Thinkpad X1 Carbon 3rd - Reducing the compressed framebuffer size

[Ville Syrjälä]

On Tue, Feb 13, 2018 at 02:38:42PM +0100, Pali Rohár wrote:
> On Tuesday 13 February 2018 15:27:26 Ville Syrjälä wrote:
> > On Tue, Feb 13, 2018 at 09:50:30AM +0100, Pali Rohár wrote:
> > > On Tuesday 06 February 2018 16:21:43 Pali Rohár wrote:
> > > > Hi! I'm periodically getting following message in dmesg on Lenovo
> > > > Thinkpad X1 Carbon 3rd generation:
> > > > 
> > > > [drm] Reducing the compressed framebuffer size. This may lead to less power savings than a non-reduced-size. Try to increase stolen memory size if available in BIOS.
> > > > 
> > > > In BIOS I already set GPU size to 512M, but this did not help. Also
> > > > update to last BIOS version did not help.
> > > > 
> > > > So why this message is periodically print in dmesg? And what can I do
> > > > with this problem?
> > > > 
> > > > And why cannot Linux kernel allocate itself more memory for GPU (if BIOS
> > > > can/could do that)? Is not 512MB for GPU enough?
> > > 
> > > And here is output from lspci, which clearly says that 512MB is already
> > > set for GPU:
> > 
> > The PCI BAR size has nothing to do with the size of the stolen memory.
> > The BAR just provides a window into the global GTT address space of the
> > GPU. Stolen memory is a contiguous chunk of physical memory carved out
> > by the BIOS.
> 
> Ok, how could I detect how much memory was stolen?
> 
> In dmesg I see following lines:
> 
> [    0.000000] e820: BIOS-provided physical RAM map:
> [    0.000000] BIOS-e820: [mem 0x0000000000000000-0x0000000000057fff] usable
> [    0.000000] BIOS-e820: [mem 0x0000000000058000-0x0000000000058fff] reserved
> [    0.000000] BIOS-e820: [mem 0x0000000000059000-0x000000000008bfff] usable
> [    0.000000] BIOS-e820: [mem 0x000000000008c000-0x000000000009ffff] reserved
> [    0.000000] BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff] reserved
> [    0.000000] BIOS-e820: [mem 0x0000000000100000-0x00000000ab908fff] usable
> [    0.000000] BIOS-e820: [mem 0x00000000ab909000-0x00000000abb08fff] type 20
> [    0.000000] BIOS-e820: [mem 0x00000000abb09000-0x00000000acbfefff] reserved
> [    0.000000] BIOS-e820: [mem 0x00000000acbff000-0x00000000acd7efff] ACPI NVS
> [    0.000000] BIOS-e820: [mem 0x00000000acd7f000-0x00000000acdfefff] ACPI data
> [    0.000000] BIOS-e820: [mem 0x00000000acdff000-0x00000000acdfffff] usable
> [    0.000000] BIOS-e820: [mem 0x00000000f80f8000-0x00000000f80f8fff] reserved
> [    0.000000] BIOS-e820: [mem 0x00000000fed1c000-0x00000000fed1ffff] reserved
> [    0.000000] BIOS-e820: [mem 0x0000000100000000-0x000000024dffffff] usable
> 
> [    0.000000] Reserving Intel graphics memory at 0x00000000ae000000-0x00000000afffffff

That's the one. Since you have a BDW the amount FBC can actually use
will be 8MiB less than what's reported here. So looks like you should
have 24MiB total, minus whatever else we end up allocating from stolen.

Check /sys/kernel/debug/dri/0/i915_gem_stolen to see what's there. Most
likely you'll have the fbdev framebuffer taking up a sizeable chunk.
You could get some back by reducing fbdev depth to 16bpp, or even 8bpp,
but I'm not convinced the fbdev gamma LUT stuff really works currently
so you might end up with bogus colors in your vts with that.

> 
> [    0.000000] Memory: 7972840K/8282704K available (6196K kernel code, 1159K rwdata, 2848K rodata, 1408K init, 688K bss, 309864K reserved, 0K cma-reserved)
> 
> > The BIOS may or may not provide a knob to change the size
> > of the stolen memory.
> 
> In BIOS Setup screen I have option to choose GPU memory and I set it to
> max 512MB. So this is not the right option...
> 
> And why cannot kernel use some continuous check of RAM itself?

Because the hardware won't allow it.

> 
> > > 
> > > $ lspci -v -s 00:02.0
> > > 00:02.0 VGA compatible controller: Intel Corporation HD Graphics 5500 (rev 09) (prog-if 00 [VGA controller])
> > >         Subsystem: Lenovo HD Graphics 5500
> > >         Flags: bus master, fast devsel, latency 0, IRQ 46
> > >         Memory at e0000000 (64-bit, non-prefetchable) [size=16M]
> > >         Memory at c0000000 (64-bit, prefetchable) [size=512M]
> > >         I/O ports at 3000 [size=64]
> > >         [virtual] Expansion ROM at 000c0000 [disabled] [size=128K]
> > >         Capabilities: [90] MSI: Enable+ Count=1/1 Maskable- 64bit-
> > >         Capabilities: [d0] Power Management version 2
> > >         Capabilities: [a4] PCI Advanced Features
> > >         Kernel driver in use: i915
> > >         Kernel modules: i915
> > > 
> > > -- 
> > > Pali Rohár
> > > pali.rohar at gmail.com
> > > _______________________________________________
> > > dri-devel mailing list
> > > dri-devel at lists.freedesktop.org
> > > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> > 
> 
> -- 
> Pali Rohár
> pali.rohar at gmail.com

-- 
Ville Syrjälä
Intel OTC