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@xxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxx wrote: >>>>>> From: Ville Syrjälä <ville.syrjala@xxxxxxxxxxxxxxx> >>>>>> >>>>>> 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@xxxxxxxxxxxxxxx> >>>>> 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 _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
