Strange effect with i915 backlight controller

[Daniel Mack <zonque@gmail.com>]

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Hi,

I'm facing a bug on a Samsung X20 notebook which features an i915 
chipset (output of 'lspci -v' attached).

The effect is that setting the backlight to odd values causes the value 
to be misinterpreted. Harald Hoyer (cc:) had the same thing on a Netbook 
(I don't recall which model it was).

So this will turn the backlight to full brightness:

# cat /sys/class/backlight/intel_backlight/max_brightness
29750
# echo 29750 > /sys/class/backlight/intel_backlight/brightness

However, writing 29749 will turn the display backlight off, and 29748 
appears to be the next valid lower value.

It seems like the IS_PINEVIEW() branch in 
drivers/gpu/drm/i915/intel_panel.c: intel_panel_actually_set_backlight() 
could do the right thing, but this code is written for an entirely 
different model, right?

I can reproduce this on 3.0 and 3.1 vanilla as well as with the current 
mainline git.

Let me know if there is any patch that I can test.


Thanks,
Daniel

[-- Attachment #2: lspci-v-x20.txt --]
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00:00.0 Host bridge: Intel Corporation Mobile 915GM/PM/GMS/910GML Express Processor to DRAM Controller (rev 03)
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, fast devsel, latency 0
	Capabilities: [e0] Vendor Specific Information: Len=09 <?>
	Kernel driver in use: agpgart-intel

00:02.0 VGA compatible controller: Intel Corporation Mobile 915GM/GMS/910GML Express Graphics Controller (rev 03) (prog-if 00 [VGA controller])
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, fast devsel, latency 0, IRQ 16
	Memory at b0080000 (32-bit, non-prefetchable) [size=512K]
	I/O ports at 1800 [size=8]
	Memory at c0000000 (32-bit, prefetchable) [size=256M]
	Memory at b0000000 (32-bit, non-prefetchable) [size=256K]
	Expansion ROM at <unassigned> [disabled]
	Capabilities: [d0] Power Management version 2
	Kernel driver in use: i915
	Kernel modules: intelfb, i915

00:02.1 Display controller: Intel Corporation Mobile 915GM/GMS/910GML Express Graphics Controller (rev 03)
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: fast devsel
	Memory at 42000000 (32-bit, non-prefetchable) [disabled] [size=512K]
	Capabilities: [d0] Power Management version 2

00:1c.0 PCI bridge: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) PCI Express Port 1 (rev 03) (prog-if 00 [Normal decode])
	Flags: bus master, fast devsel, latency 0
	Bus: primary=00, secondary=01, subordinate=01, sec-latency=0
	I/O behind bridge: 00003000-00003fff
	Memory behind bridge: b4000000-b7ffffff
	Prefetchable memory behind bridge: 00000000d0000000-00000000d3ffffff
	Capabilities: [40] Express Root Port (Slot+), MSI 00
	Capabilities: [80] MSI: Enable+ Count=1/1 Maskable- 64bit-
	Capabilities: [90] Subsystem: Samsung Electronics Co Ltd Device c01a
	Capabilities: [a0] Power Management version 2
	Capabilities: [100] Virtual Channel
	Capabilities: [180] Root Complex Link
	Kernel driver in use: pcieport
	Kernel modules: shpchp

00:1d.0 USB Controller: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) USB UHCI #1 (rev 03) (prog-if 00 [UHCI])
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 0, IRQ 23
	I/O ports at 1820 [size=32]
	Kernel driver in use: uhci_hcd

00:1d.1 USB Controller: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) USB UHCI #2 (rev 03) (prog-if 00 [UHCI])
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 0, IRQ 19
	I/O ports at 1840 [size=32]
	Kernel driver in use: uhci_hcd

00:1d.2 USB Controller: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) USB UHCI #3 (rev 03) (prog-if 00 [UHCI])
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 0, IRQ 18
	I/O ports at 1860 [size=32]
	Kernel driver in use: uhci_hcd

00:1d.3 USB Controller: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) USB UHCI #4 (rev 03) (prog-if 00 [UHCI])
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 0, IRQ 16
	I/O ports at 1880 [size=32]
	Kernel driver in use: uhci_hcd

00:1d.7 USB Controller: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) USB2 EHCI Controller (rev 03) (prog-if 20 [EHCI])
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 0, IRQ 23
	Memory at b0040000 (32-bit, non-prefetchable) [size=1K]
	Capabilities: [50] Power Management version 2
	Capabilities: [58] Debug port: BAR=1 offset=00a0
	Kernel driver in use: ehci_hcd

