On 01/02/2023 00:18, Douglas Anderson wrote:
In commit 7d8e9a90509f ("drm/msm/dsi: move DSI host powerup to modeset
time"), we moved powering up DSI hosts to modeset time. This wasn't
because it was an elegant design, but there were no better options.
That commit actually ended up breaking ps8640, and thus was born
commit ec7981e6c614 ("drm/msm/dsi: don't powerup at modeset time for
parade-ps8640") as a temporary hack to un-break ps8640 by moving it to
the old way of doing things. It turns out that ps8640 _really_ doesn't
like its pre_enable() function to be called after
dsi_mgr_bridge_power_on(). Specifically (from experimentation, not
because I have any inside knowledge), it looks like the assertion of
"RST#" in the ps8640 runtime resume handler seems like it's not
allowed to happen after dsi_mgr_bridge_power_on()
Recently, Dave Stevenson's series landed allowing bridges some control
over pre_enable ordering. The meaty commit for our purposes is commit
4fb912e5e190 ("drm/bridge: Introduce pre_enable_prev_first to alter
bridge init order"). As documented by that series, if a bridge doesn't
set "pre_enable_prev_first" then we should use the old ordering.
Now that we have the commit ("drm/bridge: tc358762: Set
pre_enable_prev_first") we can go back to the old ordering, which also
allows us to remove the ps8640 special case.
One last note is that even without reverting commit 7d8e9a90509f
("drm/msm/dsi: move DSI host powerup to modeset time"), if you _just_
revert the ps8640 special case and try it out then it doesn't seem to
fail anymore. I spent time bisecting / debugging this and it turns out
to be mostly luck, so we still want this patch to make sure it's
solid. Specifically the reason it sorta works these days is because
we implemented wait_hpd_asserted() in ps8640 now, plus the magic of
"pm_runtime" autosuspend. The fact that we have wait_hpd_asserted()
implemented means that we actually power the bridge chip up just a wee
bit earlier and then the bridge happens to stay on because of
autosuspend and thus ends up powered before dsi_mgr_bridge_power_on().
Cc: Dave Stevenson <dave.stevenson@xxxxxxxxxxxxxxx>
Cc: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
Cc: Abhinav Kumar <quic_abhinavk@xxxxxxxxxxx>
Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
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Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
--
With best wishes
Dmitry
