Check RAM usage for i915 Intel GPU

I have a Lenovo Thikpad P14s with hybrid graphics : integrated Intel GPU and additional nVidia CPU.

I used sudo prime-select intel to set it to use only the integrated GPU.

There's a BIOS setting to give 256 or 512 MB RAM to the GPU, which I set to 512.

I want to check the real memory usage by the GPU.

Let's check a few things :

$ prime-select query
intel

$ lspci |grep VGA
00:02.0 VGA compatible controller: Intel Corporation CometLake-U GT2 [UHD Graphics] (rev 02)

$ sudo lspci -v -s 00:02.0
00:02.0 VGA compatible controller: Intel Corporation CometLake-U GT2 [UHD Graphics] (rev 02) (prog-if 00 [VGA controller])
    Subsystem: Lenovo CometLake-U GT2 [UHD Graphics]
    Flags: bus master, fast devsel, latency 0, IRQ 158
    Memory at e2000000 (64-bit, non-prefetchable) [size=16M]
    Memory at 80000000 (64-bit, prefetchable) [size=512M]
    I/O ports at 4000 [size=64]
    Expansion ROM at 000c0000 [virtual] [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

$ glxinfo | grep -E 'Device:|Video memory:'
    Device: Mesa Intel(R) UHD Graphics (CML GT2) (0x9b41)
    Video memory: 15640MB

$ sudo lshw -C display
  *-display                 
       description: VGA compatible controller
       product: CometLake-U GT2 [UHD Graphics]
       vendor: Intel Corporation
       physical id: 2
       bus info: pci@0000:00:02.0
       logical name: /dev/fb0
       version: 02
       width: 64 bits
       clock: 33MHz
       capabilities: pciexpress msi pm vga_controller bus_master cap_list rom fb
       configuration: depth=32 driver=i915 latency=0 resolution=2560,1440
       resources: irq:158 memory:e2000000-e2ffffff memory:80000000-9fffffff ioport:4000(size=64) memory:c0000-dffff
  *-display UNCLAIMED
       description: 3D controller
       product: GP108GLM [Quadro P520]
       vendor: NVIDIA Corporation
       physical id: 0
       bus info: pci@0000:2d:00.0
       version: a1
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress bus_master cap_list
       configuration: latency=0
       resources: memory:e1000000-e1ffffff memory:c0000000-cfffffff memory:d0000000-d1ffffff ioport:3000(size=128)

lspci -v reports 512 MB, which should be correct.

glxinfo reports the whole RAM, which is obviously incorrect.

I don't know how to calculate each range's size in lshw output, but it seems to report the same ranges as lspci -v.

I also thought about the total memory being less than 16 GB, as reported by the free command, or if we ask the kernel directly :

grep MemTotal /proc/meminfo
MemTotal:       16015528 kB

The kernel sees 16015528 / 1024 = 15640 total MB

While 16 GB should be 16 * 1024 = 16384 total MB

So 16384 - 15640 = 744 MB are unaccounted for, might this be due to them being reserved by the GPU ? But why 744 instead of 512 ? Are some other memory ranges reserved and not seen by the kernel ?

By the way, I checked that the kernel reports kB as 1024 bytes (see https://superuser.com/questions/1737654/what-is-the-true-meaning-of-the-unit-kb-in-proc-meminfo) and eveything RAM related is usually in powers of 1024 too (as in, 16 GB total should be 16 * 1024 MB).