Re: [PATCH v3] drm: document drm_mode_get_connector

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Fri, 20 Nov 2020 08:57:33 +0000
Simon Ser <contact@emersion.fr> wrote:

> Document how to perform a GETCONNECTOR ioctl. Document the various
> struct fields. Also document how to perform a forced probe, and when
> should user-space do it.
> 
> Signed-off-by: Simon Ser <contact@emersion.fr>
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Pekka Paalanen <ppaalanen@gmail.com>
> ---
>  include/uapi/drm/drm_mode.h | 78 ++++++++++++++++++++++++++++++++++---
>  1 file changed, 73 insertions(+), 5 deletions(-)
> 
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index f29c1d37be67..3979389fcc4f 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -368,27 +368,95 @@ enum drm_mode_subconnector {
>  #define DRM_MODE_CONNECTOR_WRITEBACK	18
>  #define DRM_MODE_CONNECTOR_SPI		19
>  
> +/**
> + * struct drm_mode_get_connector - Get connector metadata.
> + *
> + * User-space can perform a GETCONNECTOR ioctl to retrieve information about a
> + * connector. User-space is expected to retrieve encoders, modes and properties
> + * by performing this ioctl at least twice: the first time to retrieve the
> + * number of elements, the second time to retrieve the elements themselves.
> + *
> + * To retrieve the number of elements, set @count_props and @count_encoders to
> + * zero, set @count_modes to 1, and set @modes_ptr to a temporary struct
> + * drm_mode_modeinfo element.

How are the counts actually returned?

> + *
> + * To retrieve the elements, allocate arrays for @encoders_ptr, @modes_ptr,
> + * @props_ptr and @prop_values_ptr, then set @count_modes, @count_props and
> + * @count_encoders to their capacity.
> + *
> + * Performing the ioctl only twice may be racy: the number of elements may have
> + * changed with a hotplug event in-between the two ioctls. User-space is
> + * expected to retry the last ioctl until the number of elements stabilizes.
> + * The kernel won't fill any array which doesn't have the expected length.

How does userspace realize the kernel didn't fill in the arrays?

> + *
> + * **Force-probing a connector**
> + *
> + * If the @count_modes field is set to zero, the kernel will perform a forced
> + * probe on the connector to refresh the connector status, modes and EDID.
> + * A forced-probe can be slow and the ioctl will block. A force-probe can cause
> + * flickering and temporary freezes, so it should not be performed
> + * automatically.
> + *
> + * User-space shouldn't need to force-probe connectors in general: the kernel
> + * will automatically take care of probing connectors that don't support
> + * hot-plug detection when appropriate. However, user-space may force-probe
> + * connectors on user request (e.g. clicking a "Scan connectors" button, or
> + * opening a UI to manage screens).

This is well written.

> + */
>  struct drm_mode_get_connector {
> -
> +	/** @encoders_ptr: Pointer to ``__u32`` array of object IDs. */
>  	__u64 encoders_ptr;
> +	/** @modes_ptr: Pointer to struct drm_mode_modeinfo array. */
>  	__u64 modes_ptr;
> +	/** @props_ptr: Pointer to ``__u32`` array of property IDs. */
>  	__u64 props_ptr;
> +	/** @prop_values_ptr: Pointer to ``__u64`` array of property values. */
>  	__u64 prop_values_ptr;
>  
> +	/** @count_modes: Number of modes. */
>  	__u32 count_modes;
> +	/** @count_props: Number of properties. */
>  	__u32 count_props;
> +	/** @count_encoders: Number of encoders. */
>  	__u32 count_encoders;
>  
> -	__u32 encoder_id; /**< Current Encoder */
> -	__u32 connector_id; /**< Id */
> +	/** @encoder_id: Object ID of the current encoder. */
> +	__u32 encoder_id;

This is an out value, not an in value, right?
It's not immediately obvious whether any members here are in, out or
in/out values.

> +	/** @connector_id: Object ID of the connector. */
> +	__u32 connector_id;
> +	/**
> +	 * @connector_type: Type of the connector.
> +	 *
> +	 * See DRM_MODE_CONNECTOR_* defines.
> +	 */
>  	__u32 connector_type;
> +	/**
> +	 * @connector_type_id: Type-specific connector number.
> +	 *
> +	 * This is not an object ID. This is a per-type connector number. Each
> +	 * (type, type_id) combination is unique across all connectors of a DRM
> +	 * device.
> +	 */
>  	__u32 connector_type_id;

Naming facepalm, oh well...

>  
> +	/**
> +	 * @connection: Status of the connector.
> +	 *
> +	 * See enum drm_connector_status.
> +	 */
>  	__u32 connection;
> -	__u32 mm_width;  /**< width in millimeters */
> -	__u32 mm_height; /**< height in millimeters */
> +	/** @mm_width: Width of the connected sink in millimeters. */
> +	__u32 mm_width;
> +	/** @mm_height: Height of the connected sink in millimeters. */
> +	__u32 mm_height;

These are actually more complicated than this, aren't they?

They could be zero for unknown, or both smaller than 20 (???) to
signify only aspect ratio? I've no idea, I just remember something
funny like that from EDID, do these have the same oddities as EDID?

> +	/**
> +	 * @subpixel: Subpixel order of the connected sink.
> +	 *
> +	 * See enum subpixel_order.
> +	 */
>  	__u32 subpixel;
>  
> +	/** @pad: Padding, must be zero. */
>  	__u32 pad;
>  };
>  

Even with these questions open, this is already a huge improvement!


Thanks,
pq

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