00:1e.0 PCI bridge: Intel Corporation 82801 Mobile PCI Bridge (rev d3) (prog-if 01 [Subtractive decode])
	Flags: bus master, fast devsel, latency 0
	Bus: primary=00, secondary=02, subordinate=06, sec-latency=32
	I/O behind bridge: 00004000-00004fff
	Memory behind bridge: b8000000-b80fffff
	Prefetchable memory behind bridge: 0000000044000000-0000000049ffffff
	Capabilities: [50] Subsystem: Gammagraphx, Inc. (or missing ID) Device 0000

00:1e.2 Multimedia audio controller: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) AC'97 Audio Controller (rev 03)
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 0, IRQ 17
	I/O ports at 1c00 [size=256]
	I/O ports at 18c0 [size=64]
	Memory at b0040800 (32-bit, non-prefetchable) [size=512]
	Memory at b0040400 (32-bit, non-prefetchable) [size=256]
	Capabilities: [50] Power Management version 2
	Kernel driver in use: snd_intel8x0
	Kernel modules: snd-intel8x0

00:1e.3 Modem: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) AC'97 Modem Controller (rev 03) (prog-if 00 [Generic])
	Subsystem: Samsung Electronics Co Ltd Device 2115
	Flags: medium devsel, IRQ 20
	I/O ports at 2400 [size=256]
	I/O ports at 2000 [size=128]
	Capabilities: [50] Power Management version 2
	Kernel modules: snd-intel8x0m

00:1f.0 ISA bridge: Intel Corporation 82801FBM (ICH6M) LPC Interface Bridge (rev 03)
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 0
	Kernel modules: iTCO_wdt, intel-rng

00:1f.1 IDE interface: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) IDE Controller (rev 03) (prog-if 8a [Master SecP PriP])
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 0, IRQ 18
	I/O ports at 01f0 [size=8]
	I/O ports at 03f4 [size=1]
	I/O ports at 0170 [size=8]
	I/O ports at 0374 [size=1]
	I/O ports at 1810 [size=16]
	Kernel driver in use: ata_piix

00:1f.3 SMBus: Intel Corporation 82801FB/FBM/FR/FW/FRW (ICH6 Family) SMBus Controller (rev 03)
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: medium devsel, IRQ 5
	I/O ports at 18a0 [size=32]
	Kernel modules: i2c-i801

02:05.0 Ethernet controller: Broadcom Corporation BCM4401-B0 100Base-TX (rev 02)
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, fast devsel, latency 32, IRQ 22
	Memory at b8000000 (32-bit, non-prefetchable) [size=8K]
	[virtual] Expansion ROM at 44000000 [disabled] [size=64K]
	Capabilities: [40] Power Management version 2
	Kernel driver in use: b44
	Kernel modules: b44

02:07.0 Network controller: Intel Corporation PRO/Wireless 2200BG [Calexico2] Network Connection (rev 05)
	Subsystem: Intel Corporation Samsung P35 integrated WLAN
	Flags: bus master, medium devsel, latency 32, IRQ 20
	Memory at b8002000 (32-bit, non-prefetchable) [size=4K]
	Capabilities: [dc] Power Management version 2
	Kernel driver in use: ipw2200
	Kernel modules: ipw2200

02:09.0 CardBus bridge: Ricoh Co Ltd RL5c476 II (rev b3)
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 168, IRQ 16
	Memory at 000da000 (32-bit, non-prefetchable) [size=4K]
	Bus: primary=02, secondary=03, subordinate=06, sec-latency=176
	Memory window 0: 50000000-53fff000 (prefetchable)
	Memory window 1: 4c000000-4ffff000
	I/O window 0: 00004400-000044ff
	I/O window 1: 00004000-000040ff
	16-bit legacy interface ports at 0001
	Kernel driver in use: yenta_cardbus
	Kernel modules: yenta_socket

02:09.1 FireWire (IEEE 1394): Ricoh Co Ltd R5C552 IEEE 1394 Controller (rev 08) (prog-if 10 [OHCI])
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 64, IRQ 17
	Memory at b8003000 (32-bit, non-prefetchable) [size=2K]
	Capabilities: [dc] Power Management version 2
	Kernel driver in use: firewire_ohci
	Kernel modules: firewire-ohci

02:09.2 SD Host controller: Ricoh Co Ltd R5C822 SD/SDIO/MMC/MS/MSPro Host Adapter (rev 17)
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: bus master, medium devsel, latency 32, IRQ 18
	Memory at b8003800 (32-bit, non-prefetchable) [size=256]
	Capabilities: [80] Power Management version 2
	Kernel driver in use: sdhci-pci
	Kernel modules: sdhci-pci

02:09.3 System peripheral: Ricoh Co Ltd R5C592 Memory Stick Bus Host Adapter (rev 08)
	Subsystem: Samsung Electronics Co Ltd Device c01a
	Flags: medium devsel, IRQ 255
	Memory at b8003c00 (32-bit, non-prefetchable) [size=256]
	Capabilities: [80] Power Management version 2


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