Re: [PATCH 2/2] gpu/drm: ingenic: Add workaround for disabled drivers

[Paul Cercueil]

Hi Greg,

Le ven., août 6 2021 at 12:17:55 +0200, Greg Kroah-Hartman 
<gregkh@xxxxxxxxxxxxxxxxxxx> a écrit :
> On Thu, Aug 05, 2021 at 10:05:27PM +0200, Paul Cercueil wrote:
> >  Hi Greg,
> >
> >  Le jeu., août 5 2021 at 21:35:34 +0200, Greg Kroah-Hartman
> >  <gregkh@xxxxxxxxxxxxxxxxxxx> a écrit :
> >  > On Thu, Aug 05, 2021 at 09:21:09PM +0200, Paul Cercueil wrote:
> >  > >  When the drivers of remote devices (e.g. HDMI chip) are 
> > disabled in
> >  > > the
> >  > >  config, we want the ingenic-drm driver to be able to probe
> >  > > nonetheless
> >  > >  with the other devices (e.g. internal LCD panel) that are 
> > enabled.
> >  > >
> >  > >  Signed-off-by: Paul Cercueil <paul@xxxxxxxxxxxxxxx>
> >  > >  ---
> >  > >   drivers/gpu/drm/ingenic/ingenic-drm-drv.c | 12 ++++++++++++
> >  > >   1 file changed, 12 insertions(+)
> >  > >
> >  > >  diff --git a/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
> >  > > b/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
> >  > >  index d261f7a03b18..5e1fdbb0ba6b 100644
> >  > >  --- a/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
> >  > >  +++ b/drivers/gpu/drm/ingenic/ingenic-drm-drv.c
> >  > >  @@ -1058,6 +1058,18 @@ static int ingenic_drm_bind(struct 
> > device
> >  > > *dev, bool has_components)
> >  > >   	for (i = 0; ; i++) {
> >  > >   		ret = drm_of_find_panel_or_bridge(dev->of_node, 0, i, 
> > &panel,
> >  > > &bridge);
> >  > >   		if (ret) {
> >  > >  +			/*
> >  > >  +			 * Workaround for the case where the drivers for the
> >  > >  +			 * remote devices are not enabled. When that happens,
> >  > >  +			 * drm_of_find_panel_or_bridge() returns -EPROBE_DEFER
> >  > >  +			 * endlessly, which prevents the ingenic-drm driver from
> >  > >  +			 * working at all.
> >  > >  +			 */
> >  > >  +			if (ret == -EPROBE_DEFER) {
> >  > >  +				ret = driver_deferred_probe_check_state(dev);
> >  > >  +				if (ret == -ENODEV || ret == -ETIMEDOUT)
> >  > >  +					continue;
> >  > >  +			}
> >  >
> >  > So you are mucking around with devices on other busses within this
> >  > driver?  What could go wrong?  :(
> >
> >  I'm doing the same thing as everybody else. This is the DRM driver, 
> > and
> >  there is a driver for the external HDMI chip which gives us a DRM 
> > bridge
> >  that we can obtain from the device tree.
>
> But then why do you need to call this function that is there for a 
> bus,
> not for a driver.

The documentation disagrees with you :)

And, if that has any weight, this solution was proposed by Rob.

> >  > Please use the existing driver core functionality for this type of
> >  > thing, it is not unique, no need for this function to be called.
> >
> >  I'm not sure you understand what I'm doing here. This driver calls
> >  drm_of_find_panel_or_bridge(), without guarantee that the driver 
> > for the
> >  remote device (connected via DT graph) has been enabled in the 
> > kernel
> >  config. In that case it will always return -EPROBE_DEFER and the 
> > ingenic-drm
> >  driver will never probe.
> >
> >  This patch makes sure that the driver can probe if the HDMI driver 
> > has been
> >  disabled in the kernel config, nothing more.
>
> That should not be an issue as you do not care if the config is 
> enabled,
> you just want to do something in the future if the driver shows up,
> right?

Well, the DRM subsystem doesn't really seem to handle hotplug of 
hardware. Right now all the drivers for the connected hardware need to 
probe before the main DRM driver. So I need to know that a remote 
device (connected via DT graph) will never probe.

Give me a of_graph_remote_device_driver_will_never_probe() and I'll use 
that.

> Much like the device link code, have you looked at that?

I don't see how that would help in any way. The device link code would 
allow me to set a dependency between the remote hardware (HDMI chip, 
provider) and the LCD controller (consumer), but I already have that 
dependency though the DT graph. What I need is a way for the consumer 
to continue probing if the provider is not going to probe.

Cheers,
-Paul