Re: [PATCH 2/2] drm/i915: Handle inaccurate time conversion issues

[Jani Nikula <jani.nikula@linux.intel.com>]

On Fri, 28 Nov 2014, Daniel Vetter <daniel.vetter@ffwll.ch> wrote:
> So apparently jiffies<->nsec<->ktime isn't accurate or something. At
> elast if we timeout there's occasionally still a few hundred us left
> (in a 2 second timeout).
>
> Stuff I've tried and thrown out again:
> - Sampling the before timestamp before jiffies. Doesn't improve test
>   path rate at all.
> - Using jiffies. Way to inaccurate, which means way too much drift
>   with signals plus automatic ioctl restarting in userspace. In
>   hindsight we should have used an absolute timeout, but hey we need
>   something for v3 of the i915 gem wait interfaces ;-)
> - Trying to figure out where accuracy gets lost. gl testcase really
>   don't care all that much about this (as long as isn't not massively
>   off), it's just that the testcase gets a bit upset if it receives an
>   EITME with timeout > 0.
>
> So as long as we're in the ballbark it's good enough. So patch
> everything up if we're at most one jiffies off. I get's me a solid
> test again.
>
> This regression is probably introduced in
>
> commit 5ed0bdf21a85d78e04f89f15ccf227562177cbd9
> Author: Thomas Gleixner <tglx@linutronix.de>
> Date:   Wed Jul 16 21:05:06 2014 +0000
>
>     drm: i915: Use nsec based interfaces
>
>     Use ktime_get_raw_ns() and get rid of the back and forth timespec
>     conversions.
>
>     Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>     Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
>     Signed-off-by: John Stultz <john.stultz@linaro.org>
>
> Probably because I'm too lazy to confirm myself and still waiting for
> QA ;-)
>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: John Stultz <john.stultz@linaro.org>
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82749
> Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

Pushed to drm-intel-next-fixes, thanks for the patch.

BR,
Jani.

> ---
>  drivers/gpu/drm/i915/i915_gem.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index 002730d1409b..abdc5bcdbd0d 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -1302,6 +1302,16 @@ int __i915_wait_seqno(struct intel_engine_cs *ring, u32 seqno,
>  		s64 tres = *timeout - (now - before);
>  
>  		*timeout = tres < 0 ? 0 : tres;
> +
> +		/*
> +		 * Apparently ktime isn't accurate enough and occasionally has a
> +		 * bit of mismatch in the jiffies<->nsecs<->ktime loop. So patch
> +		 * things up to make the test happy. We allow up to 1 jiffy.
> +		 *
> +		 * This is a regrssion from the timespec->ktime conversion.
> +		 */
> +		if (ret == -ETIME && *timeout < jiffies_to_usecs(1)*1000)
> +			*timeout = 0;
>  	}
>  
>  	return ret;
> -- 
> 1.9.3
>
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-- 
Jani Nikula, Intel Open Source Technology Center
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