linux-kernel - Re: [PATCH 1/2] dt-bindings: drm/bridge: ti-sn65dsi86: Replace #pwm-cells

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Date:   Fri, 2 Oct 2020 13:42:00 -0700
From:   Doug Anderson <dianders@...omium.org>
To:     Bjorn Andersson <bjorn.andersson@...aro.org>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Rob Herring <robh+dt@...nel.org>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Neil Armstrong <narmstrong@...libre.com>,
        Laurent Pinchart <Laurent.pinchart@...asonboard.com>,
        Jonas Karlman <jonas@...boo.se>,
        Jernej Skrabec <jernej.skrabec@...l.net>,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>
Subject: Re: [PATCH 1/2] dt-bindings: drm/bridge: ti-sn65dsi86: Replace #pwm-cells

Hi,

On Wed, Sep 30, 2020 at 3:40 PM Bjorn Andersson
<bjorn.andersson@...aro.org> wrote:
>
> While the signal on GPIO4 to drive the backlight controller indeed is
> pulse width modulated its purpose is specifically to control the
> brightness of a backlight.

I'm a bit on the fence about this.  I guess you're doing this because
it avoids some -EPROBE_DEFER cycles in Linux?  It does seem to have a
few downsides, though.

1. It means a bit of re-inventing the wheel.  It's not a very big
wheel, though, I'll give you.  ...but it's still something.

2. I'm not sure why you'd want to, but in theory one could use this
PWM for some other purposes.  It really is just a generic PWM.  Your
change prevents that.

> Drop the #pwm-cells and instead expose a new property to configure the
> granularity of the backlight PWM signal.
>
> Signed-off-by: Bjorn Andersson <bjorn.andersson@...aro.org>
> ---
>  .../devicetree/bindings/display/bridge/ti,sn65dsi86.yaml | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> index f8622bd0f61e..e380218b4646 100644
> --- a/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> +++ b/Documentation/devicetree/bindings/display/bridge/ti,sn65dsi86.yaml
> @@ -66,9 +66,12 @@ properties:
>        1-based to match the datasheet.  See ../../gpio/gpio.txt for more
>        information.
>
> -  '#pwm-cells':
> -    const: 1
> -    description: See ../../pwm/pwm.yaml for description of the cell formats.
> +  ti,backlight-scale:
> +    description:
> +      The granularity of brightness for the PWM signal provided on GPIO4, if
> +      this property is specified.
> +    minimum: 0
> +    maximum: 65535

A few issues here:

1. Maybe call this "num-steps" instead of backlight-scale.  That's
essentially what it is, right?  Saying how many discrete steps you're
allowing in your backlight?

2. IMO you need the PWM frequency specified, since it can actually
matter.  NOTE: once you have the PWM frequency specified, you could
imagine automatically figuring out what "num-steps" was.  Really you'd
want it to be the largest possible value you could achieve with your
hardware at the specified frequency.  There's no advantage (is there?)
of providing fewer steps to the backlight client.

3. Some backlights are specified inverted.  It looks like this maps
nicely to the bridge chip, which has a bit for it.  Probably nice to
expose this?

Of course, if we were just exposing the PWM directly to Linux we could
just use the PWM backlight driver and it'd all magically work.  ;-)

-Doug