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

[Pekka Paalanen <ppaalanen@gmail.com>]

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On Wed, 12 May 2021 09:50:14 -0300
Leandro Ribeiro <leandro.ribeiro@collabora.com> wrote:

> On 5/6/21 5:50 AM, Pekka Paalanen wrote:
> > On Wed, 28 Apr 2021 18:36:50 -0300
> > Leandro Ribeiro <leandro.ribeiro@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@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.
> >   
> 
> So what about:
> 
> CRTC's have both an object ID and an index, and they should not be
> confused. The index is used in cases where a densely packed identifier
> for a CRTC is needed, for instance in a bitmask of CRTC's. (maybe a link
> to the possible_crtcs field of struct drm_mode_get_plane? as example)
> 
> DRM_IOCTL_MODE_GETRESOURCES populates a structure with an array of CRTC
> id's, and the CRTC index is its position in this array.

Sure, sounds good.

Capitalized 'ID'?


Thanks,
pq

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