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Date:   Tue, 10 Aug 2021 12:40:39 +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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Hi Greg,

Le mar., ao?t 10 2021 at 11:47:32 +0200, Greg Kroah-Hartman 
<gregkh@linuxfoundation.org> a ?crit :
> On Fri, Aug 06, 2021 at 01:01:33PM +0200, Paul Cercueil wrote:
>>  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.
> 
> But you never really know that.  That is what the recent driver core
> changes were all about, to handle this very issue.  Only when the 
> child
> device shows up will you need to care about it.
> 
>>  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.
> 
> But again, you never know that, probing is async, and could happen in 
> a
> few milliseconds, or a few hours, your driver should never care about
> this at all.
> 
> Just knowing if the kernel configuration is something is not the
> solution here, please fix this properly like all other driver
> interactions are in the kernel tree.

A proper fix means reworking the DRM core so that it supports 
hot-plugging bridges. Until then there is nothing else I can do.

-Paul