On Mon, Jul 29, 2019 at 12:58 PM Oleksandr Suvorov
<oleksandr.suvorov@xxxxxxxxxxx> wrote:
>
> On Thu, Jul 25, 2019 at 5:41 PM maxime.ripard@xxxxxxxxxxxxxxxxxx
> <maxime.ripard@xxxxxxxxxxxxxxxxxx> 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?
Still no. The "always connected" case is for outputs which are
physically always connected and typing a dummy function which would
unconditionally return connected would be silly. Like built-in panels.
This is _not_ for external screens.
-Daniel
>
> @@ -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
>
> Toradex AG
> Altsagenstrasse 5 | 6048 Horw/Luzern | Switzerland | T: +41 41 500
> 4800 (main line)
--
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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