drm/dp: Clarify that wait_hpd_asserted() is not optional for panels

In response to my patch removing the "wait for HPD" logic at the
beginning of the MSM DP transfer() callback [1], we had some debate
about what the "This is an optional function" meant in the
documentation of the wait_hpd_asserted() callback. Let's clarify.

As talked about in the MSM DP patch [1], before wait_hpd_asserted()
was introduced there was no great way for panel drivers to wait for
HPD in the case that the "built-in" HPD signal was used. Panel drivers
could only wait for HPD if a GPIO was used. At the time, we ended up
just saying that if we were using the "built-in" HPD signal that DP
AUX controllers needed to wait for HPD themselves at the beginning of
their transfer() callback. The fact that the wait for HPD at the
beginning of transfer() was awkward/problematic was the whole reason
wait_hpd_asserted() was added.

Let's make it obvious that if a DP AUX controller implements
wait_hpd_asserted() that they don't need a loop waiting for HPD at the
start of their transfer() function. We'll still allow DP controllers
to work the old way but mark it as deprecated.

[1] https://lore.kernel.org/r/20240315143621.v2.3.I535606f6d4f7e3e5588bb75c55996f61980183cd@changeidReviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240319135836.v2.1.I521dad0693cc24fe4dd14cba0c7048d94f5b6b41@changeid

include/drm/display/drm_dp_helper.h

@@ -422,7 +422,18 @@ struct drm_dp_aux {
 	 * @wait_hpd_asserted: wait for HPD to be asserted
 	 *
 	 * This is mainly useful for eDP panels drivers to wait for an eDP
-	 * panel to finish powering on. This is an optional function.
+	 * panel to finish powering on. It is optional for DP AUX controllers
+	 * to implement this function. It is required for DP AUX endpoints
+	 * (panel drivers) to call this function after powering up but before
+	 * doing AUX transfers unless the DP AUX endpoint driver knows that
+	 * we're not using the AUX controller's HPD. One example of the panel
+	 * driver not needing to call this is if HPD is hooked up to a GPIO
+	 * that the panel driver can read directly.
+	 *
+	 * If a DP AUX controller does not implement this function then it
+	 * may still support eDP panels that use the AUX controller's built-in
+	 * HPD signal by implementing a long wait for HPD in the transfer()
+	 * callback, though this is deprecated.
 	 *
 	 * This function will efficiently wait for the HPD signal to be
 	 * asserted. The `wait_us` parameter that is passed in says that we