On Fri, 17 Dec 2021, Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx) (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
