linux-kernel - Re: [PATCH v7 1/2] dt-bindings: display: bridge: Document THC63LVD1024 LVDS decoder

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Date:   Sat, 07 Apr 2018 12:56:48 +0300
From:   Laurent Pinchart <laurent.pinchart@...asonboard.com>
To:     jacopo mondi <jacopo@...ndi.org>
Cc:     Jacopo Mondi <jacopo+renesas@...ndi.org>, architt@...eaurora.org,
        a.hajda@...sung.com, airlied@...ux.ie,
        vladimir_zapolskiy@...tor.com, horms@...ge.net.au,
        magnus.damm@...il.com, geert@...ux-m68k.org,
        niklas.soderlund@...natech.se, sergei.shtylyov@...entembedded.com,
        robh+dt@...nel.org, mark.rutland@....com,
        dri-devel@...ts.freedesktop.org, linux-renesas-soc@...r.kernel.org,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v7 1/2] dt-bindings: display: bridge: Document THC63LVD1024 LVDS decoder

Hi Jacopo,

On Saturday, 7 April 2018 12:45:56 EEST jacopo mondi wrote:
> On Fri, Apr 06, 2018 at 06:40:14PM +0300, Laurent Pinchart wrote:
> > Hi Jacopo,
> > 
> > (CC'ing Mark Brown)
> 
> Hi Mark
> 
> [snip]
> 
> >> Anyway, we spent enough time on naming issues, starting from my first
> >> stupid 'pdwn' permutations then on this semi-standard names.
> >> 
> >> I'll send next version with 'powerdown-gpios' and 'oe-gpios'
> >> properties hoping that would be finally accepted by everyone.
> > 
> > I certainly won't complain (as long as you write pwdn instead of pdwn in
> > the driver :-)).
> 
> Oh, so you won't complain if I address your comments? Thank you! :D
> By the way, the dumb pdwn name comes, again, from the chip name. I can
> change it to a saner name for sure...

And I've just realized that, I thought it was a typo :-/ If it comes with the 
datasheet I'm fine with either.

> >> Same on the mandatory/optional VCC supply thing. Let's try to make
> >> next version the final one. If the optional property with the dummy
> >> regulator doesn't satisfy you and it is preferred to have a
> >> fixed-regulator anyhow in DT I'll do in next version, othewise let's try
> >> not to change it again. I'll just remark here that in the current Eagle
> >> design vcc is connected to a power rail with no regulator at all :)
> > 
> > I don't like the dummy regulator much, as it generates a dev_warn(), which
> > makes me believe that it's a hack rather than a proper solution. You might
> > want to ask Mark Brown for his opinion.
> 
> Sure: Hi Mark, a bit of context here to save you a long(er) reading.
> 
> Unsurprisingly, the chip for which I'm writing this small driver needs
> a power supply to be properly functional :) In the development board
> it is installed on, the power supply is connected to a power rail,
> with no regulator. At the same time, some other designs may instead
> include a regulator.

To be precise, with an always-on regulator that can't be software-controlled.

> I assume that's a very common situation. I started by describing the
> regulator as optional in DT bindings, and use regulator_get_optional(). If
> that function returns PTR_ERR, the regulator is then ignored in driver's
> power management routines.
> 
> In this last version I thought I was acting smart and copied what happens
> in other DRM bridges like adv7511, which use 'regulator_get()' and work
> with a dummy if no regulator is provided in DT. Laurent has the above
> doubts on using a dummy, and I actually share some of his concerns
> (that dev_warn() is how I found out adv7511 was using a dummy, actually).
> 
> To sum up: when a regulator is described as optional in DT, do you
> suggest to work with a dummy if it's not there, or use the _optional()
> version of regulator_get() and manually set it to NULL and ignore it
> in the enable/disable driver's routines?
> 
> Bonus question: Laurent likes to have the regulator described as required,
> and thus require it to be described in DT also in cases where it is not
> actually there using a 'fixed-regulator' and refuse to probe if the
> regulator is not available. Do you encourage this approach, or prefer to
> have it optional and handle it in the driver in one of the above described
> ways?

-- 
Regards,

Laurent Pinchart