On Fri, Aug 31, 2018 at 10:41:40AM +0200, Daniel Vetter wrote:
On Fri, Aug 24, 2018 at 02:16:34AM +0300, Haneen Mohammed wrote:
crtc_state is accessed by both vblank_handle() and the ordered work_struct handle vkms_crc_work_handle() to retrieve and or update the frame number for computed CRC.
Since work_struct can fail, add frame_end to account for missing frame numbers.
use atomic_t witth appropriate flags for synchronization between hrtimer callback and ordered work_struct handle since spinlock can't be used with work_struct handle and mutex can't be used with hrtimer callback.
This patch passes the following subtests from igt kms_pipe_crc_basic test: bad-source, read-crc-pipe-A, read-crc-pipe-A-frame-sequence, nonblocking-crc-pipe-A, nonblocking-crc-pipe-A-frame-sequence
Signed-off-by: Haneen Mohammed hamohammed.sa@gmail.com
So atomic_t is probably the greatest trap in the linux kernel. It sounds like the right thing, but in 99% of all case you want to use it it isn't. The trouble is that atomic_t is _very_ unordered, the only thing it guarantees is that atomic_t transactions to the _same_ variable are consistent. But anything else can be reordered at will.
This is very confusing since the new C++ atomic standards has fully ordered atomics as the default, and you expressedly need to ask for the weakly ordered ones. In linux you need to sprinkle epic amounts of smb_barrier* and similar things around them to make atomic_t behave like a "normal" C++ atomic type.
tldr; atomic_t is good for special refcounting needs, when the normal refcount_t doesn't cut it. Not much else.
What usually should be done:
- Use normal u64 (to match the vblank counter size) instead of atomic_t here.
- Make sure all access is protect by an appropriate spinlock.
I see, thanks for the explanation!
hm would the patch be fine if I just switch atomic_t to u64 without using spinlock? since that won't be possible with the work_struct.
Thanks, Haneen
drivers/gpu/drm/vkms/vkms_crc.c | 33 ++++++++++++++++++++++++++++++-- drivers/gpu/drm/vkms/vkms_crtc.c | 13 +++++++++++-- drivers/gpu/drm/vkms/vkms_drv.h | 6 ++++-- 3 files changed, 46 insertions(+), 6 deletions(-)
diff --git a/drivers/gpu/drm/vkms/vkms_crc.c b/drivers/gpu/drm/vkms/vkms_crc.c index ed47d67cecd6..4a1ba5b7886a 100644 --- a/drivers/gpu/drm/vkms/vkms_crc.c +++ b/drivers/gpu/drm/vkms/vkms_crc.c @@ -34,6 +34,15 @@ static uint32_t _vkms_get_crc(struct vkms_crc_data *crc_data) return crc; }
+/**
- vkms_crc_work_handle - ordered work_struct to compute CRC
- @work: work_struct
- Work handler for computing CRCs. work_struct scheduled in
- an ordered workqueue that's periodically scheduled to run by
- _vblank_handle() and flushed at vkms_atomic_crtc_destroy_state().
- */
void vkms_crc_work_handle(struct work_struct *work) { struct vkms_crtc_state *crtc_state = container_of(work, @@ -45,8 +54,18 @@ void vkms_crc_work_handle(struct work_struct *work) output); struct vkms_crc_data *primary_crc = NULL; struct drm_plane *plane;
- u32 crc32 = 0;
- u32 frame_start, frame_end;
- frame_start = atomic_read(&crtc_state->frame_start);
- frame_end = atomic_read(&crtc_state->frame_end);
- /* _vblank_handle() hasn't updated frame_start yet */
- if (!frame_start) {
I think if we go with u64 we can ignore the issues for wrap-arround, since that will simply never happen. But a comment would be good.
Aside from the atomic_t issue I think this looks good. -Daniel
return;} else if (frame_start == frame_end) {
atomic_set(&crtc_state->frame_start, 0);return;}
drm_for_each_plane(plane, &vdev->drm) { struct vkms_plane_state *vplane_state;
@@ -67,7 +86,17 @@ void vkms_crc_work_handle(struct work_struct *work) if (primary_crc) crc32 = _vkms_get_crc(primary_crc);
- drm_crtc_add_crc_entry(crtc, true, crtc_state->n_frame, &crc32);
- frame_end = drm_crtc_accurate_vblank_count(crtc);
- /* queue_work can fail to schedule crc_work; add crc for
* missing frames*/- while (frame_start <= frame_end)
drm_crtc_add_crc_entry(crtc, true, frame_start++, &crc32);- /* to avoid using the same value again */
- atomic_set(&crtc_state->frame_end, frame_end);
- atomic_set(&crtc_state->frame_start, 0);
}
static int vkms_crc_parse_source(const char *src_name, bool *enabled) diff --git a/drivers/gpu/drm/vkms/vkms_crtc.c b/drivers/gpu/drm/vkms/vkms_crtc.c index 9d0b1a325a78..a170677acd46 100644 --- a/drivers/gpu/drm/vkms/vkms_crtc.c +++ b/drivers/gpu/drm/vkms/vkms_crtc.c @@ -22,8 +22,17 @@ static void _vblank_handle(struct vkms_output *output) DRM_ERROR("vkms failure on handling vblank");
if (state && output->crc_enabled) {
state->n_frame = drm_crtc_accurate_vblank_count(crtc);queue_work(output->crc_workq, &state->crc_work);
u32 frame = drm_crtc_accurate_vblank_count(crtc);/* update frame_start only if a queued vkms_crc_work_handle has* read the data*/if (!atomic_read(&state->frame_start))atomic_set(&state->frame_start, frame);ret = queue_work(output->crc_workq, &state->crc_work);if (!ret)DRM_WARN("failed to queue vkms_crc_work_handle");}
spin_unlock(&output->lock);
diff --git a/drivers/gpu/drm/vkms/vkms_drv.h b/drivers/gpu/drm/vkms/vkms_drv.h index 2017a2ccc43d..4a3956a0549e 100644 --- a/drivers/gpu/drm/vkms/vkms_drv.h +++ b/drivers/gpu/drm/vkms/vkms_drv.h @@ -39,12 +39,14 @@ struct vkms_plane_state {
- vkms_crtc_state - Driver specific CRTC state
- @base: base CRTC state
- @crc_work: work struct to compute and add CRC entries
- @n_frame: frame number for computed CRC
- @n_frame_start: start frame number for computed CRC
*/
- @n_frame_end: end frame number for computed CRC
struct vkms_crtc_state { struct drm_crtc_state base; struct work_struct crc_work;
- unsigned int n_frame;
- atomic_t frame_start;
- atomic_t frame_end;
};
struct vkms_output {
2.17.1
-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch