Message-ID: <5374E1B5.2020408@vodafone.de>
Date:	Thu, 15 May 2014 17:48:05 +0200
From:	Christian König <deathsimple@...afone.de>
To:	Maarten Lankhorst <maarten.lankhorst@...onical.com>,
	airlied@...ux.ie
CC:	nouveau@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org
Subject: Re: [RFC PATCH v1 08/16] drm/radeon: use common fence implementation
 for fences

Am 15.05.2014 16:18, schrieb Maarten Lankhorst:
> op 15-05-14 15:19, Christian König schreef:
>> Am 15.05.2014 15:04, schrieb Maarten Lankhorst:
>>> op 15-05-14 11:42, Christian König schreef:
>>>> Am 15.05.2014 11:38, schrieb Maarten Lankhorst:
>>>>> op 15-05-14 11:21, Christian König schreef:
>>>>>> Am 15.05.2014 03:06, schrieb Maarten Lankhorst:
>>>>>>> op 14-05-14 17:29, Christian König schreef:
>>>>>>>>> +    /* did fence get signaled after we enabled the sw irq? */
>>>>>>>>> +    if 
>>>>>>>>> (atomic64_read(&fence->rdev->fence_drv[fence->ring].last_seq) 
>>>>>>>>> >= fence->seq) {
>>>>>>>>> +        radeon_irq_kms_sw_irq_put(fence->rdev, fence->ring);
>>>>>>>>> +        return false;
>>>>>>>>> +    }
>>>>>>>>> +
>>>>>>>>> +    fence->fence_wake.flags = 0;
>>>>>>>>> +    fence->fence_wake.private = NULL;
>>>>>>>>> +    fence->fence_wake.func = radeon_fence_check_signaled;
>>>>>>>>> + __add_wait_queue(&fence->rdev->fence_queue, 
>>>>>>>>> &fence->fence_wake);
>>>>>>>>> +    fence_get(f);
>>>>>>>> That looks like a race condition to me. The fence needs to be 
>>>>>>>> added to the wait queue before the check, not after.
>>>>>>>>
>>>>>>>> Apart from that the whole approach looks like a really bad idea 
>>>>>>>> to me. How for example is lockup detection supposed to happen 
>>>>>>>> with this? 
>>>>>>> It's not a race condition because fence_queue.lock is held when 
>>>>>>> this function is called.
>>>>>> Ah, I see. That's also the reason why you moved the wake_up_all 
>>>>>> out of the processing function.
>>>>> Correct. :-)
>>>>>>> Lockup's a bit of a weird problem, the changes wouldn't allow 
>>>>>>> core ttm code to handle the lockup any more,
>>>>>>> but any driver specific wait code would still handle this. I did 
>>>>>>> this by design, because in future patches the wait
>>>>>>> function may be called from outside of the radeon driver. The 
>>>>>>> official wait function takes a timeout parameter,
>>>>>>> so lockups wouldn't be fatal if the timeout is set to something 
>>>>>>> like 30*HZ for example, it would still return
>>>>>>> and report that the function timed out.
>>>>>> Timeouts help with the detection of the lockup, but not at all 
>>>>>> with the handling of them.
>>>>>>
>>>>>> What we essentially need is a wait callback into the driver that 
>>>>>> is called in non atomic context without any locks held.
>>>>>>
>>>>>> This way we can block for the fence to become signaled with a 
>>>>>> timeout and can then also initiate the reset handling if necessary.
>>>>>>
>>>>>> The way you designed the interface now means that the driver 
>>>>>> never gets a chance to wait for the hardware to become idle and 
>>>>>> so never has the opportunity to the reset the whole thing.
>>>>> You could set up a hangcheck timer like intel does, and end up 
>>>>> with a reliable hangcheck detection that doesn't depend on cpu 
>>>>> waits. :-) Or override the default wait function and restore the 
>>>>> old behavior.
>>>>
>>>> Overriding the default wait function sounds better, please 
>>>> implement it this way.
>>>>
>>>> Thanks,
>>>> Christian. 
>>>
>>> Does this modification look sane?
>> Adding the timeout is on my todo list for quite some time as well, so 
>> this part makes sense.
>>
>>> +static long __radeon_fence_wait(struct fence *f, bool intr, long 
>>> timeout)
>>> +{
>>> +    struct radeon_fence *fence = to_radeon_fence(f);
>>> +    u64 target_seq[RADEON_NUM_RINGS] = {};
>>> +
>>> +    target_seq[fence->ring] = fence->seq;
>>> +    return radeon_fence_wait_seq_timeout(fence->rdev, target_seq, 
>>> intr, timeout);
>>> +}
>> When this call is comming from outside the radeon driver you need to 
>> lock rdev->exclusive_lock here to make sure not to interfere with a 
>> possible reset.
> Ah thanks, I'll add that.
>
>>>      .get_timeline_name = radeon_fence_get_timeline_name,
>>>      .enable_signaling = radeon_fence_enable_signaling,
>>>      .signaled = __radeon_fence_signaled,
>> Do we still need those callback when we implemented the wait callback?
> .get_timeline_name is used for debugging (trace events).
> .signaled is the non-blocking call to check if the fence is signaled 
> or not.
> .enable_signaling is used for adding callbacks upon fence completion, 
> the default 'fence_default_wait' uses it, so
> when it works no separate implementation is needed unless you want to 
> do more than just waiting.
> It's also used when fence_add_callback is called. i915 can be patched 
> to use it. ;-)

I just meant enable_signaling, the other ones are fine with me. The 
problem with enable_signaling is that it's called with a spin lock held, 
so we can't sleep.

While resetting the GPU could be moved out into a timer the problem here 
is that I can't lock rdev->exclusive_lock in such situations.

This means when i915 would call into radeon to enable signaling for a 
fence we can't make sure that there is not GPU reset running on another 
CPU. And touching the IRQ registers while a reset is going on is a 
really good recipe to lockup the whole system.

Christian.

>
> ~Maarten

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