drm/bridge: parade-ps8640: Handle DP AUX more properly

While it works, for the most part, to assume that the panel has
finished probing when devm_of_dp_aux_populate_ep_devices() returns,
it's a bit fragile. This is talked about at length in commit
a1e3667a ("drm/bridge: ti-sn65dsi86: Promote the AUX channel to
its own sub-dev").

When reviewing the ps8640 code, I managed to convince myself that it
was OK not to worry about it there and that maybe it wasn't really
_that_ fragile. However, it turns out that it really is. Simply
hardcoding panel_edp_probe() to return -EPROBE_DEFER was enough to put
the boot process into an infinite loop. I believe this manages to trip
the same issues that we used to trip with the main MSM code where
something about our actions trigger Linux to re-probe previously
deferred devices right away and each time we try again we re-trigger
Linux to re-probe.

Let's fix this using the callback introduced in the patch ("drm/dp:
Callbacks to make it easier for drivers to use DP AUX bus properly").
When using the new callback, we have to be a little careful. The
probe_done() callback is no longer always called in the context of
our probe routine. That means we can't rely on being able to return
-EPROBE_DEFER from it. We re-jigger the order of things a bit to
account for that.

With this change, the device still boots (though obviously the panel
doesn't come up) if I force panel-edp to always return
-EPROBE_DEFER. If I fake it and make the panel probe exactly once it
also works.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20220510122726.v3.4.Ia6324ebc848cd40b4dbd3ad3289a7ffb5c197779@changeid

drivers/gpu/drm/bridge/parade-ps8640.c

@@ -537,7 +537,7 @@ static const struct drm_bridge_funcs ps8640_bridge_funcs = {
 	.pre_enable = ps8640_pre_enable,
 };
 
-static int ps8640_bridge_host_attach(struct device *dev, struct ps8640 *ps_bridge)
+static int ps8640_bridge_get_dsi_resources(struct device *dev, struct ps8640 *ps_bridge)
 {
 	struct device_node *in_ep, *dsi_node;
 	struct mipi_dsi_device *dsi;
@@ -576,13 +576,40 @@ static int ps8640_bridge_host_attach(struct device *dev, struct ps8640 *ps_bridg
 	dsi->format = MIPI_DSI_FMT_RGB888;
 	dsi->lanes = NUM_MIPI_LANES;
 
-	return devm_mipi_dsi_attach(dev, dsi);
+	return 0;
+}
+
+static int ps8640_bridge_link_panel(struct drm_dp_aux *aux)
+{
+	struct ps8640 *ps_bridge = aux_to_ps8640(aux);
+	struct device *dev = aux->dev;
+	struct device_node *np = dev->of_node;
+	int ret;
+
+	/*
+	 * NOTE about returning -EPROBE_DEFER from this function: if we
+	 * return an error (most relevant to -EPROBE_DEFER) it will only
+	 * be passed out to ps8640_probe() if it called this directly (AKA the
+	 * panel isn't under the "aux-bus" node). That should be fine because
+	 * if the panel is under "aux-bus" it's guaranteed to have probed by
+	 * the time this function has been called.
+	 */
+
+	/* port@1 is ps8640 output port */
+	ps_bridge->panel_bridge = devm_drm_of_get_bridge(dev, np, 1, 0);
+	if (IS_ERR(ps_bridge->panel_bridge))
+		return PTR_ERR(ps_bridge->panel_bridge);
+
+	ret = devm_drm_bridge_add(dev, &ps_bridge->bridge);
+	if (ret)
+		return ret;
+
+	return devm_mipi_dsi_attach(dev, ps_bridge->dsi);
 }
 
 static int ps8640_probe(struct i2c_client *client)
 {
 	struct device *dev = &client->dev;
-	struct device_node *np = dev->of_node;
 	struct ps8640 *ps_bridge;
 	int ret;
 	u32 i;
@@ -623,6 +650,14 @@ static int ps8640_probe(struct i2c_client *client)
 	if (!ps8640_of_panel_on_aux_bus(&client->dev))
 		ps_bridge->bridge.ops = DRM_BRIDGE_OP_EDID;
 
+	/*
+	 * Get MIPI DSI resources early. These can return -EPROBE_DEFER so
+	 * we want to get them out of the way sooner.
+	 */
+	ret = ps8640_bridge_get_dsi_resources(&client->dev, ps_bridge);
+	if (ret)
+		return ret;
+
 	ps_bridge->page[PAGE0_DP_CNTL] = client;
 
 	ps_bridge->regmap[PAGE0_DP_CNTL] = devm_regmap_init_i2c(client, ps8640_regmap_config);
@@ -665,35 +700,19 @@ static int ps8640_probe(struct i2c_client *client)
 	if (ret)
 		return ret;
 
-	devm_of_dp_aux_populate_ep_devices(&ps_bridge->aux);
+	ret = devm_of_dp_aux_populate_bus(&ps_bridge->aux, ps8640_bridge_link_panel);
 
-	/* port@1 is ps8640 output port */
-	ps_bridge->panel_bridge = devm_drm_of_get_bridge(dev, np, 1, 0);
-	if (IS_ERR(ps_bridge->panel_bridge))
-		return PTR_ERR(ps_bridge->panel_bridge);
-
-	drm_bridge_add(&ps_bridge->bridge);
-
-	ret = ps8640_bridge_host_attach(dev, ps_bridge);
-	if (ret)
-		goto err_bridge_remove;
-
-	return 0;
+	/*
+	 * If devm_of_dp_aux_populate_bus() returns -ENODEV then it's up to
+	 * usa to call ps8640_bridge_link_panel() directly. NOTE: in this case
+	 * the function is allowed to -EPROBE_DEFER.
+	 */
+	if (ret == -ENODEV)
+		return ps8640_bridge_link_panel(&ps_bridge->aux);
 
-err_bridge_remove:
-	drm_bridge_remove(&ps_bridge->bridge);
 	return ret;
 }
 
-static int ps8640_remove(struct i2c_client *client)
-{
-	struct ps8640 *ps_bridge = i2c_get_clientdata(client);
-
-	drm_bridge_remove(&ps_bridge->bridge);
-
-	return 0;
-}
-
 static const struct of_device_id ps8640_match[] = {
 	{ .compatible = "parade,ps8640" },
 	{ }
@@ -702,7 +721,6 @@ MODULE_DEVICE_TABLE(of, ps8640_match);
 
 static struct i2c_driver ps8640_driver = {
 	.probe_new = ps8640_probe,
-	.remove = ps8640_remove,
 	.driver = {
 		.name = "ps8640",
 		.of_match_table = ps8640_match,