From: Daniel Vetter Date: Thu, 5 Apr 2018 15:44:46 +0000 (+0200) Subject: drm/atomic: better doc for implicit vs explicit fencing X-Git-Tag: v4.19~298^2~45^2~178 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=30d23f220c75cf58584b19929bd8460b4edc9771;p=platform%2Fkernel%2Flinux-rpi.git drm/atomic: better doc for implicit vs explicit fencing Note that a pile of drivers don't seem to take implicit fencing into account, or at least don't call drm_atoimc_set_fence_for_plane(). Cc'ing relevant people, or at least some. Some drivers also look like they don't disable implicit fencing (e.g. amdgpu) because the explicit fences and implicit fences are handled by entirely independent code paths. I also wonder whether we shouldn't just make the recommended helpers the default ones, since a lot of drivers don't bother to handle the implicit fences at all it seems. The helpers won't blow up even for non-GEM drivers or GEM drivers which don't fill out the gem bo pointers in struct drm_framebuffer. v2: Comments from Eric. Cc: Gerd Hoffmann Cc: Alex Deucher Cc: Harry Wentland Cc: Sinclair Yeh Cc: Thomas Hellstrom Cc: Gustavo Padovan Cc: Maarten Lankhorst Cc: Sean Paul Reviewed-by: Eric Anholt Signed-off-by: Daniel Vetter Link: https://patchwork.freedesktop.org/patch/msgid/20180405154449.23038-7-daniel.vetter@ffwll.ch --- diff --git a/drivers/gpu/drm/drm_atomic.c b/drivers/gpu/drm/drm_atomic.c index 3d9ae05..9bdd677 100644 --- a/drivers/gpu/drm/drm_atomic.c +++ b/drivers/gpu/drm/drm_atomic.c @@ -1496,6 +1496,14 @@ EXPORT_SYMBOL(drm_atomic_set_fb_for_plane); * Otherwise, if &drm_plane_state.fence is not set this function we just set it * with the received implicit fence. In both cases this function consumes a * reference for @fence. + * + * This way explicit fencing can be used to overrule implicit fencing, which is + * important to make explicit fencing use-cases work: One example is using one + * buffer for 2 screens with different refresh rates. Implicit fencing will + * clamp rendering to the refresh rate of the slower screen, whereas explicit + * fence allows 2 independent render and display loops on a single buffer. If a + * driver allows obeys both implicit and explicit fences for plane updates, then + * it will break all the benefits of explicit fencing. */ void drm_atomic_set_fence_for_plane(struct drm_plane_state *plane_state, diff --git a/include/drm/drm_modeset_helper_vtables.h b/include/drm/drm_modeset_helper_vtables.h index 3e76ca8..35e2a3a 100644 --- a/include/drm/drm_modeset_helper_vtables.h +++ b/include/drm/drm_modeset_helper_vtables.h @@ -1004,11 +1004,14 @@ struct drm_plane_helper_funcs { * This function must not block for outstanding rendering, since it is * called in the context of the atomic IOCTL even for async commits to * be able to return any errors to userspace. Instead the recommended - * way is to fill out the fence member of the passed-in + * way is to fill out the &drm_plane_state.fence of the passed-in * &drm_plane_state. If the driver doesn't support native fences then * equivalent functionality should be implemented through private * members in the plane structure. * + * Drivers which always have their buffers pinned should use + * drm_gem_fb_prepare_fb() for this hook. + * * The helpers will call @cleanup_fb with matching arguments for every * successful call to this hook. * diff --git a/include/drm/drm_plane.h b/include/drm/drm_plane.h index 9563bd2..26fa50c 100644 --- a/include/drm/drm_plane.h +++ b/include/drm/drm_plane.h @@ -80,8 +80,15 @@ struct drm_plane_state { /** * @fence: * - * Optional fence to wait for before scanning out @fb. Do not write this - * directly, use drm_atomic_set_fence_for_plane() + * Optional fence to wait for before scanning out @fb. The core atomic + * code will set this when userspace is using explicit fencing. Do not + * write this directly for a driver's implicit fence, use + * drm_atomic_set_fence_for_plane() to ensure that an explicit fence is + * preserved. + * + * Drivers should store any implicit fence in this from their + * &drm_plane_helper.prepare_fb callback. See drm_gem_fb_prepare_fb() + * and drm_gem_fb_simple_display_pipe_prepare_fb() for suitable helpers. */ struct dma_fence *fence;