linux-kernel - Re: [RFC PATCH v1 08/16] drm/radeon: use common fence implementation for fences

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Message-ID: <5374CC9A.9090905@canonical.com>
Date:	Thu, 15 May 2014 16:18:02 +0200
From:	Maarten Lankhorst <maarten.lankhorst@...onical.com>
To:	Christian König <deathsimple@...afone.de>,
	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

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. ;-)

~Maarten
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