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

[Paul Cercueil <paul@crapouillou.net>]

Hi Daniel,

Le mar., août 10 2021 at 11:35:43 +0200, Daniel Vetter 
<daniel@ffwll.ch> a écrit :
> 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@linuxfoundation.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@linuxfoundation.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@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.
> 
> Is this actually a legit use-case?
> 
> Like you have hw with a bunch of sub-devices linked, and you decided 
> to
> disable some of them, which makes the driver not load.

Yes. I'm facing that issue with a board that has a LCD panel and a HDMI 
controller (IT66121). I have a "flasher" program for all the Ingenic 
boards, that's basically just a Linux kernel + initramfs booted over 
USB (device). I can't realistically enable every single driver for all 
the hardware that's on these boards while still having a tiny 
footprint. And I shouldn't have to care about it either.

> Why should we care? Is that hdmi driver really that big that we have 
> to
> support this use-case?

DRM maintainers work with what embedded devs would call "infinite 
resources". It annoys me that CONFIG_DRM pulls the I2C code even though 
I may just have a LCD panel, and it annoys me that I have to enable 
support for hardware that I'm not even planning to use, just so that 
the DRM driver works for the hardware I do want to use.

> I know it's possible to do this, that doesn't mean it's a good idea.
> There's inifinitely more randconfigs that don't boot on my machine 
> here
> for various reasons than the ones that do boot. We don't have "fixes" 
> for
> all of these to make things still work, despite user misconfiguring 
> their
> kernel.

I understand, you can't really expect random configs to work every 
time. But it should still be possible to disable drivers for *optional* 
hardware in the config and end up with a working system.

Cheers,
-Paul