Op 10-07-17 om 14:18 schreef Ville Syrjälä: > On Mon, Jul 10, 2017 at 11:31:55AM +0200, Maarten Lankhorst wrote: >> 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 deadlocks I've seen recently didn't necessarily involve > struct_mutex IIRC. Just the modeset locks. > >> 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? > This would avoid both struct_mutex and modeset locks in the display > reset path, so I guess it should help with struct_mutex issues > as well. > I think fixing i915 to not require struct_mutex for vma pinning/unpinning will be a better use of our time, and it should also fix all deadlocks. :) And it's far better than duplicating drm_atomic_commit functionality in our reset handlers. _______________________________________________ dri-devel mailing list dri-devel@xxxxxxxxxxxxxxxxxxxxx https://lists.freedesktop.org/mailman/listinfo/dri-devel
