Re: linux-4.4 bisected: kwin5 stuck on kde5 loading screen with radeon

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Fri, Jan 15, 2016 at 11:34:08AM +0100, Vlastimil Babka wrote:
> Hi,
> 
> since kernel 4.4 I'm unable to login to kde5 desktop (on openSUSE 
> Tumbleweed). There's a screen with progressbar showing the startup, 
> which normally fades away after reaching 100%. But with kernel 4.4, the 
> progress gets stuck somewhere between 1/2 and 3/4 (not always the same).
> Top shows that kwin is using few % of CPU's but mostly sleeps in poll().
> When I kill it from another console, I see that everything has actually 
> started up, just the progressbar screen was obscuring it. The windows 
> obviously don't have decorations etc. Starting kwin manually again shows 
> me again the progressbar screen at the same position.

Hmm. Sounds like it could then be waiting for a vblank in the distant
future. There's that 1<<23 limit in the code though, but even with that
we end up with a max wait of ~38 hours assuming a 60Hz refresh rate.

Stuff to try might include enabling drm.debug=0x2f, though that'll
generate a lot of stuff. Another option would be to use the drm vblank
tracepoints to try and catch what seq number it's waiting for and
where we're at currently. Or I suppose you could just hack
up drm_wait_vblank() to print an error message or something if the
requested seq number is in the future by, say, more than a few seconds,
and if that's the case then we could try to figure out why that happens.

> 
> I have suspected that kwin is waiting for some event, but nevertheless 
> tried bisecting the kernel between 4.3 and 4.4, which lead to:
> 
> # first bad commit: [4dfd64862ff852df7b1198d667dda778715ee88f] drm: Use 
> vblank timestamps to guesstimate how many vblanks were missed
> 
> I can confirm that 4.4 works if I revert the following commits:
> 63154ff230fc9255cc507af6277cd181943c50a1 "drm/amdgpu: Fixup hw vblank 
> counter/ts for new drm_update_vblank_count() (v3)"
> 
> d1145ad1e41b6c33758a856163198cb53bb96a50 "drm/radeon: Fixup hw vblank 
> counter/ts for new drm_update_vblank_count() (v2)"

The sha1s don't seem to match what I have, so not sure which kernel tree
you have, but looking at the radeon commit at least one thing
immediately caught my attention;

+                       /* Bump counter if we are at >= leading edge of vblank,
+                        * but before vsync where vpos would turn negative and
+                        * the hw counter really increments.
+                        */
+                       if (vpos >= 0)
+                               count++;

It's rather hard to see what it's really doing since the custom flags to
the get_scanout_position now cause it return non-standard things. But if
I'm reading things correctly it should really say something like:

if (vpos >= 0 && vpos < (vsync_start - vblank_start))
	count++;

Hmm. Actually even that might not be correct since it could be using the
"fake" vblank start here, so might be it'd need to be something like:

if (vpos >= 0 && vpos < (vsync_start - vblank_start + lb_vblank_lead_lines)
	count++;

Also might be worth a shot to just ignore the hw frame counter. Eg.:

index e266ffc520d2..db728580549a 100644
--- a/drivers/gpu/drm/radeon/radeon_drv.c
+++ b/drivers/gpu/drm/radeon/radeon_drv.c
@@ -492,7 +492,6 @@ static struct drm_driver kms_driver = {
        .lastclose = radeon_driver_lastclose_kms,
        .set_busid = drm_pci_set_busid,
        .unload = radeon_driver_unload_kms,
-       .get_vblank_counter = radeon_get_vblank_counter_kms,
        .enable_vblank = radeon_enable_vblank_kms,
        .disable_vblank = radeon_disable_vblank_kms,
        .get_vblank_timestamp = radeon_get_vblank_timestamp_kms,
diff --git a/drivers/gpu/drm/radeon/radeon_irq_kms.c b/drivers/gpu/drm/radeon/radeon_irq_kms.c
index 979f3bf65f2c..3c5fcab74152 100644
--- a/drivers/gpu/drm/radeon/radeon_irq_kms.c
+++ b/drivers/gpu/drm/radeon/radeon_irq_kms.c
@@ -152,11 +152,6 @@ int radeon_driver_irq_postinstall_kms(struct drm_device *dev)
 {
        struct radeon_device *rdev = dev->dev_private;
 
-       if (ASIC_IS_AVIVO(rdev))
-               dev->max_vblank_count = 0x00ffffff;
-       else
-               dev->max_vblank_count = 0x001fffff;
-
        return 0;
 }

assuming I'm reading the code correctly.

> 
> 31ace027c9f1f8e0a2b09bbf961e4db7b1f6cf19 "drm: Don't zero vblank 
> timestamps from the irq handler"
> 
> ac0567a4b132fa66e3edf3f913938af9daf7f916 "drm: Add DRM_DEBUG_VBL()"
> 
> 4dfd64862ff852df7b1198d667dda778715ee88f "drm: Use vblank timestamps to 
> guesstimate how many vblanks were missed"
> 
> All clean reverts, just needs some fixup on top to use abs() instead of 
> abs64() due to 79211c8ed19c055ca105502c8733800d442a0ae6.
> 
> Unfortunately I don't know if this is a kernel problem or kwin problem. 
> I tried to CC maintainers of both, advices what to try or what info to 
> provide welcome. The card is "CAICOS" with 1GB memory.
> 
> Thanks,
> Vlastimil

-- 
Ville Syrjälä
Intel OTC