gpu/drm/bridge/cadence: avoid flush_scheduled_work() usage

Like commit c4f135d6 ("workqueue: Wrap flush_workqueue() using a
macro") says, flush_scheduled_work() is dangerous and will be forbidden.
We are on the way for removing all flush_scheduled_work() callers from
the kernel, and this patch is for removing flush_scheduled_work() call
 from cadence driver.

Since cdns-mhdp8546 driver uses 4 works

  mhdp->modeset_retry_work
  mhdp->hpd_work
  mhdp->hdcp.check_work
  mhdp->hdcp.prop_work

I assume that flush_scheduled_work() in cdns_mhdp_remove() needs to wait
for only these 4 works.

Since mhdp->modeset_retry_work already uses cancel_work_sync(), I assume
that flush_scheduled_work() needs to wait for only 3 works. But I came to
wonder whether mhdp->hdcp.check_work should be flushed or cancelled.

While flush_scheduled_work() waits for completion of works which were
already queued to system_wq, mhdp->hdcp.check_work is a delayed work.
That is, this work won't be queued to system_wq unless timeout expires.

Current code will wait for mhdp->hdcp.check_work only if timeout already
expired. If timeout is not expired yet, flush_scheduled_work() will fail
to cancel mhdp->hdcp.check_work, and cdns_mhdp_hdcp_check_work() which is
triggered by mhdp->hdcp.check_work will schedule hdcp->check_work, which
is too late for flush_scheduled_work() to wait for completion of
cdns_mhdp_hdcp_prop_work().

But since I couldn't get comments on how do we want to handle this race
window [1], this patch chose "do nothing" for mhdp->hdcp.check_work and
mhdp->hdcp.prop_work. That is, I assume that flush_scheduled_work() in
cdns_mhdp_remove() needs to wait for only mhdp->hpd_work work.

Link: https://lkml.kernel.org/r/943273cb-c2ec-24e3-5edb-64eacc6e2d30@I-love.SAKURA.ne.jp [1]
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Robert Foss <robert.foss@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/216591bc-28bb-0453-10bb-59e268dff540@I-love.SAKURA.ne.jp

drivers/gpu/drm/bridge/cadence/cdns-mhdp8546-core.c

@@ -2605,7 +2605,8 @@ static int cdns_mhdp_remove(struct platform_device *pdev)
 	pm_runtime_disable(&pdev->dev);
 
 	cancel_work_sync(&mhdp->modeset_retry_work);
-	flush_scheduled_work();
+	flush_work(&mhdp->hpd_work);
+	/* Ignoring mhdp->hdcp.check_work and mhdp->hdcp.prop_work here. */
 
 	clk_disable_unprepare(mhdp->clk);