[Intel-gfx] Can recent i915 support more than 8192x8192 screen?

[Marcin Owsiany]

Hello,

*TL;DR*: how can I set a 8960x2880 screen (not display) size on a T580? A
patch for i915 that I found on the internets does not seem to work.

Full story:

I'm a rather happy user of ThinkPad T580 which comes with a
high-density 3840x2160 LCD, and the following graphics hardware.

00:02.0 VGA compatible controller: Intel Corporation Device 5917 (rev 07)
(prog-if 00 [VGA controller])
        Subsystem: Lenovo Device 225a
        Flags: bus master, fast devsel, latency 0, IRQ 142
        Memory at e7000000 (64-bit, non-prefetchable) [size=16M]
        Memory at c0000000 (64-bit, prefetchable) [size=256M]
        I/O ports at e000 [size=64]
        [virtual] Expansion ROM at 000c0000 [disabled] [size=128K]
        Capabilities: [40] Vendor Specific Information: Len=0c <?>
        Capabilities: [70] Express Root Complex Integrated Endpoint, MSI 00
        Capabilities: [ac] MSI: Enable+ Count=1/1 Maskable- 64bit-
        Capabilities: [d0] Power Management version 2
        Capabilities: [100] Process Address Space ID (PASID)
        Capabilities: [200] Address Translation Service (ATS)
        Capabilities: [300] Page Request Interface (PRI)
        Kernel driver in use: i915
        Kernel modules: i915

Unfortunately attaching it to an external normal-density 2560x1440 display
means I need to apply scaling. Combined with the side-by-side arrangement
of monitors, this means I'd need to set screen size to 8960x2880. However
this does not work:

 $ xrandr --fb 8960x2880
 xrandr: screen cannot be larger than 8192x8192 (desired size 8960x2880)


I found this thread on reddit
<https://www.reddit.com/r/linux/comments/6bghzm/increasing_maximum_xorg_virtual_screen_resolution/>
about the same problem, where a user posted a simple patch claimed to be
supplied by someone on #intel-gfx. Unfortunately it does not work (or at
least is not sufficient) - after applying it xrandr does claim
that 16384x16384 is possible, but actually trying to use more
than 8192x8192 fails with an error (unfortunately I lost the exact message).

My current workaround is to pretend that the displays are arranged
vertically, but even after a few months I'm sometimes having trouble
remembering that I need to move mouse cursor UP when I want to go to LEFT
display :-)

I wonder if someone could help me here. Even just getting a definite answer
on whether my hardware can in theory support this or not would be helpful,
since I found conflicting information.

FWIW I'm on Debian stable, Linux 4.9.8x.

regards,
Marcin
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