On Mon, Feb 08, 2016 at 02:13:26AM +0100, Mario Kleiner wrote:
> Changes to drm_update_vblank_count() in Linux 4.4 broke the
> behaviour of the pre/post modeset functions as the new update
> code doesn't deal with hw vblank counter resets inbetween calls
> to drm_vblank_pre_modeset an drm_vblank_post_modeset, as it
> should.
>
> This causes mistreatment of such hw counter resets as counter
> wraparound, and thereby large forward jumps of the software
> vblank counter which in turn cause vblank event dispatching
> and vblank waits to fail/hang --> userspace clients hang.
>
> This symptom was reported on radeon-kms to cause a infinite
> hang of KDE Plasma 5 shell's login procedure, preventing users
> from logging in.
>
> Fix this by detecting when drm_update_vblank_count() is called
> inside a pre->post modeset interval. If so, clamp valid vblank
> increments to the safe values 0 and 1, pretty much restoring
> the update behavior of the old update code of Linux 4.3 and
> earlier. Also reset the last recorded hw vblank count at call
> to drm_vblank_post_modeset() to be safe against hw that after
> modesetting, dpms on etc. only fires its first vblank irq after
> drm_vblank_post_modeset() was already called.
>
> Reported-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Mario Kleiner <mario.kleiner.de@xxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 4.4+
> Cc: michel@xxxxxxxxxxx
> Cc: vbabka@xxxxxxx
> Cc: ville.syrjala@xxxxxxxxxxxxxxx
> Cc: daniel.vetter@xxxxxxxx
> Cc: dri-devel@xxxxxxxxxxxxxxxxxxxxx
> Cc: alexander.deucher@xxxxxxx
> Cc: christian.koenig@xxxxxxx
We need to untangle the new vblank stuff that assumes solide (atomic)
drivers and all the old stuff much more. But that can be done later on.
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
> drivers/gpu/drm/drm_irq.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/drivers/gpu/drm/drm_irq.c b/drivers/gpu/drm/drm_irq.c
> index aa2c74b..5c27ad3 100644
> --- a/drivers/gpu/drm/drm_irq.c
> +++ b/drivers/gpu/drm/drm_irq.c
> @@ -222,6 +222,21 @@ static void drm_update_vblank_count(struct drm_device *dev, unsigned int pipe,
> }
>
> /*
> + * Within a drm_vblank_pre_modeset - drm_vblank_post_modeset
> + * interval? If so then vblank irqs keep running and it will likely
> + * happen that the hardware vblank counter is not trustworthy as it
> + * might reset at some point in that interval and vblank timestamps
> + * are not trustworthy either in that interval. Iow. this can result
> + * in a bogus diff >> 1 which must be avoided as it would cause
> + * random large forward jumps of the software vblank counter.
> + */
> + if (diff > 1 && (vblank->inmodeset & 0x2)) {
> + DRM_DEBUG_VBL("clamping vblank bump to 1 on crtc %u: diffr=%u"
> + " due to pre-modeset.\n", pipe, diff);
> + diff = 1;
> + }
> +
> + /*
> * Restrict the bump of the software vblank counter to a safe maximum
> * value of +1 whenever there is the possibility that concurrent readers
> * of vblank timestamps could be active at the moment, as the current
> @@ -1573,6 +1588,7 @@ void drm_vblank_post_modeset(struct drm_device *dev, unsigned int pipe)
> if (vblank->inmodeset) {
> spin_lock_irqsave(&dev->vbl_lock, irqflags);
> dev->vblank_disable_allowed = true;
> + drm_reset_vblank_timestamp(dev, pipe);
> spin_unlock_irqrestore(&dev->vbl_lock, irqflags);
>
> if (vblank->inmodeset & 0x2)
> --
> 1.9.1
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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