Re: [Nouveau] [PATCH 09/17] drm/radeon: use common fence implementation for fences

[Maarten Lankhorst]

op 22-07-14 15:45, Christian König schreef:
> Am 22.07.2014 15:26, schrieb Daniel Vetter:
>> On Tue, Jul 22, 2014 at 02:19:57PM +0200, Christian König wrote:
>>> Am 22.07.2014 13:57, schrieb Daniel Vetter:
>>>> On Tue, Jul 22, 2014 at 01:46:07PM +0200, Daniel Vetter wrote:
>>>>> On Tue, Jul 22, 2014 at 10:43:13AM +0200, Christian König wrote:
>>>>>> Am 22.07.2014 06:05, schrieb Dave Airlie:
>>>>>>> On 9 July 2014 22:29, Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxx> wrote:
>>>>>>>> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxx>
>>>>>>>> ---
>>>>>>>>   drivers/gpu/drm/radeon/radeon.h        |   15 +-
>>>>>>>>   drivers/gpu/drm/radeon/radeon_device.c |   60 ++++++++-
>>>>>>>>   drivers/gpu/drm/radeon/radeon_fence.c  |  223 ++++++++++++++++++++++++++------
>>>>>>>>   3 files changed, 248 insertions(+), 50 deletions(-)
>>>>>>>>
>>>>>>>  From what I can see this is still suffering from the problem that we
>>>>>>> need to find a proper solution to,
>>>>>>>
>>>>>>> My summary of the issues after talking to Jerome and Ben and
>>>>>>> re-reading things is:
>>>>>>>
>>>>>>> We really need to work out a better interface into the drivers to be
>>>>>>> able to avoid random atomic entrypoints,
>>>>>> Which is exactly what I criticized from the very first beginning. Good to
>>>>>> know that I'm not the only one thinking that this isn't such a good idea.
>>>>> I guess I've lost context a bit, but which atomic entry point are we
>>>>> talking about? Afaics the only one that's mandatory is the is
>>>>> fence->signaled callback to check whether a fence really has been
>>>>> signalled. It's used internally by the fence code to avoid spurious
>>>>> wakeups. Afaik that should be doable already on any hardware. If that's
>>>>> not the case then we can always track the signalled state in software and
>>>>> double-check in a worker thread before updating the sw state. And wrap
>>>>> this all up into a special fence class if there's more than one driver
>>>>> needing this.
>>>> One thing I've forgotten: The i915 scheduler that's floating around runs
>>>> its bottom half from irq context. So I really want to be able to check
>>>> fence state from irq context and I also want to make it possible
>>>> (possible! not mandatory) to register callbacks which are run from any
>>>> context asap after the fence is signalled.
>>> NAK, that's just the bad design I've talked about. Checking fence state
>>> inside the same driver from interrupt context is OK, because it's the
>>> drivers interrupt that we are talking about here.
>>>
>>> Checking fence status from another drivers interrupt context is what really
>>> concerns me here, cause your driver doesn't have the slightest idea if the
>>> called driver is really capable of checking the fence right now.
>> I guess my mail hasn't been clear then. If you don't like it we could add
>> a bit of glue to insulate the madness and bad design i915 might do from
>> radeon. That imo doesn't invalidate the overall fence interfaces.
>>
>> So what about the following:
>> - fence->enabling_signaling is restricted to be called from process
>>    context. We don't use any different yet, so would boild down to adding a
>>    WARN_ON(in_interrupt) or so to fence_enable_sw_signalling.
>>
>> - Make fence->signaled optional (already the case) and don't implement it
>>    in readon (i.e. reduce this patch here). Only downside is that radeon
>>    needs to correctly (i.e. without races or so) call fence_signal. And the
>>    cross-driver synchronization might be a bit less efficient. Note that
>>    you can call fence_signal from wherever you want to, so hopefully that
>>    doesn't restrict your implementation.
>>
>> End result: No one calls into radeon from interrupt context, and this is
>> guaranteed.
>>
>> Would that be something you can agree to?
>
> No, the whole enable_signaling stuff should go away. No callback from the driver into the fence code, only the other way around.
>
> fence->signaled as well as fence->wait should become mandatory and only called from process context without holding any locks, neither atomic nor any mutex/semaphore (rcu might be ok).
fence->wait is mandatory, and already requires sleeping.

If .signaled is not implemented there is no guarantee the fence will be
signaled sometime soon, this is also why enable_signaling exists, to
allow the driver to flush. I get it that it doesn't apply to radeon and nouveau,
but for other drivers that could be necessary, like vmwgfx.

Ironically that is also a part of the ttm fence, except it was called flush there.

I would also like to note that ttm_bo_wait currently is also a function that currently uses is_signaled from atomic_context...

For the more complicated locking worries: Lockdep is your friend, use PROVE_LOCKING and find bugs before they trigger. ;-)

>> Like I've said I think restricting the insanity other people are willing
>> to live with just because you don't like it isn't right. But it is
>> certainly right for you to insist on not being forced into any such
>> design. I think the above would achieve this.
>
> I don't think so. If it's just me I would say that I'm just to cautious and the idea is still save to apply to the whole kernel.
>
> But since Dave, Jerome and Ben seems to have similar concerns I think we need to agree to a minimum and save interface for all drivers.
>
> Christian.
>

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