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

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Message-ID: <YRJLNHXR0PhykBwL@kroah.com>
Date:   Tue, 10 Aug 2021 11:47:32 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Paul Cercueil <paul@...pouillou.net>
Cc:     Rob Herring <robh@...nel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Sam Ravnborg <sam@...nborg.org>, list@...ndingux.net,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 2/2] gpu/drm: ingenic: Add workaround for disabled drivers

On Fri, Aug 06, 2021 at 01:01:33PM +0200, Paul Cercueil wrote:
> Hi Greg,
> 
> Le ven., août 6 2021 at 12:17:55 +0200, Greg Kroah-Hartman
> <gregkh@...uxfoundation.org> 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@...uxfoundation.org> 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@...pouillou.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.

thanks,

greg k-h