[PATCH 2/2] drm/doc: document drm_mode_get_plane

[Pekka Paalanen]

On Wed, 19 May 2021 10:30:40 -0300
Leandro Ribeiro <leandro.ribeiro at collabora.com> wrote:

> On 5/6/21 6:10 AM, Pekka Paalanen wrote:
> > On Wed, 28 Apr 2021 18:36:51 -0300
> > Leandro Ribeiro <leandro.ribeiro at collabora.com> wrote:
> >   
> >> Add a small description and document struct fields of
> >> drm_mode_get_plane.
> >>
> >> Signed-off-by: Leandro Ribeiro <leandro.ribeiro at collabora.com>  
> > 
> > Hi,
> > 
> > thanks a lot for revising these.
> >   
> >> ---
> >>  include/uapi/drm/drm_mode.h | 22 ++++++++++++++++++++++
> >>  1 file changed, 22 insertions(+)
> >>
> >> diff --git a/include/uapi/drm/drm_mode.h b/include/uapi/drm/drm_mode.h
> >> index a5e76aa06ad5..8fa6495cd948 100644
> >> --- a/include/uapi/drm/drm_mode.h
> >> +++ b/include/uapi/drm/drm_mode.h
> >> @@ -312,16 +312,38 @@ struct drm_mode_set_plane {
> >>  	__u32 src_w;
> >>  };
> >>
> >> +/**
> >> + * struct drm_mode_get_plane - Get plane metadata.
> >> + *
> >> + * Userspace can perform a GETPLANE ioctl to retrieve information about a
> >> + * plane.
> >> + */
> >>  struct drm_mode_get_plane {
> >> +	/** @plane_id: Object ID of the plane. */  
> > 
> > This is an "in" field, right?
> > 
> > "in" meaning that userspace sets it to the ID of the plane it wants
> > information on.
> > 
> > "out" field is a field written by the kernel as a response.
> > 
> > I'm not sure if the kernel has a habit of documenting these, because we
> > use libdrm to abstract this so users do not need to care, but I think
> > it would be nice.
> >   
> 
> In a quick look, I couldn't find anything. But I can change the phrasing
> to the following:
> 
> "@plane_id: Object ID of the plane whose information should be
> retrieved. IN field, set by userspace."

I think "set by userspace" or "set by caller" would be enough.


> >>  	__u32 plane_id;
> >>
> >> +	/** @crtc_id: Object ID of the current CRTC. */
> >>  	__u32 crtc_id;
> >> +	/** @fb_id: Object ID of the current fb. */
> >>  	__u32 fb_id;
> >>
> >> +	/**
> >> +	 * @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. To learn how to find out the index of a certain CRTC, please see
> >> +	 * :ref:`crtc_index`.  
> > 
> > This could be shortened to something like bit N corresponds to CRTC
> > index N, and make "CRTC index N" a hyperlink.
> >   
> 
> Nice, I'll apply this change.
> 
> >> +	 */
> >>  	__u32 possible_crtcs;
> >> +	/** @gamma_size: Number of entries of the legacy gamma lookup table. */
> >>  	__u32 gamma_size;
> >>
> >> +	/** @count_format_types: Number of formats. */
> >>  	__u32 count_format_types;
> >> +	/**
> >> +	 * @format_type_ptr: Pointer to ``__u32`` array of formats that are
> >> +	 * supported by the plane. These formats do not require modifiers.
> >> +	 */
> >>  	__u64 format_type_ptr;  
> > 
> > The count/ptr fields have an interesting usage pattern, which I suppose
> > is common for all DRM ioctls. Makes me wonder if it should be documented.
> > 
> > AFAIU, count is in+out field: when set to 0, the kernel uses it to
> > return the count needed. Then userspace allocates space and calls the
> > ioctl again with the right count and ptr set to point to the allocated
> > array of count elements. This is so that kernel never allocates memory
> > on behalf of userspace for the return data, making things much simpler
> > at the cost of maybe needing to call the ioctl twice to first figure
> > out long the array should be.
> > 
> > This can be seen in libdrm code for drmModeGetPlane().
> >
> > There is certainly no point in explaining all that here, that is too
> > much. But if there was a way to annotate the count member as in+out,
> > that would be nice. And the ptr member as "in".
> >   
> 
> This is documented in the description of struct drm_mode_get_connector:
> 
> https://www.kernel.org/doc/html/latest/gpu/drm-uapi.html#c.drm_mode_get_connector
> 
> Would be enough to have something similar in struct drm_mode_get_plane?

That would be really nice!


Thanks,
pq
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