[PATCH] drm/doc: Document legacy_cursor_update better

[PATCH] drm/doc: Document legacy_cursor_update better

[Daniel Vetter]

It's the horror and shouldn't be used. Realized we're not clear on
this in a discussion with Rob about what msm is doing to better
support async commits.

Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
---
 include/drm/drm_atomic.h | 12 +++++++++++-
 1 file changed, 11 insertions(+), 1 deletion(-)

diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
index d07c851d255b..413fd0ca56a8 100644
--- a/include/drm/drm_atomic.h
+++ b/include/drm/drm_atomic.h
@@ -308,7 +308,6 @@ struct __drm_private_objs_state {
  * struct drm_atomic_state - the global state object for atomic updates
  * @ref: count of all references to this state (will not be freed until zero)
  * @dev: parent DRM device
- * @legacy_cursor_update: hint to enforce legacy cursor IOCTL semantics
  * @async_update: hint for asynchronous plane update
  * @planes: pointer to array of structures with per-plane data
  * @crtcs: pointer to array of CRTC pointers
@@ -336,6 +335,17 @@ struct drm_atomic_state {
 	 * drm_atomic_crtc_needs_modeset().
 	 */
 	bool allow_modeset : 1;
+	/**
+	 * @legacy_cursor_update:
+	 *
+	 * Hint to enforce legacy cursor IOCTL semantics.
+	 *
+	 * WARNING: This is thoroughly broken and pretty much impossible to
+	 * implement correctly. Drivers must ignore this and should instead
+	 * implement &drm_plane_helper_funcs.atomic_async_check and
+	 * &drm_plane_helper_funcs.atomic_async_commit hooks. New users of this
+	 * flag are not allowed.
+	 */
 	bool legacy_cursor_update : 1;
 	bool async_update : 1;
 	/**
-- 
2.28.0

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Re: [PATCH] drm/doc: Document legacy_cursor_update better

[Daniel Vetter]

Also cc Sean, who reviewed the msm patch to double down on the
not-so-awesome async solution (I think at least, I'm still not
entirely sure what's all going on there):

commit 2d99ced787e3d0f251fa370d2aae83cf2085a8d9
Author: Rob Clark <robdclark@chromium.org>
Date:   Thu Aug 29 09:45:16 2019 -0700

   drm/msm: async commit support

On Tue, Oct 20, 2020 at 4:39 PM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
>
> It's the horror and shouldn't be used. Realized we're not clear on
> this in a discussion with Rob about what msm is doing to better
> support async commits.
>
> Cc: Rob Clark <robdclark@gmail.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> ---
>  include/drm/drm_atomic.h | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> index d07c851d255b..413fd0ca56a8 100644
> --- a/include/drm/drm_atomic.h
> +++ b/include/drm/drm_atomic.h
> @@ -308,7 +308,6 @@ struct __drm_private_objs_state {
>   * struct drm_atomic_state - the global state object for atomic updates
>   * @ref: count of all references to this state (will not be freed until zero)
>   * @dev: parent DRM device
> - * @legacy_cursor_update: hint to enforce legacy cursor IOCTL semantics
>   * @async_update: hint for asynchronous plane update
>   * @planes: pointer to array of structures with per-plane data
>   * @crtcs: pointer to array of CRTC pointers
> @@ -336,6 +335,17 @@ struct drm_atomic_state {
>          * drm_atomic_crtc_needs_modeset().
>          */
>         bool allow_modeset : 1;
> +       /**
> +        * @legacy_cursor_update:
> +        *
> +        * Hint to enforce legacy cursor IOCTL semantics.
> +        *
> +        * WARNING: This is thoroughly broken and pretty much impossible to
> +        * implement correctly. Drivers must ignore this and should instead
> +        * implement &drm_plane_helper_funcs.atomic_async_check and
> +        * &drm_plane_helper_funcs.atomic_async_commit hooks. New users of this
> +        * flag are not allowed.
> +        */
>         bool legacy_cursor_update : 1;
>         bool async_update : 1;
>         /**
> --
> 2.28.0
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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Re: [PATCH] drm/doc: Document legacy_cursor_update better

[Thomas Zimmermann]

Hi

On 20.10.20 16:39, Daniel Vetter wrote:
> It's the horror and shouldn't be used. Realized we're not clear on
> this in a discussion with Rob about what msm is doing to better
> support async commits.
> 
> Cc: Rob Clark <robdclark@gmail.com>
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> Cc: Maxime Ripard <mripard@kernel.org>
> Cc: Thomas Zimmermann <tzimmermann@suse.de>
> Cc: David Airlie <airlied@linux.ie>
> Cc: Daniel Vetter <daniel@ffwll.ch>
> ---
>  include/drm/drm_atomic.h | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> index d07c851d255b..413fd0ca56a8 100644
> --- a/include/drm/drm_atomic.h
> +++ b/include/drm/drm_atomic.h
> @@ -308,7 +308,6 @@ struct __drm_private_objs_state {
>   * struct drm_atomic_state - the global state object for atomic updates
>   * @ref: count of all references to this state (will not be freed until zero)
>   * @dev: parent DRM device
> - * @legacy_cursor_update: hint to enforce legacy cursor IOCTL semantics
>   * @async_update: hint for asynchronous plane update
>   * @planes: pointer to array of structures with per-plane data
>   * @crtcs: pointer to array of CRTC pointers
> @@ -336,6 +335,17 @@ struct drm_atomic_state {
>  	 * drm_atomic_crtc_needs_modeset().
>  	 */
>  	bool allow_modeset : 1;
> +	/**
> +	 * @legacy_cursor_update:
> +	 *
> +	 * Hint to enforce legacy cursor IOCTL semantics.
> +	 *
> +	 * WARNING: This is thoroughly broken and pretty much impossible to
> +	 * implement correctly. Drivers must ignore this and should instead
> +	 * implement &drm_plane_helper_funcs.atomic_async_check and
> +	 * &drm_plane_helper_funcs.atomic_async_commit hooks. New users of this
> +	 * flag are not allowed.
> +	 */
>  	bool legacy_cursor_update : 1;

The text could also say why the field exists in the first place. If it's
supposed to go away, you could add an item to the TODO list.

