[Bug 90075] New: [BYT] Acer Aspire Switch 10 backlight control

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=90075

            Bug ID: 90075
           Summary: [BYT] Acer Aspire Switch 10 backlight control
           Product: DRI
           Version: DRI git
          Hardware: Other
                OS: All
            Status: NEW
          Severity: normal
          Priority: medium
         Component: DRM/Intel
          Assignee: intel-gfx-bugs at lists.freedesktop.org
          Reporter: nekit1000 at gmail.com
        QA Contact: intel-gfx-bugs at lists.freedesktop.org
                CC: intel-gfx-bugs at lists.freedesktop.org

Backlight hack/WIP patch found in #85977 doesn't work for Acer Aspire Switch 10
with Intel Atom Z3735F.

dev_priv->vbt.dsi.config->pwm_blc == 1 on it, so it seems to be BYT-CR and, as
Shohbit answered, needs to have brightness controlled by LPSS PWM. However,
unlike another Z3735F tablet (Teclast X98 Air 3G) PWM is not present in ACPI
DSDT and not detected by LPSS PWM 4.0 kernel driver (CONFIG_PWM_LPSS_PCI,
CONFIG_PWM_LPSS_PLATFORM and CONFIG_X86_INTEL_LPSS are enabled in kernel
options). lspci also doesn't show any PWM device.

In Windows, I can see Intel Atom LPIO1 PWM Controller at PCI Bus 0-30-1 (should
be probably pci 0000:00:1e.1 in Linux enumeration) in System Information Viewer
tool, but it's not present in Device Manager, so I'm not sure if it's used at
all. May there be any other method to set backlight level involved?

I tried compiling 3.14 kernel from Android-IA, and it actually tries to set
backlight via LPSS, but fails since PWM is not detected:
[    0.968667] lpio_bl_read: can't find pwm device with pwm_num 0
[    2.414817] lpio_bl_write: can't find pwm device with pwm_num 0

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