[Intel-gfx] How to fix screen resolution detection?

[Jani Nikula]

On Fri, 17 Dec 2021, Alan Stern <stern at rowland.harvard.edu> wrote:
> The screen resolution on my laptop is not reported accurately.  Here's 
> an extract from the output of xdpyinfo:
>
> screen #0:
>   dimensions:    3200x1800 pixels (847x476 millimeters)
>   resolution:    96x96 dots per inch
>
> The number of pixels is correct, but the size and resolution values 
> smack of a bogus default.  The actual width of the screen (determined 
> with a tape measure) is about 11.5 inches (291 mm), which yields a 
> resolution of 280 dots per inch (11 dots per mm), approximately.  
> Most definitely _not_ 96 dpi.
>
> Presumably X gets the size/resolution information from Wayland, which 
> gets it from the kernel, which gets it from the firmware.  So the kernel 
> driver is the logical place to start in figuring where things are going 
> wrong.  The laptop uses i915; here are the relevant lines from the 
> kernel log:
>
> [    0.000000] Linux version 5.14.9-200.fc34.x86_64 (mockbuild at bkernel02.iad2.fedoraproject.org) (gcc (GCC) 11.2.1 20210728 (Red Hat 11.2.1-1), GNU ld version 2.35.2-5.fc34) #1 SMP Thu Sep 30 11:55:35 UTC 2021
>
> [    0.463895] efifb: probing for efifb
> [    0.463913] efifb: framebuffer at 0xe0000000, using 22500k, total 22500k
> [    0.463916] efifb: mode is 3200x1800x32, linelength=12800, pages=1
> [    0.463919] efifb: scrolling: redraw
> [    0.463920] efifb: Truecolor: size=8:8:8:8, shift=24:16:8:0
> [    0.464028] Console: switching to colour frame buffer device 400x112
> [    0.474894] fb0: EFI VGA frame buffer device
>
> [    2.888858] fb0: switching to inteldrmfb from EFI VGA
> [    2.891260] Console: switching to colour dummy device 80x25
> [    2.891318] i915 0000:00:02.0: vgaarb: deactivate vga console
> [    2.902665] i915 0000:00:02.0: vgaarb: changed VGA decodes: olddecodes=io+mem,decodes=io+mem:owns=io+mem
> [    2.904833] i915 0000:00:02.0: [drm] Finished loading DMC firmware i915/skl_dmc_ver1_27.bin (v1.27)
> [    2.947359] [drm] Initialized i915 1.6.0 20201103 for 0000:00:02.0 on minor 0
> [    2.949468] ACPI: video: Video Device [GFX0] (multi-head: yes  rom: no  post: no)
> [    2.949803] input: Video Bus as /devices/LNXSYSTM:00/LNXSYBUS:00/PNP0A08:00/LNXVIDEO:00/input/input9
> [    2.964371] fbcon: i915 (fb0) is primary device
> [    2.979854] Console: switching to colour frame buffer device 400x112
> [    3.012355] i915 0000:00:02.0: [drm] fb0: i915 frame buffer device
>
> Now, I know nothing about the kernel's graphics subsystems.  How can I 
> find out what size/resolution information i915 is getting and passing to 
> Wayland?  If it's wrong, how can I fix it?

I could be wrong, but I think userspace figures the dimensions from the
EDID itself, not through a kernel API.

I actually get slightly different results from xrandr, xdpyinfo, and
edid-decode on the EDID. What does edid-decode tell you for the
dimensions? For me it's 'edid-decode /sys/class/drm/card0-DP-3/edid' but
replace the subdir with your info.

xdpyinfo also gives me 96x96 dpi, probably bogus.


BR,
Jani.


-- 
Jani Nikula, Intel Open Source Graphics Center