Re: [PATCH 0/1] This patch fixes connection detection for monitors w/o DDC.

[Oleksandr Suvorov <oleksandr.suvorov@toradex.com>]

On Thu, Jul 25, 2019 at 5:41 PM maxime.ripard@free-electrons.com
<maxime.ripard@free-electrons.com> wrote:
>
> On Thu, Jul 25, 2019 at 11:05:23AM +0000, Oleksandr Suvorov wrote:
> >
> > Even in source code of this driver there is an author's description:
> >     /*
> >      * Even if we have an I2C bus, we can't assume that the cable
> >      * is disconnected if drm_probe_ddc fails. Some cables don't
> >      * wire the DDC pins, or the I2C bus might not be working at
> >      * all.
> >      */
> >
> > That's true. DDC and VGA channels are independent, and therefore
> > we cannot decide whether the monitor is connected or not,
> > depending on the information from the DDC.
> >
> > So the monitor should always be considered connected.
>
> Well, no. Like you said, we cannot decided whether is connected or
> not.

Maxime, thanks, I agree that's a bad solution.
But I still think we should be able to define the DT node of a device for
this driver to claim the connector is always connected.
Please see my following thoughts.

> > Thus there is no reason to use connector detect callback for this
> > driver: DRM sub-system considers monitor always connected if there
> > is no detect() callback registered with drm_connector_init().
> >
> > How to reproduce the bug:
> > * setup: i.MX8QXP, LCDIF video module + gpu/drm/mxsfb driver,
> >   adv712x VGA DAC + dumb-vga-dac driver, VGA-connector w/o DDC;
> > * try to use drivers chain mxsfb-drm + dumb-vga-dac;
> > * any DRM applications consider the monitor is not connected:
> >   ===========
> >   $ weston-start
> >   $ cat /var/log/weston.log
> >       ...
> >       DRM: head 'VGA-1' found, connector 32 is disconnected.
> >       ...
> >   $ cat /sys/devices/platform/5a180000.lcdif/drm/card0/card0-VGA-1/status
> >   unknown
>
> And that's exactly what's being reported here: we cannot decide if it
> is connected or not, so it's unknown.
>
> If weston chooses to ignore connectors that are in an unknown state,
> I'd say it's weston's problem, since it's much broader than this
> particular device.

If we look at the code of drm_probe_helper.c, we can see, the
drm_helper_probe_detect_ctx() assume the cable is connected if there is no
detect() callback registered.
...
                if (funcs->detect_ctx)
                         ret = funcs->detect_ctx(connector, &ctx, force);
                 else if (connector->funcs->detect)
                         ret = connector->funcs->detect(connector, force);
                 else
                         ret = connector_status_connected;
...

The driver dumb-vga-dac supports both DT configurations:
- with DDC channel, that allows us to detect if the cable is connected;
- without DDC channel. In this case, IMHO, the driver should behave
the same way as a
  connector driver without registered detect() callback.

So what about the patch like?

@@ -81,6 +81,13 @@ dumb_vga_connector_detect(struct drm_connector
*connector, bool force)
 {
        struct dumb_vga *vga = drm_connector_to_dumb_vga(connector);

+       /*
+        * If I2C bus for DDC is not defined, asume that the cable
+        * is always connected.
+        */
+       if (PTR_ERR(vga->ddc) == -ENODEV)
+               return connector_status_connected;
+
        /*
         * Even if we have an I2C bus, we can't assume that the cable
         * is disconnected if drm_probe_ddc fails. Some cables don't

-- 
Best regards
Oleksandr Suvorov

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