Re: [PATCH 09/10] drm/vkms: totally reworked crc data tracking

[Daniel Vetter <daniel@ffwll.ch>]

On Wed, Jun 12, 2019 at 10:46:28AM -0300, Rodrigo Siqueira wrote:
> On Thu, Jun 6, 2019 at 7:28 PM Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> >
> > The crc computation worker needs to be able to get at some data
> > structures and framebuffer mappings, while potentially more atomic
> > updates are going on. The solution thus far is to copy relevant bits
> > around, but that's very tedious.
> >
> > Here's a new approach, which tries to be more clever, but relies on a
> > few not-so-obvious things:
> > - crtc_state is always updated when a plane_state changes. Therefore
> >   we can just stuff plane_state pointers into a crtc_state. That
> >   solves the problem of easily getting at the needed plane_states.
> 
> Just for curiosity, where exactly the crct_state is updated? If
> possible, could you elaborate a little bit about this?

drm_atomic_get_plane_state always gets the crtc_state too, which means if
we duplicate a plane_state, we will _always_ duplicate the crtc state too.
This is a guarantee by the atomic kms design.

> 
> > - with the flushing changes from previous patches the above also holds
> >   without races due to the next atomic update being a bit eager with
> >   cleaning up pending work - we always wait for all crc work items to
> >   complete before unmapping framebuffers.
> > - we also need to make sure that the hrtimer fires off the right
> >   worker. Keep a new distinct crc_state pointer, under the
> >   vkms_output->lock protection for this. Note that crtc->state is
> >   updated very early in the atomic commit, way before we arm the
> >   vblank event - the vblank event should always match the buffers we
> >   use to compute the crc. This also solves an issue in the hrtimer,
> >   where we've accessed drm_crtc->state without holding the right locks
> >   (we held none - oops).
> > - in the worker itself we can then just access the plane states we
> >   need, again solving a bunch of ordering and locking issues.
> >   Accessing plane->state requires locks, accessing the private
> >   vkms_crtc_state->active_planes pointer only requires that the memory
> >   doesn't get freed too early.
> >
> > The idea behind vkms_crtc_state->active_planes is that this would
> > contain all visible planes, in z-order, as a first step towards a more
> > generic blending implementation.
> >
> > Note that this patch also fixes races between prepare_fb/cleanup_fb
> > and the crc worker accessing ->vaddr.
> >
> > Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
> > Cc: Rodrigo Siqueira <rodrigosiqueiramelo@gmail.com>
> > Cc: Haneen Mohammed <hamohammed.sa@gmail.com>
> > Cc: Daniel Vetter <daniel@ffwll.ch>
> > ---
> >  drivers/gpu/drm/vkms/vkms_crc.c  | 21 +++--------
> >  drivers/gpu/drm/vkms/vkms_crtc.c | 60 +++++++++++++++++++++++++++++---
> >  drivers/gpu/drm/vkms/vkms_drv.h  |  9 ++++-
> >  3 files changed, 67 insertions(+), 23 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/vkms/vkms_crc.c b/drivers/gpu/drm/vkms/vkms_crc.c
> > index 9d15e5e85830..0d31cfc32042 100644
> > --- a/drivers/gpu/drm/vkms/vkms_crc.c
> > +++ b/drivers/gpu/drm/vkms/vkms_crc.c
> > @@ -159,11 +159,8 @@ void vkms_crc_work_handle(struct work_struct *work)
> >                                                 crc_work);
> >         struct drm_crtc *crtc = crtc_state->base.crtc;
> >         struct vkms_output *out = drm_crtc_to_vkms_output(crtc);
> > -       struct vkms_device *vdev = container_of(out, struct vkms_device,
> > -                                               output);
> >         struct vkms_crc_data *primary_crc = NULL;
> >         struct vkms_crc_data *cursor_crc = NULL;
> > -       struct drm_plane *plane;
> >         u32 crc32 = 0;
> >         u64 frame_start, frame_end;
> >         bool crc_pending;
> > @@ -184,21 +181,11 @@ void vkms_crc_work_handle(struct work_struct *work)