Best regards
Thomas

>  	bool async_update : 1;
>  	/**
> 

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer
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Re: [PATCH] drm/doc: Document legacy_cursor_update better

[Daniel Vetter]

On Tue, Oct 20, 2020 at 4:55 PM Thomas Zimmermann <tzimmermann@suse.de> wrote:
>
> Hi
>
> On 20.10.20 16:39, Daniel Vetter wrote:
> > It's the horror and shouldn't be used. Realized we're not clear on
> > this in a discussion with Rob about what msm is doing to better
> > support async commits.
> >
> > Cc: Rob Clark <robdclark@gmail.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
> > Cc: Maxime Ripard <mripard@kernel.org>
> > Cc: Thomas Zimmermann <tzimmermann@suse.de>
> > Cc: David Airlie <airlied@linux.ie>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > ---
> >  include/drm/drm_atomic.h | 12 +++++++++++-
> >  1 file changed, 11 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
> > index d07c851d255b..413fd0ca56a8 100644
> > --- a/include/drm/drm_atomic.h
> > +++ b/include/drm/drm_atomic.h
> > @@ -308,7 +308,6 @@ struct __drm_private_objs_state {
> >   * struct drm_atomic_state - the global state object for atomic updates
> >   * @ref: count of all references to this state (will not be freed until zero)
> >   * @dev: parent DRM device
> > - * @legacy_cursor_update: hint to enforce legacy cursor IOCTL semantics
> >   * @async_update: hint for asynchronous plane update
> >   * @planes: pointer to array of structures with per-plane data
> >   * @crtcs: pointer to array of CRTC pointers
> > @@ -336,6 +335,17 @@ struct drm_atomic_state {
> >        * drm_atomic_crtc_needs_modeset().
> >        */
> >       bool allow_modeset : 1;
> > +     /**
> > +      * @legacy_cursor_update:
> > +      *
> > +      * Hint to enforce legacy cursor IOCTL semantics.
> > +      *
> > +      * WARNING: This is thoroughly broken and pretty much impossible to
> > +      * implement correctly. Drivers must ignore this and should instead
> > +      * implement &drm_plane_helper_funcs.atomic_async_check and
> > +      * &drm_plane_helper_funcs.atomic_async_commit hooks. New users of this
> > +      * flag are not allowed.
> > +      */
> >       bool legacy_cursor_update : 1;
>
> The text could also say why the field exists in the first place. If it's
> supposed to go away, you could add an item to the TODO list.

"Hint to enforce legacy cursor IOCTL semantics." is the purpose of
this thing. It was used way back as pretty shoddy approach to handling
cursor updates, but is slowly dying.

Good point on adding a todo item for this, I'll respin with that added.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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[PATCH] drm/doc: Document legacy_cursor_update better

[Daniel Vetter]

It's the horror and shouldn't be used. Realized we're not clear on
this in a discussion with Rob about what msm is doing to better
support async commits.

v2: Refine existing todo item to include this (Thomas)

Cc: Sean Paul <sean@poorly.run>
Cc: Rob Clark <robdclark@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <mripard@kernel.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
---
 Documentation/gpu/todo.rst |  4 ++++
 include/drm/drm_atomic.h   | 12 +++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
index 700637e25ecd..6b224ef14455 100644
--- a/Documentation/gpu/todo.rst
+++ b/Documentation/gpu/todo.rst
@@ -105,6 +105,10 @@ converted over to the new infrastructure.
 One issue with the helpers is that they require that drivers handle completion
 events for atomic commits correctly. But fixing these bugs is good anyway.
 
+Somewhat related is the legacy_cursor_update hack, which should be replaced with
+the new atomic_async_check/commit functionality in the helpers in drivers that
+still look at that flag.
+
 Contact: Daniel Vetter, respective driver maintainers
 
 Level: Advanced
diff --git a/include/drm/drm_atomic.h b/include/drm/drm_atomic.h
index d07c851d255b..413fd0ca56a8 100644
--- a/include/drm/drm_atomic.h
+++ b/include/drm/drm_atomic.h
@@ -308,7 +308,6 @@ struct __drm_private_objs_state {
  * struct drm_atomic_state - the global state object for atomic updates
  * @ref: count of all references to this state (will not be freed until zero)
  * @dev: parent DRM device
- * @legacy_cursor_update: hint to enforce legacy cursor IOCTL semantics
  * @async_update: hint for asynchronous plane update
  * @planes: pointer to array of structures with per-plane data
  * @crtcs: pointer to array of CRTC pointers
@@ -336,6 +335,17 @@ struct drm_atomic_state {
 	 * drm_atomic_crtc_needs_modeset().
 	 */
 	bool allow_modeset : 1;
+	/**
+	 * @legacy_cursor_update:
+	 *
+	 * Hint to enforce legacy cursor IOCTL semantics.
+	 *
+	 * WARNING: This is thoroughly broken and pretty much impossible to
+	 * implement correctly. Drivers must ignore this and should instead
+	 * implement &drm_plane_helper_funcs.atomic_async_check and
+	 * &drm_plane_helper_funcs.atomic_async_commit hooks. New users of this
+	 * flag are not allowed.
+	 */
 	bool legacy_cursor_update : 1;
 	bool async_update : 1;
 	/**
-- 
2.28.0

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