Re: [PATCH v3 21/22] drm/atomic: Introduce drm_atomic_helper_duplicate_commited_state()

[Maarten Lankhorst <maarten.lankhorst@linux.intel.com>]

Op 10-07-17 om 08:43 schreef Daniel Vetter:
> On Fri, Jul 07, 2017 at 06:18:12PM +0300, Ville Syrjälä wrote:
>> On Fri, Jul 07, 2017 at 04:05:28PM +0200, Daniel Vetter wrote:
>>> On Fri, Jul 7, 2017 at 3:21 PM, Ville Syrjälä
>>> <ville.syrjala@linux.intel.com> wrote:
>>>> On Fri, Jul 07, 2017 at 02:03:38PM +0200, Daniel Vetter wrote:
>>>>> On Thu, Jul 06, 2017 at 11:24:41PM +0300, ville.syrjala@linux.intel.com wrote:
>>>>>> From: Ville Syrjälä <ville.syrjala@linux.intel.com>
>>>>>>
>>>>>> For i915 GPU reset handling we'll want to be able to duplicate the state
>>>>>> that was last commited to the hardware. For that purpose let's start to
>>>>>> track the commited state for each object and provide a way to duplicate
>>>>>> the commmited state into a new drm_atomic_state. The locking for
>>>>>> .commited_state must to be provided by the driver.
>>>>>>
>>>>>> drm_atomic_helper_duplicate_commited_state() duplicates the state
>>>>>> to both old_state and new_state. For the purposes of i915 GPU reset we
>>>>>> would only need one of them, but we actually need two top level states;
>>>>>> one for disabling everything (which would need the duplicated state to
>>>>>> be old_state), and another to reenable everything (which would need the
>>>>>> duplicated state to be new_state). So to make it less comples I figured
>>>>>> I'd just always duplicate both. Might want to rethink this if for no
>>>>>> other reason that reducing the chances of memory allocation failure.
>>>>>> Due to the double state duplication we need
>>>>>> drm_atomic_helper_clean_commited_state() to clean up the duplicated
>>>>>> old_state since that's not handled by the normal drm_atomic_state
>>>>>> cleanup code.
>>>>>>
>>>>>> TODO: do we want this in the helper, or maybe it should be just in i915?
>>>>>>
>>>>>> v2: s/commited/committed/ everywhere (checkpatch)
>>>>>>     Handle state duplication errors better
>>>>>> v3: Even more care in dealing with memory allocation errors
>>>>>>     Handle private objs too
>>>>>>     Deal with the potential ordering issues between swap_state()
>>>>>>     and hw_done() by keeping track of which state was swapped in
>>>>>>     last
>>>>>>
>>>>>> Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
>>>>> I still don't get why we need to duplicate the committed state for gpu
>>>>> reset. When I said I'm not against adding all that complexity long-term I
>>>>> meant when we actually really need it. Imo g4x gpu reset isn't a good
>>>>> justification for that, reworking the atomic world for that seems
>>>>> massively out of proportion.
>>>> Well, I still don't see what's so "massive" about a couple of extra state
>>>> pointers hanging around.
>>>>
>>>> Also while doing that state duplication stuff, my old idea of
>>>> splitting the crtc disable and enable phases into separate atomic
>>>> commits popped up again in my head. For that being able to duplicate
>>>> arbitrary states would seem like a nice thing to have. So the
>>>> refactoring I had to do can have other uses.
>>> I fully realize that you're unhappy with how atomic ended up getting
>>> merged and that you think it's a grave mistake. And maybe it is, maybe
>>> it isn't, I have no idea.
>> I don't think I ever said that. I've said that it has certain design
>> problems that we ought to fix. This one being one, and another being
>> to separate the user state from the internal state. The latter I think
>> we'll have to tackle rather soon thanks to some new hardware in the
>> pipeline, or we need to come up with some other way to achieve the
>> same effect.
>>
>>> But right now we have _nothing_ asking for
>>> that reorg afaik, and using gen4 reset to justify it is in my opinion
>>> simply not solid engineering practice. Maybe we need this in the
>>> future, and then we can add it, but not before. Refactoring stuff to
>>> prettify the architecture isn't really useful work.
>> Neither is having to throw out code that already exists and works. If
>> you're so worried about time being wasted on pre-g4x GPU reset, then
>> we could just as well merge my code and move on to more productive
>> endeavors.
> I'm worried about future time wasted on this, not current time.
>
>>>>> Why exactly can't we do this simpler? I still don't get that part. Is
>>>>> there really no solution that doesn't break atomic's current assumption
>>>>> that commits are fully ordered on a given crtc?
>>>> From the point of view of the old and new states it doesn't actually
