linux-kernel - Re: [RFC PATCH 1/6] dt-bindings: display: imx: Add EPDC

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Message-ID: <20220314230446.5cc52121@aktux>
Date:   Mon, 14 Mar 2022 23:04:46 +0100
From:   Andreas Kemnade <andreas@...nade.info>
To:     Jonathan Neuschäfer <j.neuschaefer@....net>
Cc:     p.zabel@...gutronix.de, airlied@...ux.ie, daniel@...ll.ch,
        robh+dt@...nel.org, shawnguo@...nel.org, s.hauer@...gutronix.de,
        kernel@...gutronix.de, festevam@...il.com, linux-imx@....com,
        maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, dri-devel@...ts.freedesktop.org,
        devicetree@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, alistair@...stair23.me,
        samuel@...lland.org, josua.mayer@....eu,
        letux-kernel@...nphoenux.org
Subject: Re: [RFC PATCH 1/6] dt-bindings: display: imx: Add EPDC

On Sat, 12 Mar 2022 20:23:48 +0100
Jonathan Neuschäfer <j.neuschaefer@....net> wrote:

> Hello Andreas,
> 
> Sorry for the delay, I finally got around to having a look at the
> patchset.
> 
> Some comments from skimming the patches below, and in my other replies.
> 
> 
> On Sun, Feb 06, 2022 at 09:00:11AM +0100, Andreas Kemnade wrote:
> > Add a binding for the Electrophoretic Display Controller found at least
> > in the i.MX6.
> > The timing subnode is directly here to avoid having display parameters
> > spread all over the plate.
> > 
> > Supplies are organized the same way as in the fbdev driver in the
> > NXP/Freescale kernel forks. The regulators used for that purpose,
> > like the TPS65185, the SY7636A and MAX17135 have typically a single bit to
> > start a bunch of regulators of higher or negative voltage with a
> > well-defined timing. VCOM can be handled separately, but can also be
> > incorporated into that single bit.
> > 
> > Signed-off-by: Andreas Kemnade <andreas@...nade.info>
> > ---
> >  .../bindings/display/imx/fsl,mxc-epdc.yaml    | 159 ++++++++++++++++++
> >  1 file changed, 159 insertions(+)
> >  create mode 100644 Documentation/devicetree/bindings/display/imx/fsl,mxc-epdc.yaml
> > 
> > diff --git a/Documentation/devicetree/bindings/display/imx/fsl,mxc-epdc.yaml b/Documentation/devicetree/bindings/display/imx/fsl,mxc-epdc.yaml
> > new file mode 100644
> > index 000000000000..7e0795cc3f70
> > --- /dev/null
> > +++ b/Documentation/devicetree/bindings/display/imx/fsl,mxc-epdc.yaml
> > @@ -0,0 +1,159 @@
> > +# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
> > +%YAML 1.2
> > +---  
> [...]
> > +  - vscan-holdoff
> > +  - sdoed-width
> > +  - sdoed-delay
> > +  - sdoez-width
> > +  - sdoez-delay
> > +  - gdclk-hp-offs
> > +  - gdsp-offs
> > +  - gdoe-offs
> > +  - gdclk-offs
> > +  - num-ce  
> 
> These parameters should perhaps have sane defaults in the driver, and be
> optional in the DT.
> 
First of all I think I should document them better (as said in an
earlier review mail)

I doubt there are sane defaults, in vendor kernels, there is typically a
definition of these parameters and a video mode per display.

> 
> > +
> > +additionalProperties: false
> > +
> > +examples:
> > +  - |
> > +    #include <dt-bindings/clock/imx6sl-clock.h>
> > +    #include <dt-bindings/interrupt-controller/arm-gic.h>
> > +
> > +    epdc: epdc@...4000 {  
> [...]
> > +
> > +        timing {
> > +                clock-frequency = <80000000>;
> > +                hactive = <1448>;
> > +                hback-porch = <16>;
> > +                hfront-porch = <102>;
> > +                hsync-len = <28>;
> > +                vactive = <1072>;
> > +                vback-porch = <4>;
> > +                vfront-porch = <4>;
> > +                vsync-len = <2>;
> > +        };
> > +    };  
> 
> The way you did it here, the timing parameters are directly under the
> EPDC node in the DT, but I wonder if it would be better to have a
> separate node for the display panel, which can then provide the timing
> parameters either in the DT or in the panel driver (selected by compatible
> string of the panel).
> 
IMHO it makes sense to store these timing parameters together with the
timing parameters from above. If that all somehow comes from a panel
driver, we need to design an interface for it. So for simplicity I
added the stuff just to the EPDC node.

Vendor kernel has this:
struct imx_epdc_fb_mode {
    struct fb_videomode *vmode;
    int vscan_holdoff;
    int sdoed_width;
    int sdoed_delay;
    int sdoez_width;
    int sdoez_delay;
    int gdclk_hp_offs;
    int gdsp_offs;
    int gdoe_offs;
    int gdclk_offs;
    int num_ce;
};

So things are basically combined here.

Regards,
Andreas