[PATCH] drm/bochs: Add screen blanking support

[Takashi Iwai]

On Tue, 20 Apr 2021 19:47:31 +0200,
Thomas Zimmermann wrote:
> 
> Hi
> 
> Am 20.04.21 um 18:56 schrieb Takashi Iwai:
> > On bochs DRM driver, the execution of "setterm --blank force" results
> > in a frozen screen instead of a blank screen.  It's due to the lack of
> > the screen blanking support in its code.
> >
> > Actually, the QEMU bochs vga side can switch to the blanking mode when
> > the bit 0x20 is cleared on VGA_ATT_IW register (0x3c0), which updates
> > ar_index in QEMU side.  So, essentially, we'd just need to clear the
> > bit at pipe disable callback; that's what this patch does essentially.
> >
> > However, a tricky part is that the QEMU vga code does treat VGA_ATT_IW
> > register always as "flip-flop"; the first write is for index and the
> > second write is for the data like palette.  Meanwhile, in the current
> > bochs DRM driver, the flip-flop wasn't considered, and it calls only
> > the register update once with the value 0x20.
> >
> 
> Unless bochs does things very different, the index should first be
> reset by reading 0x3da. Then write the index, then the data.
> 
> https://web.stanford.edu/class/cs140/projects/pintos/specs/freevga/vga/vgareg.htm#attribute

Thanks for the pointer!

It seems that QEMU stdvga actually implements the discard of flip flop
bit by reading 0x3da.  Meanwhile, the write of the data isn't needed
in this case because we do care only about the enablement bit 0x20 of
ar_index.

I'll resubmit v2 patch.


Takashi