On 28/06/16 11:48, Chris Wilson wrote:
On Tue, Jun 28, 2016 at 01:37:30PM +0300, Imre Deak wrote:
Since wait_for_atomic doesn't re-check the wait-for condition after
expiry of the timeout it can fail when called from non-atomic context
even if the condition is set correctly before the expiry. Fix this by
using the non-atomic wait_for instead.
wait_for_atomic is indeed only safe to be called from atomic context.
Likewise, wait_for is only safe to called from !atomic context.
I noticed this via the PLL locking timing out incorrectly, with this fix
I couldn't reproduce the problem.
Fixes: 0351b93992aa ("drm/i915: Do not lie about atomic timeout granularity")
The bug would be using wait_for_atomic from non-atomic context, and so
older.
CC: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
CC: Tvrtko Ursulin <tvrtko.ursulin@xxxxxxxxx>
Signed-off-by: Imre Deak <imre.deak@xxxxxxxxx>
---
drivers/gpu/drm/i915/intel_dpll_mgr.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/drivers/gpu/drm/i915/intel_dpll_mgr.c b/drivers/gpu/drm/i915/intel_dpll_mgr.c
index c0eff15..e130c3e 100644
--- a/drivers/gpu/drm/i915/intel_dpll_mgr.c
+++ b/drivers/gpu/drm/i915/intel_dpll_mgr.c
@@ -1374,8 +1374,8 @@ static void bxt_ddi_pll_enable(struct drm_i915_private *dev_priv,
I915_WRITE(BXT_PORT_PLL_ENABLE(port), temp);
POSTING_READ(BXT_PORT_PLL_ENABLE(port));
- if (wait_for_atomic_us((I915_READ(BXT_PORT_PLL_ENABLE(port)) &
- PORT_PLL_LOCK), 200))
+ if (wait_for_us((I915_READ(BXT_PORT_PLL_ENABLE(port)) & PORT_PLL_LOCK),
+ 200))
Does this work with CONFIG_I915_DEBUG and CONFIG_DEBUG_ATOMIC_SLEEP ?
CONFIG_PREEMPT_COUNT is also required.
There were a bunch of these WARNs triggering in various places. I think
I had patches to fix them but at the same time Mika had a more
comprehensive work in progress for the whole area. I suppose that just
got delayed to much.
AFAIR the meat of the discussion was what is more important - sleep
granularity or timeout accuracy. I preferred the former to avoid waiting
for too long for operations which are normally much quicker than a
jiffie and normally succeed.
Another issue if wait_for_us for sleeps < 10us is not the most efficient
implementation. So another idea I had is to implement those via the
wait_for_atomic but without the in_atomic WARN. And obviously now after
Imre found this with the extra cond check as well.
So I think Imre's patches are good in principle, should go in, and
probably afterwards we can talk about improving wait_for_us for timeouts
under 10us and potentially the timeout precision as well.
Regards,
Tvrtko
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