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

[Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>]

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.