Re: [PATCH v2 1/1] drm/doc: document drm_mode_get_plane

From: Pekka Paalanen <ppaalanen@gmail.com>
To: Leandro Ribeiro <leandro.ribeiro@collabora.com>
Cc: airlied@linux.ie, kernel@collabora.com, dri-devel@lists.freedesktop.org
Subject: Re: [PATCH v2 1/1] drm/doc: document drm_mode_get_plane
Date: Tue, 27 Apr 2021 10:40:24 +0300	[thread overview]
Message-ID: <20210427104024.4e8aa086@eldfell> (raw)
In-Reply-To: <550b87d0-d180-32cb-349e-1ff000c0530f@collabora.com>

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On Mon, 26 Apr 2021 14:30:53 -0300
Leandro Ribeiro <leandro.ribeiro@collabora.com> wrote:

> On 4/26/21 7:58 AM, Simon Ser wrote:
> > On Monday, April 26th, 2021 at 9:36 AM, Pekka Paalanen <ppaalanen@gmail.com> wrote:
> >   
> >>>> This should probably explain what the bits in the mask correspond to.
> >>>> As in, which CRTC does bit 0 refer to, and so on.  
> >>>
> >>> What about:
> >>>
> >>> "possible_crtcs: Bitmask of CRTC's compatible with the plane. CRTC's are
> >>> created and they receive an index, which corresponds to their position
> >>> in the bitmask. CRTC with index 0 will be in bit 0, and so on."  
> >>
> >> This would still need to explain where can I find this index.  
> >   
> 
> What do you mean?
> 
> > This closed merge request had some docs about possible CRTCs:
> > 
> > https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/102
> >   
> I'm afraid I don't know exactly what you expect to be documented here
> that is still missing. Could you please elaborate?
> 
> Thanks a lot!

The documentation you add is talking about "CRTC index". What defines a
CRTC object's index? How do I determine what index a CRTC object has?

The answer is, AFAIK, that the index is never stored explicitly
anywhere. You have to get the DRM resources structure, which has an
array for CRTC IDs. The index is the index to that array, IIRC. So if
one does not already know this, it is going to be really hard to figure
out what the "index" is. It might even be confused with the object ID,
which it is not but the ID might by complete accident be less than 32
so it would look ok at first glance.

If the index is already explained somewhere else, a reference to that
documentation would be enough.

Thanks,
pq

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