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

[Leandro Ribeiro <leandro.ribeiro@collabora.com>]

On 4/27/21 6:00 AM, Daniel Vetter wrote:
> On Tue, Apr 27, 2021 at 10:40:24AM +0300, Pekka Paalanen wrote:
>> 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.
> 
> I think if we do this we should have a DOC: section in the drm_mode.h uapi
> header which explains how the index is computed, and then we reference
> that everywhere. Because otherwise there's going to be a _lot_ of
> duplication of this all over. Kernel-internally we solve this by just
> referencing drm_foo_index() family of functions, but for the uapi there's
> really nothing, so needs text.
> 
> -Daniel
>
Ok, I've sent a v3 with a small section to document how to get the index
of a CRTC object from userspace perspective. But I could only find two
comments that would benefit from it (at least in "Userland interfaces"
page).

Thanks!
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