Re: [PATCH v3 4/4] drm/bridge: parade-ps8640: Handle DP AUX more properly

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

On 10/05/2022 22:29, Douglas Anderson wrote:
> 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
> a1e3667a9835 ("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>

> ---
> 
> Changes in v3:
> - Adapt to v3 changes in aux bus.
> - Use devm_drm_bridge_add() to simplify.
> 
> Changes in v2:
> - Rewrote atop new method introduced by patch #1.
> 
>   drivers/gpu/drm/bridge/parade-ps8640.c | 74 ++++++++++++++++----------
>   1 file changed, 46 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/parade-ps8640.c b/drivers/gpu/drm/bridge/parade-ps8640.c
> index e2467e58b5b7..ff4227f6d800 100644
> --- a/drivers/gpu/drm/bridge/parade-ps8640.c
> +++ b/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,


-- 
With best wishes
Dmitry