Re: [PATCH 0/3] drm/exynos: modify usage of wait for vblank

[Inki Dae <inki.dae@xxxxxxxxxxx>]

2012/12/5 Inki Dae <inki.dae@xxxxxxxxxxx>

2012/12/5 Prathyush K <prathyush@xxxxxxxxxxxx>

On Wed, Dec 5, 2012 at 11:40 AM, Inki Dae <inki.dae@xxxxxxxxxxx> wrote:

2012/12/5 Inki Dae <inki.dae@xxxxxxxxxxx>

2012/12/5 Prathyush K <prathyush.k@xxxxxxxxxxx>

This patchset fixes a few issues with use of wait for vblank in
exynos drm.

Please apply these three patches without "drm/exynos: release fb pended by page flip"
patch.

Patch 1: modify wait for vsync functions to use wait queues
This modifies the wait_for_vblank functions to use wait queues
thus ensuring that the current task goes to sleep without taking
up CPU while waiting.

Patch 2: move wait_for_vblank to manager_ops
Currently, we do not call wait for vblank if encoder is in DPMS_OFF
state inside exynos_drm_encoder_complete_scanout function. This is
because wait for vblank is treated as an overlay op. 

This can be
modified to a manager_op thus removing the above check for DPMS_OFF.
This fixes a crash while removing a fb.
While removing the current fb (crtc->fb == fb) the encoder
dpms off is called and then the framebuffer is removed. So in
this case, the wait for vblank is not called thus leading to a crash
(since fb might get removed before dma is finished)

Patch 3: do not disable crtc if already off
We should not disable the overlay if the crtc is in DPMS_OFF state.
Also, the disable function should not call DPMS_OFF of the crtc.
This is required to ensure we are able to wait for vblank
before freeing any framebuffers after disabling the crtc.

With these three patches and without "drm/exynos: release fb pended by page flip"
I could not find any crash/page_fault in drm with fimd/hdmi during hotplug
and page flips.

 

It seems better than old one and also including some exception codes, Patch 3 we did't consider. Ok, we will test these patches and merge them instead of old one if no problem.

 

Tested and worked fine. But with this patch and without "drm/exynos: release fb pended by page flip", we would still have potential issue to page fault like below,
1. dma is accessing no current fb
2. the fb to be released is not current fb so the fb would be released without disabling crtc when drm is released.
3. but the memory region to no current fb is being accessed by the dma so the page fault would be induced. This is a rare case but possible issue.

Hi Mr. Dae,

But we would not release the fb till we get a vblank (i.e. wait for vblank is called before removing every framebuffer).

Taking your example itself:

fb0 is current fb i.e. (crtc->fb = fb0).

But the dma is accessing from fb1.

We call remove fb1. 

In this case, as you said, crtc will not be disabled as fb1 is not current fb.

But we wait for vblank before removing fb1. Thus we ensure that dma from fb1 is complete and dma from fb0 has started.

So it is safe to remove fb1.

Please, see the below codes,

drm_release()
        drm_fb_release()
                drm_framebuffer_remove()
                        /* assume that current fb is fb1 and dma is accessing fb0 but trying to release fb0 at here */
                        /* this situation could be reproduced with pageflip test. */
                        fb0 is not crtc->fb so the crtc isn't disabled
                        ...
                        drm_framebuffer_unreference(fb0);  <- fb0 will be released without wait_for_vblank().

The wait_for_vblank() is called by exynos_drm_encoder_complete_scanout() when fb->func->destroy() is called so the wait_for_vblank() will be called after fb0 is released.
Is there another place that wait_for_vblank() is called?

Ah, sorry~. exynos_drm_encoder_completescanout() is called prior to gem releasing. Right, no problem. :)

Thanks,
Inki Dae

 

Thanks,
Inki Dae

Hope I correctly understood the issue you mentioned.

 

Anyway I removed the patch, "drm/exynos: release fb pended by page flip". And I hope this issue can be resolved by Daniel's fixup later.

And I think it's better to separate your patch set into 4 patches like below,
1. the patch including only exynos drm framework part.
2. the patch including only fimd driver part.
3. the patch including only hdmi driver part.
4. the patch including exception code.(previous Patch 3)

Please, re-send them.

Sure. I'll post them again as requested.

Regards,

Prathyush

 

Thanks,
Inki Dae

Thanks,

Inki Dae

 

Prathyush K (3)
  drm/exynos: modify wait for vsync functions to use wait queues
  drm/exynos: move wait_for_vblank to manager_ops
  drm/exynos: do not disable crtc if already off

 drivers/gpu/drm/exynos/exynos_drm_crtc.c    |    6 +++-
 drivers/gpu/drm/exynos/exynos_drm_drv.h     |   20 ++------------
 drivers/gpu/drm/exynos/exynos_drm_encoder.c |   15 +++--------
 drivers/gpu/drm/exynos/exynos_drm_fimd.c    |   37 ++++++++++++++++++--------
 drivers/gpu/drm/exynos/exynos_drm_hdmi.c    |   22 ++++++++--------
 drivers/gpu/drm/exynos/exynos_drm_hdmi.h    |    2 +-
 drivers/gpu/drm/exynos/exynos_mixer.c       |   37 +++++++++++++++++---------
 7 files changed, 73 insertions(+), 66 deletions(-)

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