Re: [PATCH v9 1/4] drm: of: Add drm_of_get_dsi_bus helper function

[Maxime Ripard <maxime@cerno.tech>]

Hi,

On Wed, Jan 11, 2023 at 10:30:09AM -0600, Chris Morgan wrote:
> From: Chris Morgan <macromorgan@hotmail.com>
> 
> Add helper function to find DSI host for devices where DSI panel is not
> a minor of a DSI bus (such as the Samsung AMS495QA01 panel or the
> official Raspberry Pi touchscreen display).
> 
> Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
> Signed-off-by: Maya Matuszczyk <maccraft123mc@gmail.com>
> ---
>  drivers/gpu/drm/drm_of.c | 70 ++++++++++++++++++++++++++++++++++++++++
>  include/drm/drm_of.h     | 10 ++++++
>  2 files changed, 80 insertions(+)
> 
> diff --git a/drivers/gpu/drm/drm_of.c b/drivers/gpu/drm/drm_of.c
> index 7bbcb999bb75..4ebb5bc4b595 100644
> --- a/drivers/gpu/drm/drm_of.c
> +++ b/drivers/gpu/drm/drm_of.c
> @@ -10,6 +10,7 @@
>  #include <drm/drm_crtc.h>
>  #include <drm/drm_device.h>
>  #include <drm/drm_encoder.h>
> +#include <drm/drm_mipi_dsi.h>
>  #include <drm/drm_of.h>
>  #include <drm/drm_panel.h>
>  
> @@ -493,3 +494,72 @@ int drm_of_get_data_lanes_count_ep(const struct device_node *port,
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(drm_of_get_data_lanes_count_ep);
> +
> +/**
> + * drm_of_get_dsi_bus - find the DSI bus for a given device
> + * @dev: parent device of display (SPI, I2C)
> + * @info: DSI device info to be updated with correct DSI node
> + *
> + * Gets parent DSI bus for a DSI device controlled through a bus other
> + * than MIPI-DCS (SPI, I2C, etc.) using the Device Tree.
> + *
> + * Returns pointer to mipi_dsi_host if successful, -EINVAL if the
> + * request is unsupported, -EPROBE_DEFER if the DSI host is found but
> + * not available, or -ENODEV otherwise.
> + */
> +struct mipi_dsi_host *drm_of_get_dsi_bus(struct device *dev,
> +					 struct mipi_dsi_device_info *info)
> +{
> +	struct mipi_dsi_host *dsi_host;
> +	struct device_node *endpoint, *dsi_host_node;
> +
> +	/*
> +	 * Exit immediately if we attempt to call this function when
> +	 * DRM_MIPI_DSI is not enabled, in the event CONFIG_OF is
> +	 * enabled.
> +	 */
> +	if (!IS_ENABLED(CONFIG_DRM_MIPI_DSI))
> +		return ERR_PTR(-EINVAL);

The commit log isn't super clear on why this is needed, but it would be
more consistent to add an ifdef and only compile the entire function if
DRM_MIPI_DSI is there, just like you did for OF already.

> +	/*
> +	 * Get first endpoint child from device.
> +	 */
> +	endpoint = of_graph_get_next_endpoint(dev->of_node, NULL);
> +	if (!endpoint)
> +		return ERR_PTR(-ENODEV);
> +
> +	/*
> +	 * Follow the first endpoint to get the DSI host node.
> +	 */
> +	dsi_host_node = of_graph_get_remote_port_parent(endpoint);

There's no need to hold the reference to endpoint after that call. call
of_node_put(endpoint) here, and it will simplify the error path.

> +	if (!dsi_host_node)
> +		goto error;
> +
> +	/*
> +	 * Get the DSI host from the DSI host node. If we get an error
> +	 * or the return is null assume we're not ready to probe just
> +	 * yet. Release the DSI host node since we're done with it.
> +	 */
> +	dsi_host = of_find_mipi_dsi_host_by_node(dsi_host_node);
> +	of_node_put(dsi_host_node);
> +	if (IS_ERR_OR_NULL(dsi_host)) {
> +		of_node_put(endpoint);
> +		return ERR_PTR(-EPROBE_DEFER);
> +	}
> +
> +	/*
> +	 * Set the node of the mipi_dsi_device_info to the correct node
> +	 * and then release the endpoint node since we're done with it.
> +	 */
> +	info->node = of_graph_get_remote_port(endpoint);

Ah, you're using it there.

I think I'd rework the function to:

- retrieve the endpoint
- retrieve the remote port, give up the endpoint
- retrieve the remote port parent

Also, I'm not entirely sure what you had in mind, but info might not be
there at all and it would be fine imho.

Maxime