>>>> break that. The commits done from the reset path are essentially
>>>> invisible to the normal modeset operation.
>>> You insert something into a fully ordered queue. That does break the
>>> entire concept and needs a pile of locks and stuff to make it work.
>> Exactly one lock. Well two if you could the spinlock to protect the
>> committed_state pointer update from parallel updates touching the same
>> kms object. That latter one could be removed if atomic wouldn't allow
>> parallel commits to touch the same object.
>>
>>> Yes it's doable, but it's a redesign with all the implications of
>>> subtle breakage all over.
>> What? It doesn't really even do anything unless you do the
>> duplicate_committed state(). Everything else is just assigning pointers.
>> So unless there's some really obvious bug somewhere it can't break
>> anything outside the GPU reset path. And really the only way to break
>> to GPU reset path is to have actual bugs in the normal display commit
>> code.
> It's the gpu reset I'm worried about. There's no point in fixing it if it
> immediately breaks again.
>
>>> While we do have open bugs for the current
>>> design. Rewriting the world to fix a bug needs seriously better
>>> justification imo.
>>>
>>>> The one alternative proposed idea of allowing gem and kms sides go
>>>> out of whack scares me a bit. I think that might land us in more
>>>> trouble when I finally get around to making the video overlay a
>>>> drm_plane.
>>> We've run perfectly fine with this idea for years.
>> Not perfectly. I've had to fix it several times. And I don't think I was
>> the only one.
> The problem is that no one tests against gen4, and everyone forgets that
> it exists. That's why I'd like something with the minimal amount of
> invasiveness, since that would at least be easier to patch up when we
> inevitably break it. Also, something entirely contained to i915
> conceptually, without imposing more restrictions on shared code.
>>>> And I think trying to keep the GPU reset paths as similar as possible
>>>> between all the platforms would be a nice thing. Just whacking
>>>> everything on the head with a hammer on one platform but not on
>>>> another one seems to me like extra variation in behaviour that we
>>>> don't necessarily want.
>>>>
>>>> But like I said, if someone can come up with a better solution I
>>>> probably wouldn't object too much. It's not going to be coming from me
>>>> though since I have plenty of other things to do and vacation time is
>>>> coming up very soon. So unless someone else comes up with something nice
>>>> soon I think we should just go with my solution because a) it's already
>>>> available, and b) works quite decently from what I can see.
>>> I guess I'll have to retype the old thing in the new world, but it
>>> really shouldn't be more than the quick draft I've laid down in the
>>> old thread. This here is imo no-go with all the core changes, and even
>>> just done within i915 I think it's highly dubious that it provides a
>>> real benefit, since defacto it means we'll have to abandon the atomic
>>> helpers entirely.
>> Now I think you're just being difficult for the sake of it. Have you
>> looked at the code at all? It's fully done from the atomic helpers
>> right now. And even moving the committed state tracking to i915
>> wouldn't require abandoning the helpers. We could just update the
>> committed state pointers when we call hw_done(), and we'd have to
>> update the state seqno/age timestamp when we call swap_state().
>> That's all there is to this.
> I'm concerned with the maintainenace burden you impose on all future i915
> atomic work, that's all. Yes it looks simple to you and right now, but
> it's another little thing to keep working, and we're really good at
> breaking stuff all the time.
>
> But if you strongly think this is the best possible approach overall,
> taking into account long-term impact, then go ahead with implementing this
> in i915. Adding it the concept of a committed state and being able to
> duplicate that and squeeze another commit in to the shared atomic helpers
> doesn't make sense imo.
> -Daniel

I think the problem is about struct_mutex usage by atomic commit during reset.
GPU reset has to wait for all previous atomic updates to complete, but
cleanup_planes and prepare_plane_fb both require struct_mutex, which can lead
to a deadlock. #99093

The real fix is not taking struct_mutex during atomic commit, which will mean
no deadlock can happen.

Is this the bug being fixed here or am I missing something?

~Maarten

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