[PATCH] drm/doc: Document that modifiers are always required for fb

[Pekka Paalanen]

On Wed,  2 Sep 2020 12:24:40 +0200
Daniel Vetter <daniel.vetter at ffwll.ch> wrote:

> Even for legacy userspace, since otherwise GETFB2 is broken and if you
> switch between modifier-less and modifier-aware compositors, smooth
> transitions break.
> 
> Also it's just best practice to make sure modifiers are invariant for
> a given drm_fb, and that a modifier-aware kms drivers only has one
> place to store them, ignoring any old implicit bo flags or whatever
> else might float around.
> 
> Motivated by some irc discussion with Bas about amdgpu modifier
> support.
> 
> Fixes: 455e00f1412f ("drm: Add getfb2 ioctl")
> Cc: Daniel Stone <daniels at collabora.com>
> Cc: Juston Li <juston.li at intel.com>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
> Cc: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
> Cc: Marek Olšák <maraeo at gmail.com>
> Cc: "Wentland, Harry" <harry.wentland at amd.com>
> Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> ---
>  include/drm/drm_mode_config.h | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h
> index a18f73eb3cf6..5ffbb4ed5b35 100644
> --- a/include/drm/drm_mode_config.h
> +++ b/include/drm/drm_mode_config.h
> @@ -58,6 +58,12 @@ struct drm_mode_config_funcs {
>  	 * actual modifier used if the request doesn't have it specified,
>  	 * ie. when (@mode_cmd->flags & DRM_MODE_FB_MODIFIERS) == 0.
>  	 *
> +	 * IMPORTANT: These implied modifiers for legacy userspace must be
> +	 * stored in struct &drm_framebuffer, including all relevant metadata
> +	 * like &drm_framebuffer.pitches and &drm_framebuffer.offsets if the
> +	 * modifier enables additional planes beyond the fourcc pixel format
> +	 * code. This is required by the GETFB2 ioctl.
> +	 *
>  	 * If the parameters are deemed valid and the backing storage objects in
>  	 * the underlying memory manager all exist, then the driver allocates
>  	 * a new &drm_framebuffer structure, subclassed to contain
> @@ -915,6 +921,13 @@ struct drm_mode_config {
>  	 * @allow_fb_modifiers:
>  	 *
>  	 * Whether the driver supports fb modifiers in the ADDFB2.1 ioctl call.
> +	 *
> +	 * IMPORTANT:
> +	 *
> +	 * If this is set the driver must fill out the full implicit modifier
> +	 * information in their &drm_mode_config_funcs.fb_create hook for legacy
> +	 * userspace which does not set modifiers. Otherwise the GETFB2 ioctl is
> +	 * broken for modifier aware userspace.
>  	 */
>  	bool allow_fb_modifiers;
>  

Hi,

are there any drivers that would infer this information at
modeset/pageflip/atomic ioctl time instead of AddFB/AddFB2 time?

Userspace may be creating the FB once per buffer and keep re-using
that over several render/display cycles. If a driver was changing the
"effective modifiers" dynamically, userspace could break.


Thanks,
pq
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