Re: [PATCH RFC] arm64: dts: mediatek: mt8195-cherry: Remove keyboard-backlight node

["Nícolas F. R. A. Prado" <nfraprado@collabora.com>]

On Tue, Jul 16, 2024 at 11:24:44AM +0200, AngeloGioacchino Del Regno wrote:
> Il 15/07/24 18:09, Nícolas F. R. A. Prado ha scritto:
> > Commit 970c3a6b7aa3 ("mfd: cros_ec: Register keyboard backlight
> > subdevice") introduced support for detecting keyboard backlight
> > fuctionality through communication with the ChromeOS EC. This means that
> > the DT node is no longer used. Remove the unneeded node.
> > 
> > Signed-off-by: Nícolas F. R. A. Prado <nfraprado@collabora.com>
> > ---
> > Different CrosEC FW versions could potentially not support discovering
> > the keyboard backlight functionality, but I've tested both a recent
> > 
> >    tomato_v2.0.23149-099cd3e539 tomato_15699.72.0 2024-01-03
> > 
> > and an old
> > 
> >    tomato_v2.0.10686-234e646fd8 tomato_14268.0.0 2021-10-07
> > 
> > version on mt8195-cherry-tomato and on both relying only on the
> > discoverability works. I've tested on both tomato-r2 and tomato-r3. I
> > have not tested on dojo, however, as I don't have access to it.
> > 
> 
> Dojo will work anyway because those machines do share the same base FW... but
> anyway, I'm not sure that this is the right thing to do.
> 
> The commit that you mentioned says that it is meant to make that "work on machines
> without specific ACPI or OF support for the keyboard backlight", but not that the
> intention is to stop using either ACPI nor DT nodes for that.

Yes, because as I understand it not every EC might support this protocol. So
that commit just added an additional way to probe the keyboard backlight.

So we don't need to stop using the DT to probe it. But in practice we have
already stopped, as long as the EC supports the protocol (which from my testing
is always for these platforms), since that is tried first. Meaning the DT node
is now useless.

The only point in keeping the DT node would be to use it as a fallback in case
the discovery with the EC fails or breaks. But I have never seen a DT node be
there just as fallback, so it doesn't feel right to me either.

> 
> The DT kselftest is relatively young, and I suspect that anyway this is not the
> only affected device, so the justification is only barely valid.

I didn't include the failing test as part of the commit message proper as I
don't think it should justify this change. I added it just to clarify my
motivation. The test showed me that something unexpected was happening. After
looking into it I thought that a DT node that is no longer used to probe has no
point in staying around, so that's the justification that I added to the commit
message.

> 
> Don't misunderstand me, I'm not saying that I'm not okay with this, but I'd like to
> have more opinions about this.
> 
> If we choose to go this way, ideally we should remove this from all of the upstream
> Chromebook devicetrees (not only MediaTek, clearly!) so that would require a bit
> more effort to test here and there.

Note that the cherry DT is the only DT upstream with the
google,cros-kbd-led-backlight compatible. So it's really only tomato and dojo
that need to be tested.

Thanks,
Nícolas