i.e. >>> introduce an intermediate IO MemoryRegion that does the swapping and then >>> forward it >>> onto the VRAM MemoryRegion? >> >> Thanks for the pointer, i'll check whether that would work. For now i >> think i'll remove the Artist patch from the series, so we can apply the >> other patches, and i'll re-submit Artist when it's done. I guess the >> rewrite to use a MemRegion is a bit bigger. But i would to get the other >> patches in especially the LASI Stuff as both Helge and i have a lot of >> stuff depending on that. > > I've looked into it again and changed my mind. There are at least the > following > functions that the Artist chip does before a Read/Write is passed to/from > VRAM: > > - endianess conversion (actually configurable via some register, but i don't > know how and hardwired it depending on CMAP / FB access) > > - The Address passed on the System bus are the X/Y coordinates added to the FB > base address in the selected buffer instead of the VRAM offset for pixel > data. > I think that's configurable via the some registers, but i don't know how. > Unfortunately there's absolutely no documentation about Artist available and > everything was developed by reverse engineering. > > - bitmap to Byte/Word conversion (not implemented yet for the VRAM window, > only > for the I/O register window) > > So in my opinion it's way to much effort to squeeze all of that into the > memory > space, and it is not really a Memory range that's just behind a bus bridge.
Hi Sven, Certainly in some cases it isn't possible to model devices in QEMU exactly as real hardware, although I think that some of the ideas above could be used to improve the implementation without too much extra effort. Then again from my work on QEMU I completely understand that sometimes this can be difficult with older, more esoteric devices. Ultimately after review that decision has to come from the maintainer(s) for the relevant devices/machines, so I guess that would be Richard and/or Gerd in this case? ATB, Mark.
