[PATCH] drm: of: Improve error handling in bridge/panel detection

[Thierry Reding]

On Thu, Apr 07, 2022 at 11:34:08AM +0200, Paul Kocialkowski wrote:
> With the previous rework of drm_of_find_panel_or_bridge only
> -EPROBE_DEFER is returned while previous behavior allowed -ENODEV
> to be returned when the port/endpoint is either missing or unavailable.
> 
> Make the default return code of the function -ENODEV to handle this and
> only return -EPROBE_DEFER in find_panel_or_bridge when the of device is
> available but not yet registered. Also return the error code whenever
> the remote node exists to avoid checking for child nodes.
> 
> Checking child nodes could result in -EPROBE_DEFER returned by
> find_panel_or_bridge with an unrelated child node that would overwrite
> a legitimate -ENODEV from find_panel_or_bridge if the remote node from
> the of graph is unavailable. This happens because find_panel_or_bridge
> has no way to distinguish between a legitimate panel/bridge node that
> isn't yet registered and an unrelated node.
> 
> Add comments around to clarify this behavior.
> 
> Signed-off-by: Paul Kocialkowski <paul.kocialkowski at bootlin.com>
> Fixes: 67bae5f28c89 ("drm: of: Properly try all possible cases for bridge/panel detection")
> Cc: Bjorn Andersson <bjorn.andersson at linaro.org>
> Cc: Thierry Reding <thierry.reding at gmail.com>
> Cc: Linus Walleij <linus.walleij at linaro.org>
> 
> ---
>  drivers/gpu/drm/drm_of.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)

This also fixes the regression that I was seeing on Tegra.

Tested-by: Thierry Reding <treding at nvidia.com>
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