[PATCH 0/2] drm/tegra: Fix panel support on Venice 2 and Nyan

[Dmitry Osipenko]

20.12.2021 13:48, Thierry Reding пишет:
> From: Thierry Reding <treding at nvidia.com>
> 
> Hi,
> 
> this is an alternative proposal to fix panel support on Venice 2 and
> Nyan. Dmitry had proposed a different solution that involved reverting
> the I2C/DDC registration order and would complicate things by breaking
> the encapsulation of the driver by introducing a global (though locally
> scoped) variable[0].
> 
> This set of patches avoids that by using the recently introduced DP AUX
> bus infrastructure. The result is that the changes are actually less
> intrusive and not a step back. Instead they nicely remove the circular
> dependency that previously existed and caused these issues in the first
> place.
> 
> To be fair, this is not perfect either because it requires a device tree
> change and hence isn't technically backwards-compatible. However, given
> that the original device tree was badly broken in the first place, I
> think we can make an exception, especially since it is not generally a
> problem to update device trees on the affected devices.
> 
> Secondly, this relies on infrastructure that was introduced in v5.15 and
> therefore will be difficult to backport beyond that. However, since this
> functionality has been broken since v5.13 and all of the kernel versions
> between that and v5.15 are EOL anyway, there isn't much that we can do
> to fix the interim versions anyway.
> 
> Adding Doug and Laurent since they originally designed the AUX bus
> patches in case they see anything in here that would be objectionable.
> 
> Thierry
> 
> [0]: https://lore.kernel.org/dri-devel/20211130230957.30213-1-digetx@gmail.com/
> 
> Thierry Reding (2):
>   drm/tegra: dpaux: Populate AUX bus
>   ARM: tegra: Move panels to AUX bus
> 
>  arch/arm/boot/dts/tegra124-nyan-big.dts   | 15 +++++++++------
>  arch/arm/boot/dts/tegra124-nyan-blaze.dts | 15 +++++++++------
>  arch/arm/boot/dts/tegra124-venice2.dts    | 14 +++++++-------
>  drivers/gpu/drm/tegra/Kconfig             |  1 +
>  drivers/gpu/drm/tegra/dpaux.c             |  7 +++++++
>  5 files changed, 33 insertions(+), 19 deletions(-)
> 

It should "work" since you removed the ddc-i2c-bus phandle from the
panel nodes, and thus, panel->ddc won't be used during panel-edp driver
probe. But this looks like a hack rather than a fix.

I'm not sure why and how devm_of_dp_aux_populate_ep_devices() usage
should be relevant here. The drm_dp_aux_register() still should to
invoked before devm_of_dp_aux_populate_ep_devices(), otherwise
panel->ddc adapter won't be registered.

The panel->ddc isn't used by the new panel-edp driver unless panel is
compatible with "edp-panel". Hence the generic_edp_panel_probe() should
either fail or crash for a such "edp-panel" since panel->ddc isn't fully
instantiated, AFAICS.