> >         if (!crc_pending)
> >                 return;
> >
> > -       drm_for_each_plane(plane, &vdev->drm) {
> > -               struct vkms_plane_state *vplane_state;
> > -               struct vkms_crc_data *crc_data;
> > +       if (crtc_state->num_active_planes >= 1)
> > +               primary_crc = crtc_state->active_planes[0]->crc_data;
> >
> > -               vplane_state = to_vkms_plane_state(plane->state);
> > -               crc_data = vplane_state->crc_data;
> > -
> > -               if (drm_framebuffer_read_refcount(&crc_data->fb) == 0)
> > -                       continue;
> > -
> > -               if (plane->type == DRM_PLANE_TYPE_PRIMARY)
> > -                       primary_crc = crc_data;
> > -               else
> > -                       cursor_crc = crc_data;
> > -       }
> > +       if (crtc_state->num_active_planes == 2)
> > +               cursor_crc = crtc_state->active_planes[1]->crc_data;
> >
> >         if (primary_crc)
> >                 crc32 = _vkms_get_crc(primary_crc, cursor_crc);
> > diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c
> > index 48a793ba4030..14156ed70415 100644
> > --- a/drivers/gpu/drm/vkms/vkms_crtc.c
> > +++ b/drivers/gpu/drm/vkms/vkms_crtc.c
> > @@ -1,6 +1,7 @@
> >  // SPDX-License-Identifier: GPL-2.0+
> >
> >  #include "vkms_drv.h"
> > +#include <drm/drm_atomic.h>
> >  #include <drm/drm_atomic_helper.h>
> >  #include <drm/drm_probe_helper.h>
> >
> > @@ -9,7 +10,7 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
> >         struct vkms_output *output = container_of(timer, struct vkms_output,
> >                                                   vblank_hrtimer);
> >         struct drm_crtc *crtc = &output->crtc;
> > -       struct vkms_crtc_state *state = to_vkms_crtc_state(crtc->state);
> > +       struct vkms_crtc_state *state;
> >         u64 ret_overrun;
> >         bool ret;
> >
> > @@ -23,6 +24,7 @@ static enum hrtimer_restart vkms_vblank_simulate(struct hrtimer *timer)
> >         if (!ret)
> >                 DRM_ERROR("vkms failure on handling vblank");
> >
> > +       state = output->crc_state;
> >         if (state && output->crc_enabled) {
> >                 u64 frame = drm_crtc_accurate_vblank_count(crtc);
> >
> > @@ -124,10 +126,9 @@ static void vkms_atomic_crtc_destroy_state(struct drm_crtc *crtc,
> >
> >         __drm_atomic_helper_crtc_destroy_state(state);
> >
> > -       if (vkms_state) {
> > -               WARN_ON(work_pending(&vkms_state->crc_work));
> > -               kfree(vkms_state);
> > -       }
> > +       WARN_ON(work_pending(&vkms_state->crc_work));
> > +       kfree(vkms_state->active_planes);
> > +       kfree(vkms_state);
> >  }
> >
> >  static void vkms_atomic_crtc_reset(struct drm_crtc *crtc)
> > @@ -156,6 +157,52 @@ static const struct drm_crtc_funcs vkms_crtc_funcs = {
> >         .verify_crc_source      = vkms_verify_crc_source,
> >  };
> >
> > +static int vkms_crtc_atomic_check(struct drm_crtc *crtc,
> > +                                  struct drm_crtc_state *state)
> > +{
> > +       struct vkms_crtc_state *vkms_state = to_vkms_crtc_state(state);
> > +       struct drm_plane *plane;
> > +       struct drm_plane_state *plane_state;
> > +       int i = 0, ret;
> > +
> > +       if (vkms_state->active_planes)
> > +               return 0;
> > +
> > +       ret = drm_atomic_add_affected_planes(state->state, crtc);
> > +       if (ret < 0)
> > +               return ret;
> > +
> > +       drm_for_each_plane_mask(plane, crtc->dev, state->plane_mask) {
> 
> In the drm_for_each_plane_mask documentation says: “ Iterate over all
> planes specified by bitmask”. I did not understand what it means by
> “specified by bitmask” and the use of this macro in this context. I

state->plane_mask is the bitmask. Each bit in there represents a plane, so
can be used as a very effective representation for a subset of planes (as
long as we don't have more than 32 planes in total).

