Hi Pekka, Thank you for reviewing this patch. > On Mon, 13 Sep 2021 16:35:26 -0700 > Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx> wrote: > > > If a driver supports this capability, it means that there would be an > > additional signalling mechanism for a page flip completion in addition > > to out_fence or DRM_MODE_PAGE_FLIP_EVENT. > > > > This capability may only be relevant for Virtual KMS drivers and is currently > > used only by virtio-gpu. Also, it can provide a potential solution for: > > https://gitlab.freedesktop.org/wayland/weston/-/issues/514 > > > > Signed-off-by: Vivek Kasireddy <vivek.kasireddy@xxxxxxxxx> > > --- > > drivers/gpu/drm/drm_ioctl.c | 3 +++ > > include/drm/drm_mode_config.h | 8 ++++++++ > > include/uapi/drm/drm.h | 1 + > > 3 files changed, 12 insertions(+) > > > > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c > > index 8b8744dcf691..8a420844f8bc 100644 > > --- a/drivers/gpu/drm/drm_ioctl.c > > +++ b/drivers/gpu/drm/drm_ioctl.c > > @@ -302,6 +302,9 @@ static int drm_getcap(struct drm_device *dev, void *data, struct > drm_file *file_ > > case DRM_CAP_CRTC_IN_VBLANK_EVENT: > > req->value = 1; > > break; > > + case DRM_CAP_RELEASE_FENCE: > > + req->value = dev->mode_config.release_fence; > > + break; > > Hi Vivek, > > is this actually necessary? > > I would think that userspace figures out the existence of the release > fence capability by seeing that the KMS property "RELEASE_FENCE_PTR" > either exists or not. [Vivek] Yeah, that makes sense. However, in order for the userspace to not see this property, we'd have to prevent drm core from exposing it; which means we need to check dev->mode_config.release_fence before attaching the property to the crtc. > > However, would we not need a client cap instead? > > If a KMS driver knows that userspace is aware of "RELEASE_FENCE_PTR" > and will use it when necessary, then the KMS driver can send the > pageflip completion without waiting for the host OS to signal the old > buffer as free for re-use. [Vivek] Right, the KMS driver can just look at whether the release_fence was added by the userspace (in the atomic commit) to determine whether it needs to wait for the old fb. > > If the KMS driver does not know that userspace can handle pageflip > completing "too early", then it has no choice but to wait until the old > buffer is really free before signalling pageflip completion. > > Wouldn't that make sense? [Vivek] Yes; DRM_CAP_RELEASE_FENCE may not be necessary to implement the behavior you suggest which makes sense. > > > Otherwise, this proposal sounds fine to me. [Vivek] Did you get a chance to review the Weston MR: https://gitlab.freedesktop.org/wayland/weston/-/merge_requests/668 Could you please take a look? Thanks, Vivek > > > Thanks, > pq > > > > default: > > return -EINVAL; > > } > > diff --git a/include/drm/drm_mode_config.h b/include/drm/drm_mode_config.h > > index 12b964540069..944bebf359d7 100644 > > --- a/include/drm/drm_mode_config.h > > +++ b/include/drm/drm_mode_config.h > > @@ -935,6 +935,14 @@ struct drm_mode_config { > > */ > > bool normalize_zpos; > > > > + /** > > + * @release_fence: > > + * > > + * If this option is set, it means there would be an additional signalling > > + * mechanism for a page flip completion. > > + */ > > + bool release_fence; > > + > > /** > > * @modifiers_property: Plane property to list support modifier/format > > * combination. > > diff --git a/include/uapi/drm/drm.h b/include/uapi/drm/drm.h > > index 3b810b53ba8b..8b8985f65581 100644 > > --- a/include/uapi/drm/drm.h > > +++ b/include/uapi/drm/drm.h > > @@ -767,6 +767,7 @@ struct drm_gem_open { > > * Documentation/gpu/drm-mm.rst, section "DRM Sync Objects". > > */ > > #define DRM_CAP_SYNCOBJ_TIMELINE 0x14 > > +#define DRM_CAP_RELEASE_FENCE 0x15 > > > > /* DRM_IOCTL_GET_CAP ioctl argument type */ > > struct drm_get_cap {
