[PATCH] drm: document drm_property_enum.value for bitfields

[PATCH] drm: document drm_property_enum.value for bitfields

[Simon Ser]

When a property has the type DRM_MODE_PROP_BITMASK, the value field
stores a bitshift, not a bitmask, which can be surprising.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Leandro Ribeiro <leandro.ribeiro@collabora.com>
---
 include/drm/drm_property.h | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/include/drm/drm_property.h b/include/drm/drm_property.h
index bbf5c1fdd7b0..f3ea7f97f372 100644
--- a/include/drm/drm_property.h
+++ b/include/drm/drm_property.h
@@ -37,6 +37,11 @@
  *
  * For enumeration and bitmask properties this structure stores the symbolic
  * decoding for each value. This is used for example for the rotation property.
+ *
+ * If the property has the type &DRM_MODE_PROP_BITMASK, @value stores a
+ * bitshift, not a bitmask. In other words, the enum entry is enabled if the
+ * bit number @value is set in the property's value. This enum entry has the
+ * bitmask ``1 << value``.
  */
 struct drm_property_enum {
 	uint64_t value;
-- 
2.32.0

Re: [PATCH] drm: document drm_property_enum.value for bitfields

[Daniel Vetter]

On Wed, Jul 21, 2021 at 06:51:30AM +0000, Simon Ser wrote:
> When a property has the type DRM_MODE_PROP_BITMASK, the value field
> stores a bitshift, not a bitmask, which can be surprising.
> 
> Signed-off-by: Simon Ser <contact@emersion.fr>
> Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> Cc: Leandro Ribeiro <leandro.ribeiro@collabora.com>
> ---
>  include/drm/drm_property.h | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/include/drm/drm_property.h b/include/drm/drm_property.h
> index bbf5c1fdd7b0..f3ea7f97f372 100644
> --- a/include/drm/drm_property.h
> +++ b/include/drm/drm_property.h
> @@ -37,6 +37,11 @@
>   *
>   * For enumeration and bitmask properties this structure stores the symbolic
>   * decoding for each value. This is used for example for the rotation property.
> + *
> + * If the property has the type &DRM_MODE_PROP_BITMASK, @value stores a
> + * bitshift, not a bitmask. In other words, the enum entry is enabled if the
> + * bit number @value is set in the property's value. This enum entry has the
> + * bitmask ``1 << value``.

Please move this into an inline comment to make it clear that this is
specifically about @value. With that:

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

I was also pondering whether we have a nice place to link to for
"property's value" but really they're just uint64_t all over the place,
and only stored in the drm_mode_object for non-atomic properties. So
wording sounds like the best option we have.

>   */
>  struct drm_property_enum {
>  	uint64_t value;
> -- 
> 2.32.0
> 
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch

Re: [PATCH] drm: document drm_property_enum.value for bitfields

[Pekka Paalanen]

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On Wed, 21 Jul 2021 13:43:05 +0200
Daniel Vetter <daniel@ffwll.ch> wrote:

> On Wed, Jul 21, 2021 at 06:51:30AM +0000, Simon Ser wrote:
> > When a property has the type DRM_MODE_PROP_BITMASK, the value field
> > stores a bitshift, not a bitmask, which can be surprising.
> > 
> > Signed-off-by: Simon Ser <contact@emersion.fr>
> > Cc: Pekka Paalanen <pekka.paalanen@collabora.com>
> > Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
> > Cc: Leandro Ribeiro <leandro.ribeiro@collabora.com>
> > ---
> >  include/drm/drm_property.h | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/include/drm/drm_property.h b/include/drm/drm_property.h
> > index bbf5c1fdd7b0..f3ea7f97f372 100644
> > --- a/include/drm/drm_property.h
> > +++ b/include/drm/drm_property.h
> > @@ -37,6 +37,11 @@
> >   *
> >   * For enumeration and bitmask properties this structure stores the symbolic
> >   * decoding for each value. This is used for example for the rotation property.
> > + *
> > + * If the property has the type &DRM_MODE_PROP_BITMASK, @value stores a
> > + * bitshift, not a bitmask. In other words, the enum entry is enabled if the
> > + * bit number @value is set in the property's value. This enum entry has the
> > + * bitmask ``1 << value``.  
> 
> Please move this into an inline comment to make it clear that this is
> specifically about @value. With that:
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
> 

With that

Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>


Thanks,
pq

> I was also pondering whether we have a nice place to link to for
> "property's value" but really they're just uint64_t all over the place,
> and only stored in the drm_mode_object for non-atomic properties. So
> wording sounds like the best option we have.
> 
> >   */
> >  struct drm_property_enum {
> >  	uint64_t value;
> > -- 
> > 2.32.0
> > 
> >   
> 


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