Re: [PATCH v6 11/26] drm/bridge: samsung-dsim: use devm_drm_of_get_bridge[_by_node] to find the out_bridge

[Dmitry Baryshkov]

On Thu, Feb 06, 2025 at 07:14:26PM +0100, Luca Ceresoli wrote:
> In order to support panels described either via graph links or via a
> subnode (e.g. "panel@0"), this driver uses low-level deprecated functions
> to find the next bridge. The resulting logic is complex and duplicates code
> already present in the DRM bridge core. Switch to the new APIs in DRM
> bridge core that allow to do the same in a much cleaner way.
> 
> Note there are two slight changes in the new logic intended to improve the
> final result:
> 
>  * the old code looks for a subnode with any name except "port" or "ports",
>    while the new code uses the node passed as a parameter
> 
>  * the old code looks for a subnode first and falls back to a graph link,
>    while the new code uses the reverse order because graph links are the
>    recommended device tree representation now
> 
> The first change makes the code more robust by avoiding the risk of using
> an unrelated node which is not describing a panel and not names "port" or
> "ports".
> 
> The second change is not expected to expose regressions because, in the
> cases where both a subnode and a graph link are used to describe a panel,
> the graph link should point to the subnode itself, such as in
> arch/arm64/boot/dts/qcom/sdm850-lenovo-yoga-c630.dts
> 
> As a further cleanup, use a temporary variable to assign dsi->out_bridge
> only on success. This avoids the risk of leaving a non-NULL value in
> dsi->out_bridge when samsung_dsim_host_attach() fails.
> 
> Suggested-by: Dmitry Baryshkov <dmitry.baryshkov@xxxxxxxxxx>
> Signed-off-by: Luca Ceresoli <luca.ceresoli@xxxxxxxxxxx>
> 
> ---
> 
> This patch was added in v6.
> ---
>  drivers/gpu/drm/bridge/samsung-dsim.c | 55 ++++++-----------------------------
>  1 file changed, 9 insertions(+), 46 deletions(-)
> 
> diff --git a/drivers/gpu/drm/bridge/samsung-dsim.c b/drivers/gpu/drm/bridge/samsung-dsim.c
> index f8b4fb8357659018ec0db65374ee5d05330639ae..bbd0a4f5a3f52b61bf48f10d6e8ca741bffa5e46 100644
> --- a/drivers/gpu/drm/bridge/samsung-dsim.c
> +++ b/drivers/gpu/drm/bridge/samsung-dsim.c
> @@ -1704,55 +1704,16 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
>  	const struct samsung_dsim_plat_data *pdata = dsi->plat_data;
>  	struct device *dev = dsi->dev;
>  	struct device_node *np = dev->of_node;
> -	struct device_node *remote;
> -	struct drm_panel *panel;
> +	struct drm_bridge *out_bridge;
>  	int ret;
>  
> -	/*
> -	 * Devices can also be child nodes when we also control that device
> -	 * through the upstream device (ie, MIPI-DCS for a MIPI-DSI device).
> -	 *
> -	 * Lookup for a child node of the given parent that isn't either port
> -	 * or ports.
> -	 */

Please leave the comment in place (maybe rewrite it slightly).

> -	for_each_available_child_of_node(np, remote) {
> -		if (of_node_name_eq(remote, "port") ||
> -		    of_node_name_eq(remote, "ports"))
> -			continue;
> +	out_bridge = devm_drm_of_get_bridge(dev, np, 1, 0);
> +	if (IS_ERR(out_bridge) && PTR_ERR(out_bridge) != -EPROBE_DEFER)

Can it actually return EPROBE_DEFER?

> +		out_bridge = devm_drm_of_get_bridge_by_node(dev, device->dev.of_node);
>  
> -		goto of_find_panel_or_bridge;
> -	}
> -
> -	/*
> -	 * of_graph_get_remote_node() produces a noisy error message if port
> -	 * node isn't found and the absence of the port is a legit case here,
> -	 * so at first we silently check whether graph presents in the
> -	 * device-tree node.
> -	 */
> -	if (!of_graph_is_present(np))
> -		return -ENODEV;
> -
> -	remote = of_graph_get_remote_node(np, 1, 0);
> -
> -of_find_panel_or_bridge:
> -	if (!remote)
> -		return -ENODEV;
> -
> -	panel = of_drm_find_panel(remote);
> -	if (!IS_ERR(panel)) {
> -		dsi->out_bridge = devm_drm_panel_bridge_add(dev, panel);
> -	} else {
> -		dsi->out_bridge = of_drm_find_bridge(remote);
> -		if (!dsi->out_bridge)
> -			dsi->out_bridge = ERR_PTR(-EINVAL);
> -	}
> -
> -	of_node_put(remote);
> -
> -	if (IS_ERR(dsi->out_bridge)) {
> -		ret = PTR_ERR(dsi->out_bridge);
> -		DRM_DEV_ERROR(dev, "failed to find the bridge: %d\n", ret);
> -		return ret;
> +	if (IS_ERR(out_bridge)) {
> +		DRM_DEV_ERROR(dev, "failed to find the bridge: %ld\n", PTR_ERR(out_bridge));
> +		return PTR_ERR(out_bridge);
>  	}
>  
>  	DRM_DEV_INFO(dev, "Attached %s device (lanes:%d bpp:%d mode-flags:0x%lx)\n",
> @@ -1784,6 +1745,8 @@ static int samsung_dsim_host_attach(struct mipi_dsi_host *host,
>  	dsi->format = device->format;
>  	dsi->mode_flags = device->mode_flags;
>  
> +	dsi->out_bridge = out_bridge;
> +

Please move the assignment closer to the original place.

>  	return 0;
>  }
>  
> 
> -- 
> 2.34.1
> 

-- 
With best wishes
Dmitry