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

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Message-ID: <CAG3jFytoE9hWvq2e2Caqn4qP_RuEOnm4r9VQ85ffbAcguSLf+w@mail.gmail.com>
Date:   Tue, 23 Mar 2021 10:10:43 +0100
From:   Robert Foss <robert.foss@...aro.org>
To:     Adrien Grassein <adrien.grassein@...il.com>
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 Adrien,

Sorry about the slow reply, but I just received the documentation from
the vendor. So let's dig in to the HPD issue.

> +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

> +
> +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.

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.


[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