[PATCH v7 2/2] drm/bridge: Introduce LT8912B DSI to HDMI bridge

[Robert Foss]

> > Are you saying that pin 63 never is high, or that an irq/isr routine
> > isn't getting executed?
> >
> > > When reading the value of the HPD pin, I always get 1 (and no
> > > transition occurs when plugging / unplugging a cable).
> > > The HPD IRQ is done on the HDMI connector driver [5].
> > > I think a register configuration should be done to enable the IRQ pin
> > > or maybe there is a nug in electronics.
> >
> > After looking at the documentation a bit more, I think we can ignore
> > pin63 and instead have a look at pin14. It is the HDMI TX HPD Control
> > pin. It has a 100k pull-down, so it should be active high.
>
> pin63 is always active high.
> pin14 is connected to the HDMI logic (pin 19 of the HDMI connector)
> with a 100k pull-down.

Thanks, that makes sense. pin14 is connected directly to the physical connector.

>
> >
> > I also found some different I2C addresses than what you've used, I
> > assume the device is available on both addresses.
> >
> > Chip control registers, address:0x90
> > CEC control registers, address 0x92
> >
> Strange, configuration seems to be working well with the address used
> in my driver.

Some chips support alternative addresses, I guess we can assume that
is the case here.

>
> > > The HPD pin is linked to a 2.2k pullup resistor (maybe it's wrong)
> >
> > The datasheet isn't entirely clear about if pin14 has an internal 100k
> > pull-down, or if they recommend adding a 100k pull-down.
> >
> > But this does seem like an issue.
>
> pin14 can't be used directly. I guess it's used by the internal logic
> of the chip to generate the HPD (pin63) signal.

Ack. Ok, false alert with pin14 then.

> >
> > >
> > > >
> > > > > >
> > > > > > > +
> > > > > > > +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.
> > > >
> > > > Looking at the dt-binding patch, it does not seem to list any
> > > > interrupts. So that should be added. I think the irq support from [3]
> > > > & [4] can be pretty much copied.
> > > >
> > > > Then we can come back and replace the polling code with the IRQ driven
> > > > code from [2].
> > >
> > > My board uses a "max7323" GPIO expander and the HPD pin is linked to it.
> > > I test this GPIO expander by soldering a pull up resistor and an
> > > interrupt on it and an interrupt was correctly triggered in both
> > > max7323 driver and hdmi-connector;
> > > So I guess that my DTB configuration is correct.
> > > I made my DBT configuration available:
> > >   - hdmi-connector node: [6]
> > >   - lt8912b node: |7]
> > >   - max7323 node: [8].
> >
> > Looking at [7] I think that you would want to add something like:
> >
> > hdmi-bridge at 48 {
> >         interrupts-extended = <&max7323 $LT8912B_PIN_14 IRQ_TYPE_EDGE_RISING>;
> > }

Ok, so pin63 is the output from lt8912b we should be listening to.
And looking at lt9611 it seems to be low=active, so IRQ_TYPE_EDGE_FALLING.

         interrupts-extended = <&max7323 $LT8912B_PIN_63 IRQ_TYPE_EDGE_FALLING>;


> >
> > And of course add the corresponding parts from [2] and [3].
> >
> > >
> > >
> > > >
> > > > >
> > > > > >
> > > > > > [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
> > >
> > > [5] https://github.com/torvalds/linux/blob/master/drivers/gpu/drm/bridge/display-connector.c#L199
> > > [6] https://github.com/grassead/linux-next/blob/master/arch/arm64/boot/dts/freescale/imx8mq-nitrogen.dts#L37
> > > [7] https://github.com/grassead/linux-next/blob/master/arch/arm64/boot/dts/freescale/imx8mq-nitrogen.dts#L249
> > > [8] https://github.com/grassead/linux-next/blob/master/arch/arm64/boot/dts/freescale/imx8mq-nitrogen.dts#L291
> > >
> > >
> > > Thanks,
>
> Maybe the conclusion is that we cannot have the HPD working.

If we can read bit7 @ 0xC1, all that we are missing is getting the
interrupt to fire after a HPD event.