Re: [PATCH v3 5/6] hppa: Add emulation of Artist graphics

[Sven Schnelle <svens@stackframe.org>]

Hi Mark,

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.

Regards
Sven