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Date:   Fri, 06 Aug 2021 13:01:33 +0200
From:   Paul Cercueil <paul@crapouillou.net>
Subject: Re: [PATCH 2/2] gpu/drm: ingenic: Add workaround for disabled drivers
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Rob Herring <robh@kernel.org>,
        "Rafael J . Wysocki" <rafael@kernel.org>,
        David Airlie <airlied@linux.ie>,
        Daniel Vetter <daniel@ffwll.ch>,
        Sam Ravnborg <sam@ravnborg.org>, list@opendingux.net,
        linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org,
        dri-devel@lists.freedesktop.org
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        <20210805192110.90302-3-paul@crapouillou.net> <YQw9hjZll4QmYVLX@kroah.com>
        <3HUDXQ.7RBGD4FUHR2F@crapouillou.net> <YQ0MU/GcLkPLiy5C@kroah.com>
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Hi Greg,

Le ven., ao?t 6 2021 at 12:17:55 +0200, Greg Kroah-Hartman 
<gregkh@linuxfoundation.org> a ?crit :
> On Thu, Aug 05, 2021 at 10:05:27PM +0200, Paul Cercueil wrote:
>>  Hi Greg,
>> 
>>  Le jeu., ao?t 5 2021 at 21:35:34 +0200, Greg Kroah-Hartman
>>  <gregkh@linuxfoundation.org> a ?crit :
>>  > 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@crapouillou.net>
>>  > >  ---
>>  > >   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