[PATCH v2] drm: document drm_mode_get_connector

[Daniel Vetter]

On Thu, Nov 19, 2020 at 2:52 PM Simon Ser <contact at 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 at emersion.fr>
> Cc: Daniel Vetter <daniel at ffwll.ch>
> Cc: Pekka Paalanen <ppaalanen at gmail.com>
> ---
>  include/uapi/drm/drm_mode.h | 76 ++++++++++++++++++++++++++++++++++---
>  1 file changed, 71 insertions(+), 5 deletions(-)
>
> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> index 5ad10ab2a577..cd97c5671c75 100644
> --- a/include/uapi/drm/drm_mode.h
> +++ b/include/uapi/drm/drm_mode.h
> @@ -368,27 +368,93 @@ 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.
> + *
> + * 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.
> + *
> + * **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.
> + *
> + * 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).

I think we should warn here that force probe can disrupt the UI, hence
why it should only be done on user request and not automatically.

> + */
>  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;
> +       /** @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;
>
> +       /**
> +        * @connection: Status of the connector.
> +        *
> +        * See enum drm_connector_status.

Please add & so it becomes a link in the generated docs (and pls check
the link goes to the right place).

> +        */
>         __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;
> +       /**
> +        * @subpixel: Subpixel order of the connected sink.
> +        *
> +        * See enum subpixel_order.

Same here.

> +        */
>         __u32 subpixel;
>
> +       /** @pad: Padding. */

I think this should have a "Must be zero"?

With the nits addressed: Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

Thanks a lot for doing this!
>         __u32 pad;
>  };
>
> --
> 2.29.2
>
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch