# Re: \[PATCH\] Added backlight driver for Acer Aspire 4736

- From: joeyli \<jlee@suse.com\>
- Date: 2012-03-14 05:53:01 
- List: linux-fbdev
- Page: https://ratatoskr.run/linux-fbdev/2012/03/3835581
- Thread: https://ratatoskr.run/linux-fbdev/2012/03/3835160/t.md

```text
於 三，2012-03-14 於 08:13 +0530，Pradeep Subrahmanion 提到：
> Hi Joey , 
> 
> > Per my understood, EC firmware should change brightness but didn't do
> > that, another
> > way is touch i915 register in _BCM.
> 
>   how do we do this ? you mean change the _BCM implementation ? 

"BIOS guy" should do something like this:

 Method (AINT, 2, NotSerialized)
{
...
        If (LEqual (Arg0, One))
        {
            Store (Divide (Multiply (Arg1, 0xFF), 0x64, ), BCLP)
            Or (BCLP, 0x80000000, BCLP)		<== touch BCLP register
            Store (0x02, ASLC)
        }

    Method (_BCM, 1, NotSerialized)
    {
        If (LAnd (LGreaterEqual (Arg0, Zero), LLessEqual (Arg0, 0x64)))
        {
            AINT (One, Arg0)     <== call AINT method
            Store (Arg0, BRTL)
        }
    }


Just for reference, they should do that when EC didn't wire to
backlight. 

> > 
> > Acer machine provide a broken _BCM implementation and they didn't test
> > it.
> > 
> > > > > By ' ACPI interface' , I mean 'acpi_video0' inside the
> > > > > /sys/class/backlight. I havn't tried the /sys/class/backlight interface
> > > > > directly . I will try that also.
> > > > 
> > > > So writing values into /sys/class/backlight/acpi_video0/brightness does 
> > > > nothing?
> > > 
> > > 
> > > No change in value when writing
> > > to /sys/class/backlight/acpi_video0/brightness.
> > > 
> > > Another thing is that when i did boot with acpi_backlight = 'acer_wmi' ,
> > > in new kernel (3.3.0-rc7) , it shows following messages , 
> > > 
> > > [    8.350825] wmi: Mapper loaded
> > > [   10.363975] acer_wmi: Acer Laptop ACPI-WMI Extras
> > > [   10.396186] acer_wmi: Function bitmap for Communication Device: 0x91
> > > [   10.396385] acer_wmi: Brightness must be controlled by generic video
> > > driver
> > > 
> > > Also there was no interface inside /sys/class/backlight for acer_wmi. 
> > > 
> > 
> > Yes, acer_wmi support backlight control with AMW0 interface, your
> > machine didn't have AMW0 interface.
> > 
> > Normally, backlight should control by standard acpi interface.
> > 
> > > I also tried writing directly to Embedded controller register .But no
> > > change.
> > 
> > The machine has broken _BCM method, because EC should do something after
> > _BCM changed EC register.
> 
> Thanks , 
> 
> Pradeep Subrahmanion

Why they didn't find _BCM not work?

My guess is:

Because the backlight control is through WDDM driver on Windows platform
but not through standard ACPI method _BCM. They only test Windows
platform, so, they didn't find _BCM broken.

And, they also didn't really follow Microsoft WDDM spec:

 http://msdn.microsoft.com/en-us/windows/hardware/gg487382.aspx

Per spec,
ODM should keep _BCM works fine for any other OS didn't support WDDM
driver, but they didn't.

At last year, I told Acer PM one time for this issue, they said will
check but finally didn't response me.


Thanks
Joey Lee

```