Re: [PATCH v4 2/7] dt-bindings: ti-lmu: Modify dt bindings for the LM3697

From: Rob Herring <robh@kernel.org>
To: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc: Dan Murphy <dmurphy@ti.com>, Pavel Machek <pavel@ucw.cz>,
	devicetree@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-leds@vger.kernel.org, lee.jones@linaro.org,
	tony@atomide.com
Subject: Re: [PATCH v4 2/7] dt-bindings: ti-lmu: Modify dt bindings for the LM3697
Date: Thu, 25 Oct 2018 14:54:44 -0500	[thread overview]
Message-ID: <20181025195444.GA12737@bogus> (raw)
In-Reply-To: <ade3121a-58ab-c0c3-6605-cf7d6a638da0@gmail.com>

On Thu, Oct 25, 2018 at 08:27:18PM +0200, Jacek Anaszewski wrote:
> On 10/25/2018 08:07 PM, Dan Murphy wrote:
> > Rob
> > 
> > On 10/24/2018 09:54 AM, Rob Herring wrote:
> >> On Wed, Oct 24, 2018 at 07:07:57AM -0500, Dan Murphy wrote:
> >>> Pavel
> >>>
> >>> On 10/24/2018 04:04 AM, Pavel Machek wrote:
> >>>> Hi!
> >>>>
> >>>>> The LM3697 is a single function LED driver. The single function LED
> >>>>> driver needs to reside in the LED directory as a dedicated LED driver
> >>>>> and not as a MFD device.  The device does have common brightness and ramp
> >>>>
> >>>> So it is single function LED driver. That does not mean it can not
> >>>> share bindings with the rest. Where the bindings live is not imporant.
> >>>>
> >>>
> >>> It can share bindings that are correctly done, not ones that are incomplete and incorrect.
> >>>
> >>> Where bindings live is important to new Linux kernel developers and product 
> >>> developers looking for the proper documentation on the H/W bindings.
> >>>
> >>>>> reside in the Documentation/devicetree/bindings/leds directory and follow the
> >>>>> current LED and general bindings guidelines.
> >>>>
> >>>> What you forgot to tell us in the changelog:
> >>>
> >>> I can add this to the changelog.
> >>>
> >>>>
> >>>>> +Optional child properties:
> >>>>> +	- runtime-ramp-up-msec: Current ramping from one brightness level to
> >>>>> +				the a higher brightness level.
> >>>>> +				Range from 2048 us - 117.44 s
> >>>>
> >>>> The other binding uses "ramp-up-msec". Tell us why you are changing this, or
> >>>> better don't change things needlessly.
> >>>>
> >>>> We don't want to be using "runtime-ramp-up-msec" for one device and
> >>>> "ramp-up-msec" for the other.
> >>>
> >>> This is another example of how the original bindings were incorrect and misleading.
> >>>
> >>> The LM3697 have 2 ramp implementations that can be used.
> >>>
> >>> Startup/Shutdown ramp and Runtime Ramp.  Same Ramp rates different registers and
> >>> different end user experience.
> >>>
> >>> So having a single node call ramp-up-msec is misleading and it does not
> >>> indicate what the H/W will do.
> >>
> >> The existing ones aren't documented (present in the example is not 
> >> documented). This seems like something that should be common rather than 
> >> TI specific. Though it also seems more like something the user would 
> >> want to control (i.e. sysfs) rather than fixed in DT.
> >>
> > 
> > Changing the runtime ramping or startup/shutdown ramping could also be done via sysfs.
> > I am not dedicated to having it in the DT file I was following prior art.
> > 
> > Jacek
> > 
> > Do you have an opinion on this?
> 
> This is this problem with the Device Tree's scope of responsibility.
> It is defined as a means for "describing the hardware", but often
> this rule is abused by the properties that fall into "configuration"
> category. E.g. default-state, retain-state-suspended from leds-gpio.txt
> or linux-default-trigger from common LED bindings.
> 
> In some cases this is justified. The question is whether it is something
> that necessarily needs to be configured on driver probing? If not, then
> I'd go for sysfs interface.

Yes. I'd also add it should be along the lines of for a given 
board it's always configured in that way or is it something you'd want 
in the BIOS of your PC.

Rob