> tried to replace it for drm_for_each_plane, but the test just break.

Hm right now this should work, because we only have one crtc, so no
problem with walking over planes we shouldn't even look at.

> Could you explain a little bit the magic behind
> drm_for_each_plane_mask?

Hope the above quick summary of what we use the bitmask for helps to get
you started.
-Daniel

> 
> > +               plane_state = drm_atomic_get_existing_plane_state(state->state,
> > +                                                                 plane);
> > +               WARN_ON(!plane_state);
> > +
> > +               if (!plane_state->visible)
> > +                       continue;
> > +
> > +               i++;
> > +       }
> > +
> > +       vkms_state->active_planes = kcalloc(i, sizeof(plane), GFP_KERNEL);
> > +       if (!vkms_state->active_planes)
> > +               return -ENOMEM;
> > +       vkms_state->num_active_planes = i;
> > +
> > +       i = 0;
> > +       drm_for_each_plane_mask(plane, crtc->dev, state->plane_mask) {
> > +               plane_state = drm_atomic_get_existing_plane_state(state->state,
> > +                                                                 plane);
> > +
> > +               if (!plane_state->visible)
> > +                       continue;
> > +
> > +               vkms_state->active_planes[i++] =
> > +                       to_vkms_plane_state(plane_state);
> > +       }
> > +
> > +       return 0;
> > +}
> > +
> >  static void vkms_crtc_atomic_enable(struct drm_crtc *crtc,
> >                                     struct drm_crtc_state *old_state)
> >  {
> > @@ -197,10 +244,13 @@ static void vkms_crtc_atomic_flush(struct drm_crtc *crtc,
> >                 crtc->state->event = NULL;
> >         }
> >
> > +       vkms_output->crc_state = to_vkms_crtc_state(crtc->state);
> > +
> >         spin_unlock_irq(&vkms_output->lock);
> >  }
> >
> >  static const struct drm_crtc_helper_funcs vkms_crtc_helper_funcs = {
> > +       .atomic_check   = vkms_crtc_atomic_check,
> >         .atomic_begin   = vkms_crtc_atomic_begin,
> >         .atomic_flush   = vkms_crtc_atomic_flush,
> >         .atomic_enable  = vkms_crtc_atomic_enable,
> > diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h
> > index 2a35299bfb89..4e7450111d45 100644
> > --- a/drivers/gpu/drm/vkms/vkms_drv.h
> > +++ b/drivers/gpu/drm/vkms/vkms_drv.h
> > @@ -49,6 +49,10 @@ struct vkms_crtc_state {
> >         struct drm_crtc_state base;
> >         struct work_struct crc_work;
> >
> > +       int num_active_planes;
> > +       /* stack of active planes for crc computation, should be in z order */
> > +       struct vkms_plane_state **active_planes;
> > +
> >         /* below three are protected by vkms_output.crc_lock */
> >         bool crc_pending;
> >         u64 frame_start;
> > @@ -62,12 +66,15 @@ struct vkms_output {
> >         struct hrtimer vblank_hrtimer;
> >         ktime_t period_ns;
> >         struct drm_pending_vblank_event *event;
> > -       bool crc_enabled;
> >         /* ordered wq for crc_work */
> >         struct workqueue_struct *crc_workq;
> >         /* protects concurrent access to crc_data */
> >         spinlock_t lock;
> >
> > +       /* protected by @lock */
> > +       bool crc_enabled;
> > +       struct vkms_crtc_state *crc_state;
> > +
> >         spinlock_t crc_lock;
> >  };
> >
> > --
> > 2.20.1
> >
> 
> 
> -- 
> 
> Rodrigo Siqueira
> https://siqueira.tech

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
_______________________________________________
dri-devel mailing list
dri-devel@lists.freedesktop.org
https://lists.freedesktop.org/mailman/listinfo/dri-devel