linux-kernel - Re: [PATCH v7 2/2] drm/bridge: Introduce LT8912B DSI to HDMI bridge

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Message-ID: <CABkfQAGvPy3DzXQnDJqm1q_rOLWR7BQTXb8z05iML3s3Mc8LJw@mail.gmail.com>
Date:   Tue, 23 Mar 2021 11:01:12 +0100
From:   Adrien Grassein <adrien.grassein@...il.com>
To:     Robert Foss <robert.foss@...aro.org>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        Andrzej Hajda <a.hajda@...sung.com>,
        Rob Herring <robh+dt@...nel.org>,
        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>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kernel test robot <lkp@...el.com>
Subject: Re: [PATCH v7 2/2] drm/bridge: Introduce LT8912B DSI to HDMI bridge

Hey Robert,

Thanks for the update.

Le mar. 23 mars 2021 à 10:10, Robert Foss <robert.foss@...aro.org> a écrit :
>
> Hey Adrien,
>
> Sorry about the slow reply, but I just received the documentation from
> the vendor. So let's dig in to the HPD issue.

No problem :)
>
> > +static enum drm_connector_status lt8912_check_cable_status(struct lt8912 *lt)
> > +{
> > +       int ret;
> > +       unsigned int reg_val;
> > +
> > +       ret = regmap_read(lt->regmap[I2C_MAIN], 0xC1, &reg_val);
> > +       if (ret)
> > +               return connector_status_unknown;
> > +
> > +       if (reg_val & BIT(7))
> > +               return connector_status_connected;
>
> Register 0xc0 & BIT(7) - HPD signal after debounce
> Register 0xc0 & BIT(6) - HPD signal for TX HPD pad

So, if I understand well, I need to write 0xc0 & BIT(6) with 1 to
enable the HPD pin.
>
> > +
> > +static int lt8912_probe(struct i2c_client *client,
> > +        const struct i2c_device_id *id)
> > +{
> > +       static struct lt8912 *lt;
> > +       int ret = 0;
> > +       struct device *dev = &client->dev;
> > +
> > +       lt = devm_kzalloc(dev, sizeof(struct lt8912), GFP_KERNEL);
> > +       if (!lt)
> > +               return -ENOMEM;
> > +
> > +       lt->dev = dev;
> > +       lt->i2c_client[0] = client;
> > +       lt->cable_status = connector_status_unknown;
> > +       lt->workq = create_workqueue("lt8912_workq");
>
> Looking at [1] and maybe even better [2], I think this polling
> approach is the wrong way to go. And with access to documentation, I
> think we should be able to sort this out.

I neither like the polling approach too. I did it to go on this issue.
I will totally remove it once the HPD issue will be resolved.
>
> Using the irq driver approach requires the interrupt pin to be
> configured. Pin 63 of the lt8912b is the IRQ output pin.
>
> In order to trigger interrupts based on it, the dt-binding would need
> to be updated[3][4] as well as whichever DTS you're using.
>

The IRQ part is working well in my DTB. It test it by adding some
electronics to emulate the HPD pin on the GPIO expander where the HPD
pin is linked.

>
> [1] https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/bridge/analogix/anx7625.c#L1751
>
> [2] https://github.com/torvalds/linux/blob/v5.11/drivers/gpu/drm/bridge/lontium-lt9611.c#L1160
>
> [3] https://github.com/torvalds/linux/blob/v5.11/Documentation/devicetree/bindings/display/bridge/lontium,lt9611.yaml#L27
>
> [4] https://github.com/torvalds/linux/blob/v5.11/Documentation/devicetree/bindings/display/bridge/lontium,lt9611.yaml#L144

Thanks a lot,
Adrien