On Wed, 5 Mar 2025, Mark Cave-Ayland wrote:
On 03/03/2025 17:52, Hab Gallagher wrote:
(added Gerd on CC)
That driver is for a VGA device which I think does not support gamma in
hardware. QEMU emulates the hardware so if it does not support gamma
then
there's no place to add it in QEMU. Therefore if you want to emulate
gamma
on VGA this should be done within the guest driver but that may be
slower.
How does this work on real hardware? What graphics device is used there
and does that support gamma in hardware? Maybe that graphics device
needs
to be emulated instead?
For understanding the MacOS side of things, I have been consulting https://
developer.apple.com/library/archive/documentation/Hardware/DeviceManagers/pci_srvcs/
pci_cards_drivers/Designing_PCI_Cards_Drivers.pdf
<https://developer.apple.com/
library/archive/documentation/Hardware/DeviceManagers/pci_srvcs/pci_cards_drivers/
Designing_PCI_Cards_Drivers.pdf>
To reduce visible flashes resulting from color table changes, the
SetGamma
routine works in conjunction with the SetEntries control routine on
indexed
devices. The SetGamma routine first loads new gamma correction data
into the
driver’s private storage; the next SetEntries control call applies the
gamma
correction as it changes the CLUT. SetGamma calls are always followed
by
SetEntries control calls on indexed devices.
For direct devices, the SetGamma routine first sets up the gamma
correction data
table. Next, it synthesizes a black-to-white linear ramp color table.
Finally, it
applies the new gamma correction to the color table and sets the data
directly in
the hardware.
As far as I can tell, the documentation is implying that somewhere between
the driver and the physical hardware, *something* must keep track of the
gamma table (even if indirectly by subsequently applying it to the palette
table) to map the raw framebuffer data to gamma-corrected colors.
Perhaps this is also of interest:
https://github.com/SolraBizna/mac_qfb_driver
<https://github.com/SolraBizna/mac_qfb_driver>The nubus declaration rom
supports
gamma correction.
This is very much of interest! Thank you. I am hoping to continue to use
the mac99 machine type, but this code is a useful reference.
The most basic support requires cscGetGamma and cscSetGamma to at least
return success: https://github.com/cebix/macemu/
commit/2676e1bd134703d888788c682fb56e07b5cf56a9
<https://github.com/cebix/macemu/
commit/2676e1bd134703d888788c682fb56e07b5cf56a9>
The patch to SheepShaver was small because most of the functionality was
already present, albeit dead code.
Surprisingly, github can't deal with CR line endings, so I can't (easily)
link to some of the code to cite it:
https://github.com/ozbenh/QemuMacDrivers/blob/master/
shared/MacDriverUtils.c
<https://github.com/ozbenh/QemuMacDrivers/blob/master/shared/
MacDriverUtils.c>
Once the gamma table is saved, it's "applied" by cscSetEntries to combine a
color palette and gamma table into the "real" color. QemuMacDrivers does
not seem to keep track of the palette at all, sending it upstream to the
driver by calling this function in a loop:
OSStatus QemuVga_SetColorEntry(UInt32 index, RGBColor *color)
{
VgaWriteB(0x3c8, index);
VgaWriteB(0x3c9, color->red >> 8);
VgaWriteB(0x3c9, color->green >> 8);
VgaWriteB(0x3c9, color->blue >> 8);
return noErr;
}
VgaWriteB seems to be doing MMIO to write those values upstream.
Similarly, mac_qfb_driver seems to use MMIO to write back both the palette
table and the gamma table to the driver, saving neither one in emulated
driver state: https://
github.com/SolraBizna/mac_qfb_driver/blob/e78ba4ccd08d254a10bad7c13d90810b17dbfd87/
src/control.cc#L48-L62 <https://github.com/SolraBizna/mac_qfb_driver/blob/
e78ba4ccd08d254a10bad7c13d90810b17dbfd87/src/control.cc#L48-L62>
Implementing the registers to store the gamma table is trivial: the tricky
part is updating the VGA framebuffer code to apply the gamma as it is
rendered. Have a look at
https://gitlab.com/qemu-project/qemu/-/blob/master/hw/display/vga.c?ref_type=heads#L1478
(vga_draw_graphic) and
https://gitlab.com/qemu-project/qemu/-/blob/master/hw/display/vga.c?ref_type=heads#L1194
(vga_draw_text) to see how this is handled.
Gerd: what do you think would be the best way to apply a gamma to the VGA
device?
I think that would be similar to what's done here:
https://github.com/SolraBizna/qemu/commit/f551de525360a26df481c5b80876845c323d9303
Maybe add such additional draw routines with alpha and only set those in
vga_draw_line_table when an alpha is set otherwise use the current ones.
That should avoid changing current code in case when alpha is not
used. Alternatively that qfb device could be merged and a PCI version
added for mac99?
Regards,
BALATON Zoltan
