[PATCH V2 4/9] drm/exynos: add exynos_dp_panel driver registration to drm driver

[Thierry Reding]

On Tue, Aug 19, 2014 at 09:02:39PM -0700, Stéphane Marchesin wrote:
> On Tue, Apr 22, 2014 at 8:26 AM, Thierry Reding
> <thierry.reding at gmail.com> wrote:
> > On Tue, Apr 22, 2014 at 08:33:23PM +0530, Ajay kumar wrote:
> >> Hi Thierry,
> >>
> >> On Tue, Apr 22, 2014 at 2:03 PM, Thierry Reding
> >> <thierry.reding at gmail.com> wrote:
> >> > On Tue, Apr 22, 2014 at 04:09:13AM +0530, Ajay Kumar wrote:
> >> >> Register exynos_dp_panel before the list of exynos crtcs and
> >> >> connectors are probed.
> >> >>
> >> >> This is needed because exynos_dp_panel should be registered to
> >> >> the drm_panel list via panel-exynos-dp probe, i.e much before
> >> >> exynos_dp_bind calls of_drm_find_panel().
> >> >>
> >> >> Signed-off-by: Ajay Kumar <ajaykumar.rs at samsung.com>
> >> >> ---
> >> >> Changes since V1:
> >> >>       Added platform_driver_unregister(&exynos_dp_panel_driver) to
> >> >>       exynos_drm_platform_remove as per Jingoo Han's correction
> >> >>
> >> >>  drivers/gpu/drm/exynos/exynos_drm_drv.c |   15 +++++++++++++++
> >> >>  drivers/gpu/drm/exynos/exynos_drm_drv.h |    1 +
> >> >>  2 files changed, 16 insertions(+)
> >> >>
> >> >> diff --git a/drivers/gpu/drm/exynos/exynos_drm_drv.c b/drivers/gpu/drm/exynos/exynos_drm_drv.c
> >> >> index 1d653f8..2db7f67 100644
> >> >> --- a/drivers/gpu/drm/exynos/exynos_drm_drv.c
> >> >> +++ b/drivers/gpu/drm/exynos/exynos_drm_drv.c
> >> >> @@ -530,12 +530,23 @@ static int exynos_drm_platform_probe(struct platform_device *pdev)
> >> >>               goto err_unregister_ipp_drv;
> >> >>  #endif
> >> >>
> >> >> +#ifdef CONFIG_DRM_PANEL_EXYNOS_DP
> >> >> +     ret = platform_driver_register(&exynos_dp_panel_driver);
> >> >> +     if (ret < 0)
> >> >> +             goto err_unregister_dp_panel;
> >> >> +#endif
> >> >
> >> > No, this is not how you're supposed to use DRM panel drivers. The idea
> >> > is that you write a standalone driver for a given panel.
> >> >
> >> > What you do here has a number of problems. For one it's a driver that's
> >> > tightly coupled to Exynos SoCs. But if I have a different SoC that uses
> >> > the same panel I want to be able to use the same driver, and not have to
> >> > rewrite the driver for my SoC.
> >> >
> >> > Another problem is that you're assuming here that the driver is built in
> >> > and it will break if you try to build either Exynos DRM or the panel
> >> > driver as a module. This is perhaps nothing you care about right now,
> >> > but eventually people will want to ship a single kernel that can run on
> >> > a number of SoCs. But if we keep adding things like this, that kernel
> >> > will keep growing in size until it no longer fits in any kind of memory.
> >> >
> >> > Thierry
> >>
> >> I completely agree with you in this!
> >>
> >> Yes, this is not acceptable, but I want to know an "acceptable"
> >> workaround for the situation below:
> >> I register the driver using module_init().
> >> And, exynos_drm gets probed much before the panel driver probe happens.
> >> So, the panel driver hasn't probed yet, but exynos_dp via exynos_drm
> >> tries to call
> >> "of_drm_find_panel" which always returns NULL.
> >
> > That's a situation that your driver needs to be able to deal with. The
> > driver registration order doesn't matter one bit. It may happen to work
> > most of the time, but as soon as one of the resources that your panel
> > driver needs isn't there when the panel is probed, then it won't be
> > registered and of_drm_find_panel() will still return NULL.
> >
> > Usually the right thing to do in that case would be to return (and
> > propagate) -EPROBE_DEFER so that your driver's probe is deferred and
> > retried when other drivers have been probed. That way it should
> > eventually get a non-NULL panel.
> 
> So I just gave this (drm_panel + probe deferring) a shot on exynos,
> and correctly reacting to -EPROBE_DEFER postpones DP initialization by
> approximately 1.5 second. Is there a good way to handle that? As it
> stands, this isn't usable.

How much is 1.5 seconds compared to the overall boot time of the device?
What exactly is causing this 1.5 second delay?

This really is a fundamental issue with deferred probing and the issue
has come up several times in the past. A couple of possible solutions
have been proposed, with the latest being here[0] I think. That ended in
a bit of a debacle, unfortunately, but on of the outcomes was that a lot
of the ordering problems could be fixed by using phandle references to
track dependencies. I'm not aware of anyone working on that right now,
presumably because everyone is busy getting features merged rather than
optimizing boot speed.

Thierry

[0]: https://lkml.org/lkml/2014/5/12/452
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