[PATCH 1/2] drm/doc: document how userspace should find out CRTC index

[Pekka Paalanen]

On Wed, 28 Apr 2021 18:36:50 -0300
Leandro Ribeiro <leandro.ribeiro at collabora.com> wrote:

> In this patch we add a section to document what userspace should do to
> find out the CRTC index. This is important as there are multiple places
> in the documentation that need this, so it's better to just point to
> this section and avoid repetition.
> 
> Signed-off-by: Leandro Ribeiro <leandro.ribeiro at collabora.com>
> ---
>  Documentation/gpu/drm-uapi.rst    | 14 ++++++++++++++
>  drivers/gpu/drm/drm_debugfs_crc.c |  9 +++++----
>  include/uapi/drm/drm.h            |  3 ++-
>  3 files changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/Documentation/gpu/drm-uapi.rst b/Documentation/gpu/drm-uapi.rst
> index 04bdc7a91d53..1aa52a6ac567 100644
> --- a/Documentation/gpu/drm-uapi.rst
> +++ b/Documentation/gpu/drm-uapi.rst
> @@ -457,6 +457,20 @@ Userspace API Structures
>  .. kernel-doc:: include/uapi/drm/drm_mode.h
>     :doc: overview
> 
> +.. _crtc_index:
> +
> +CRTC index
> +----------
> +
> +In some situations, it is important for userspace to find out the index of a

That could be said about everything, so this sentence has no
information value. Instead, you could start by stating that CRTCs have
both an object ID and an index, and they are not the same thing. CRTC
index is used in cases where a densely packed identifier for a CRTC is
needed, e.g. in bit-for-crtc masks, where using the object ID would not
work.

> +CRTC. The CRTC index should not be confused with its object id.
> +
> +In order to do this, userspace should first query the resources object

Instead of saying what userspace must do, you could just explain where
it can be observed.

> +from the device that owns the CRTC (using the DRM_IOCTL_MODE_GETRESOURCES

So here you might start with: DRM_IOCTL_MODE_GETRESOURCES populates a
structure with an array of CRTC IDs. CRTC's index is its index in that
array.

> +ioctl). The resources object contains a pointer to an array of CRTC's, and also
> +the number of entries of the array. The index of the CRTC is the same as its
> +position in this array.

Anyway, the idea here is right.

> +
>  .. kernel-doc:: include/uapi/drm/drm.h
>     :internal:
> 
> diff --git a/drivers/gpu/drm/drm_debugfs_crc.c b/drivers/gpu/drm/drm_debugfs_crc.c
> index 3dd70d813f69..9575188d97ee 100644
> --- a/drivers/gpu/drm/drm_debugfs_crc.c
> +++ b/drivers/gpu/drm/drm_debugfs_crc.c
> @@ -46,10 +46,11 @@
>   * it reached a given hardware component (a CRC sampling "source").
>   *
>   * Userspace can control generation of CRCs in a given CRTC by writing to the
> - * file dri/0/crtc-N/crc/control in debugfs, with N being the index of the CRTC.
> - * Accepted values are source names (which are driver-specific) and the "auto"
> - * keyword, which will let the driver select a default source of frame CRCs
> - * for this CRTC.
> + * file dri/0/crtc-N/crc/control in debugfs, with N being the index of the
> + * CRTC. To learn how to find out the index of a certain CRTC, please see
> + * :ref:`crtc_index`. Accepted values are source names (which are

This a bit verbose: "To learn..." It could be more concise, like
making the words "the index" a hyperlink instead of adding a whole
sentence.

> + * driver-specific) and the "auto" keyword, which will let the driver select a
> + * default source of frame CRCs for this CRTC.
>   *
>   * Once frame CRC generation is enabled, userspace can capture them by reading
>   * the dri/0/crtc-N/crc/data file. Each line in that file contains the frame
> diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h
> index 67b94bc3c885..6944f08ab1a6 100644
> --- a/include/uapi/drm/drm.h
> +++ b/include/uapi/drm/drm.h
> @@ -636,7 +636,8 @@ struct drm_gem_open {
>   * DRM_CAP_VBLANK_HIGH_CRTC
>   *
>   * If set to 1, the kernel supports specifying a CRTC index in the high bits of
> - * &drm_wait_vblank_request.type.
> + * &drm_wait_vblank_request.type. To learn how to find out the index of a
> + * certain CRTC, please see :ref:`crtc_index`.

The same here with "a CRTC index" turned into a hyperlink.


Thanks,
pq

>   *
>   * Starting kernel version 2.6.39, this capability is always set to 1.
>   */
> --
> 2.31.1
> 
> _______________________________________________
> dri-devel mailing list
> dri-devel at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

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