Re: [RFT PATCH v2 2/3] drm/msm/dsi: Stop unconditionally powering up DSI hosts at modeset

[Dmitry Baryshkov <dmitry.baryshkov@linaro.org>]

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@raspberrypi.com>
> Cc: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
> Cc: Abhinav Kumar <quic_abhinavk@quicinc.com>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>
> ---

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>

-- 
With best wishes
Dmitry