On Wed, 25 Sep 2013 06:32:10 +0200
Mario Kleiner <mario.kleiner@xxxxxxxxxxxxxxxx> wrote:
> But given the new situation, your proposal is great! If we push the
> clock readouts into the get_scanoutpos routine, we can make this robust
> without causing grief for the rt people and without the need for a new
> separate lock for display regs in intel-kms.
>
> E.g., for intel-kms:
>
> i915_get_crtc_scanoutpos(..., ktime_t *stime, ktime_t *etime)
> {
> ...
> spin_lock_irqsave(...uncore.lock);
> preempt_disable();
> *stime = ktime_get();
> position = __raw_i915_read32(dev_priv, PIPEDSL(pipe));
> *etime = ktime_get();
> preempt_enable();
> spin_unlock_irqrestore(...uncore.lock)
> ...
> }
>
> With your patchset to reduce the amount of register reads needed in that
> function, and given that forcewake handling isn't needed for these
> registers, this should make it robust again and wouldn't need new locks.
>
> Unless ktime_get is also a bad thing to do in a preempt disabled section?
ktime_get() works fine in preempt_disable sections, although it may add
some latencies, but you shouldn't need to worry about it.
I like this solution the best too, but if it does go in, I would ask to
send us the patch for adding the preempt_disable() and we can add the
preempt_disable_rt() to it. Why make mainline have a little more
overhead?
-- Steve
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