linux-kernel - Re: [PATCH v2 4/5] drm/plane: Extend damage tracking kernel-doc

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Message-ID: <vhshocGSkXgVLycHIcJIVPsN9OQokPA2NCgIBqOvIzpKRZXQjN1uEiFKVudwa-S4hpBnFPaxxYh8hCFxd-u_ahYKBamQxFzIhBkYGkND9Kc=@emersion.fr>
Date:   Thu, 16 Nov 2023 12:14:51 +0000
From:   Simon Ser <contact@...rsion.fr>
To:     Thomas Zimmermann <tzimmermann@...e.de>
Cc:     Javier Martinez Canillas <javierm@...hat.com>,
        linux-kernel@...r.kernel.org,
        Pekka Paalanen <pekka.paalanen@...labora.com>,
        Sima Vetter <daniel.vetter@...ll.ch>,
        Bilal Elmoussaoui <belmouss@...hat.com>,
        dri-devel@...ts.freedesktop.org,
        Maxime Ripard <mripard@...nel.org>,
        Erico Nunes <nunes.erico@...il.com>
Subject: Re: [PATCH v2 4/5] drm/plane: Extend damage tracking kernel-doc

On Thursday, November 16th, 2023 at 13:06, Thomas Zimmermann <tzimmermann@...e.de> wrote:

> > + * Note that there are two types of damage handling: frame damage and buffer
> > + * damage. The type of damage handling implemented depends on a driver's upload
> > + * target. Drivers implementing a per-plane or per-CRTC upload target need to
> > + * handle frame damage while drivers implementing a per-buffer upload target
> > + * need to handle buffer damage.
> > + *
> > + * The existing damage helpers only support the frame damage type, there is no
> > + * buffer age support or similar damage accumulation algorithm implemented yet.
> > + *
> > + * Only drivers handling frame damage can use the mentiored damage helpers to

Typo: mentioned

> > + * iterate over the damaged regions. Drivers that handle buffer damage, need to
> > + * set &struct drm_plane_state.ignore_damage_clips as an indication to
> > + * drm_atomic_helper_damage_iter_init() that the damage clips should be ignored.
> > + * In that case, the returned damage rectangle is the &drm_plane_state.src since
> > + * a full plane update should happen.
> > + *
> > + * For more information about the two type of damage, see:
> > + * https://registry.khronos.org/EGL/extensions/KHR/EGL_KHR_swap_buffers_with_damage.txt
> > + * https://emersion.fr/blog/2019/intro-to-damage-tracking/
> 
> One thought you might want to consider.
> 
> These URLs are helpful. The only issue I have is that frame damage and
> buffer damage are user-space concepts. The kernel bug is that damage
> handling expects the backing storage/upload buffer not to change for a
> given plane. If the upload buffer changes between page flips, the new
> upload buffer has to be updated as a whole. Hence no damage handling then.

Why would these concepts be specific to user-space? The kernel could
better handle buffer damage instead of forcing full damage, by doing
something similar to what user-space does.

Anyways:

Reviewed-by: Simon Ser <contact@...rsion.fr>