Re: [PATCH v3] drm/mediatek: mtk_dsi: Avoid EPROBE_DEFER loop with external bridge

[AngeloGioacchino Del Regno]

Il 27/01/22 16:46, AngeloGioacchino Del Regno ha scritto:
> Il 27/01/22 16:21, Chun-Kuang Hu ha scritto:
> > Hi, Angelo:
> >
> > AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx> 於
> > 2022年1月27日 週四 下午10:36寫道：
> > > DRM bridge drivers are now attaching their DSI device at probe time,
> > > which requires us to register our DSI host in order to let the bridge
> > > to probe: this recently started producing an endless -EPROBE_DEFER
> > > loop on some machines that are using external bridges, like the
> > > parade-ps8640, found on the ACER Chromebook R13.
> > >
> > > Now that the DSI hosts/devices probe sequence is documented, we can
> > > do adjustments to the mtk_dsi driver as to both fix now and make sure
> > > to avoid this situation in the future: for this, following what is
> > > documented in drm_bridge.c, move the mtk_dsi component_add() to the
> > > mtk_dsi_ops.attach callback and delete it in the detach callback;
> > > keeping in mind that we are registering a drm_bridge for our DSI,
> > > which is only used/attached if the DSI Host is bound, it wouldn't
> > > make sense to keep adding our bridge at probe time (as it would
> > > be useless to have it if mtk_dsi_ops.attach() fails!), so also move
> > > that one to the dsi host attach function (and remove it in detach).
> > >
> > > Fixes: 209264a85707 ("drm/bridge: Document the probe issue with MIPI-DSI bridges")
> >
> > The fixed tag should indicate the patch which cause the bug, but why a
> > patch just adding document would cause bug?
> > So no any patch cause bug? This patch just want to prevent a possible bug?
>
> I think you've missed my previous message on v2, so I will paste it here:
>
> unfortunately I couldn't find a valid commit for a Fixes tag. This
> started being an issue at some point, when the DRM was changed to
> adhere to the documented probe sequence: the MediaTek DSI driver was
> not the only one that got broken/affected by these changes.
>
> If you have any advice on which commit should be tagged, I'm open for
> any kind of suggestion.
>
>
> I tried to check on other drivers which got fixed for the same behavior,
> for example drm/msm, but none of them had a Fixes tag.
> When the DRM got changed to adhere to this sequence, some drm/bridge
> drivers were also changed; this has created some incompatibilities with
> some drm drivers, including drm/msm and drm/mediatek.
>
> This commit is not fixing a latent bug that was introduced in drm/mediatek
> but rather one that was induced by the new, fixed, probe flow that got
> recently documented - and to which drivers should adhere; failing to adhere
> to that will produce an endless -EPROBE_DEFER loop, due to other drivers
> (as mentioned, for example drm/bridge drivers) having been changed to use
> that probe sequence.
>
>
> Regards,
> Angelo

I've been thinking about another solution to this issue.

Would it be fine if I send a v4 that removes the Fixes tag, but adds the following?

Cc: stable@xxxxxxxxxx # v5.15+

> > Regards,
> > Chun-Kuang.
> >
> > > Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@xxxxxxxxxxxxx>
> > > Reviewed-by: Andrzej Hajda <andrzej.hajda@xxxxxxxxx>
> > > Reviewed-by: Jagan Teki <jagan@xxxxxxxxxxxxxxxxxxxx>
> > >
> > > ---
> > >   drivers/gpu/drm/mediatek/mtk_dsi.c | 167 +++++++++++++++--------------
> > >   1 file changed, 84 insertions(+), 83 deletions(-)


--
AngeloGioacchino Del Regno
Software Engineer

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